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Microbiology/Infectious Diseases II


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(MIDII #1)

What is the “magic bullet” for tuberculosis that was discovered in 1952? What short course treatment regimen followed in 1970? How is tuberculosis treated now?
With the advent of streptomycin the late 1940s and INH in 1952, TB became curable! INH is the magic bullet for TB. Rifampin followed in the 1970s.
(MIDII #2)

Discuss the history of tuberculosis.
TB was not a significant problem until the 17th and 18th centuries as urbanization and crowding in unventilated living conditions increased, but by the 19th century with industrialization, TB caused ¼ of adult deaths in Europe. Koch discovered the TB bacillus and generated the “germ theory” of disease. Prior to the discovery of antibiotics, TB was treated with sanatorium regimens and rest. Patients were ordered to have fresh air and sunshine on rooftops and solaria.
(MIDII #3)

Discuss the epidemiology of tuberculosis in Asia, Sub-Saharan Africa and Europe
Mycobacterium tuberculosis infects 1/3 of the world's population. Causes 2 million deaths per year, 2nd only to HIV as a cause of death from infectious agents worldwide among adults. The relationship between TB and HIV has exacerbated problems, with TB increasing in areas with high AIDS incidence, particularly sub-Saharan Africa. (1) Absolute numbers of cases of TB are highest in Asia where the population density is highest, BUT (2) Case rates are highest in sub-Saharan Africa: 300 per 100,000 estimated incidence rates in sub-Saharan Africa vs 100-299 per 100,000 in Asia. (3) In Europe, 10% of infected people develop active disease and only 50% of cases are cavitary (cavitary cases are infectious). Each cavitary case needs to infect 20 to maintain the constant rate of cases. Better living conditions are less conducive to airborne spread.
(MIDII #4)

Discuss the epidemiology of TB in the United States.
The US saw a steady decline of TB until 1984 with a slowly increasing incidence. Causes: (a) neglect of TB control programs; (b) increase in urban homelessness and resultant crowding in homeless shelters; (c) advent of AIDS epidemic among this population. Restored TB control program funding and decreased crowding of the homeless means that the highest background rates of TB are among immigrants from high prevalence countries. 50% of the cases in the US are among foreign born. New York, New England, and the west coast states all have greater than 50% of cases foreign born as of 2003, with 300 per 100,000 estimated incidence rates.
(MIDII #5)

Discuss the organisms responsible for causing tuberculosis.
Mycobacterium tuberculosis complex includes several species, all derived from a soil bacterium: (1) Mycobacterium tuberculosis; (2) Mycobacterium bovis, which is found in unpasteurized milk. There has been a recent rash of cases in the US among immigrants who have favorite cheeses made from unpasteurized milk sent to them from home, particularly Mexico and the Dominican Republic; (3) Mycobacterium bovis-BCG is used to treat bladder cancer; (4) Mycobacterium africanum and Mycobacterium canetti are rare causes of TB in Africa; (5) Mycobacterium microti is a pathogen for rodents.

The organisms are aerobic, non-motile, non-spore forming bacilli. They have a high cell wall content of high molecular weight lipids (mycolic acid). They have a SLOW GROWTH RATE, with a generation time of 20 hrs vs. E.coli's generation time of 20 minutes. Takes 3-8 wks for growth on solid media. This has implications for length of treatment for complete sterilization compared with most bacterial pathogens (in other words, it takes longer to kill M. tuberculosis b/c it reproduces so slowly.)
(MIDII #6)

Lungs are the portal of entry (except for M. bovis in unpasteurized dairy products from other countries). Transmission of TB requires inhalation of droplet nuclei (bacillus 5 microns) from an infectious person with active pulmonary tuberculosis (NOT just positive PPD). COUGH is most efficient at 3000 infectious droplet nuclei per cough. TALKING: similar quantity over 5 min. SNEEZING is more efficient than coughing; SINGING is intermediate between talking and coughing. The INOCULUM SIZE is relevant. TB can be transmitted in the autopsy suite from a cadaver: cutting thru lung tissue aerosolized millions of bacilli, and subsequent PPD conversion and progression to active TB is “astonishingly high.” VIRULENCE of the strain is also relevant. There was an outbreak of TB in Kentucky after minimal contact w/the index patient. The bacillus remains alive and infectious in the air for a long period. Ventilation is key in preventing transmission. Isolation of the patient is req'd as well as maintaining a mandated number of air exchanges in hospital rooms.
(MIDII #7)

Discuss PRIMARY INFECTION with M. tuberculosis (BEFORE THE IMMUNE RESPONSE!!!) Is it possible to have reactivation of one metastatic focus (TB meningitis) without the others? Can you have TB meningitis without pulmonary TB, for exam
The bacillus reaches the alveoli and replicates extracellularly in the alveolar space & intracellularly in the alveolar MΦs. There is a lack of immediate host immune response. The alveolar MΦ ingests the TB bacillus; the bacillus sits in the phagosome; the phagosome normally incorporates proton-ATPase into the membrane yielding ↓ pH & acidification within the phagosome; the acidified phagosome fuses w/the cell lysosome, exposing the bug to toxic enzymes. MTB prevents insertion of proton-ATPase into the phagosome so the phagosome never gets acidified and never merges with the lysosome. MTB multiplies for weeks, both in the initial focus in alveolar Mφs and in cells transported lymphohematogenously throughout the body. METASTATIC FOCI are well established in regional lodes (hilar, mediastinal) and then to tissues which retain the bacilli and favor multiplication, such as (a) apical posterior areas of lungs; (b) lymph nodes in the neck; (c) kidneys; (d) epiphyses of long bones; (e) vertebral bodies; (f) juxtaependymal meningeal areas adjacent to subarachnoid space. These will be areas of reactivation of disease in the future since the seeded organisms remain alive but dormant once the immune response occurs. Reactivation can occur in any of these areas of the body with or without reactivation in others: it is possible to get TB meningitis or “scrofula” without pulmonary TB.
(MIDII #8)

Discuss the development of the immune response to TB, which requires an intact cellular immune system, including CD4+ T cells.
The immune response develops 6-12 wks after initial infection with MTB. Alveolar MΦs infected w/MTB release IL-12 & IL-18, which attract and stimulate T lymphocytes (mainly CD4 cells). ALL people have a native population of CD4 cells which can recognize mycobacterial antigens that have been processed and presented by MΦs. When a CD4 meets a mycobacterial antigen presented by the MΦ it becomes activated (transformed). A transformed CD4 cell proliferates and produces a clone of similarly reactive T lymphocytes. When the population of activated lymphocytes is large enough, there is a cutaneous delayed reaction to tuberculin = tissue hypersensitivity = positive PPD test. (AIDS patients with low CD4 cell count cannot effect an immune response to tuberculin so you cannot use a PPD test on them). Activated CD4+ T cells release IFN-γ which stimulates MΦ phagocytosis and stimulates MΦs to secrete regulatory factors such as TNF-α, which is req'd for granuloma formation and increases the MΦ's ability to kill M. tuberculosis.
(MIDII #9)

What happens if an individual lacks TNF-α with regard to the immune response to M. tuberculosis infection?
TNF-α is secreted by macrophages and increases the macrophages ability to kill M. tuberculosis; it is required for granuloma formation.(1) In mice, blockade of TNF-α resulted in reactivation, high bacillary burden, persistent tuberculosis and death. (2) TNF-α KO mice infected with M. tuberculosis followed a similar course. (3)Anti-TNF-α agents for rheumatoid arthritis and auto-immune disorders cause reactivation of tuberculosis.
(MIDII #10)

Discuss the PATHOLOGY of infection with M. tuberculosis. The tissue response to MTB infection depends on activation of MΦs with secretion of lytic enzymes which cause tissue necrosis. (1) What are EPITHELIOD CELLS? LANGHANS GIANT
(1)EPITHELOID CELLS are highly stimulated MΦs. LANGHANS GIANT CELLS have fused MΦs oriented around tuberculosis antigen w/multiple nuclei lined up peripherally. (2) SMALL Ag load and HIGH tissue hypersensitivity produce organization of lymphocytes, MΦs, Langhans giant cells & fibroblasts -> result in a granuloma, a successful tissue rxn resulting in containment of infection, healing w/fibrosis, encapsulation & scar formation. (3) LARGE Ag load and HIGH tissue hypersensitivity produce few or no epitheloid cells or Langhans cells, disorganized lymphocytes, MΦs and PMNs and result in necrosis and caseation. Caseous material is acellular and inhibits multiplication of organsms due to its pH and oxygen tension but is inherently unstable, liquefies and discharges through the bronchial tree. This discharge produces a cavity in which TB bacillus multiplies to make population 5-6 logs greater than in noncavitary lesions. (4) Large OR small Ag load w/no tissue hypersensitivity produces few PMNs or mononuclear cells and HUGE numbers of bacilli. This is seen in AIDS patients w/low CD4+ counts. Implications for post treatment appearance of lung and chest x-ray in AIDS patients is a lack of fibrosis or granuloma.
(MIDII #11)

CLINICAL SYNDROMES OF TB INFECTION: Primary Infection with Resolution
This occurs in 85% of cases. Patient is asymptomatic or has a mild viral syndrome. Enlargement of hilar and peribronchial nodes at time of infection. On CXR there is a calcified granuloma, evidence that the tuberculosis infection was successfully contained. 6-12 wks later there is development of a positive PPD.
(MIDII #12)


(1)Progressive Primary Disease in Young Children
(2)Primary Infection in Adolescence/Young Adulthood
(1)PROGRESSIVE PRIMARY DISEASE: (a) Occurs in very young children (<5) who are unable to resolve the infection. Local progression w/mid or lower field pneumonitis initially, then dissemination, miliary pattern in the lungs and frequent CNS involvement. Almost always occurs in developing world countries were TB remains endemic. (b) TB PLEURISY is a hypersensitivity rxn to a small # of organisms which reach the pleura in primary infection, resulting in exudative pleural effusion. Cultures are negative b/c very few organisms are present. 90% resolve spontaneously but WWII studies of soldiers showed 65% relapse to active TB (pre-antibiotic era). TB pleurisy should be treated! (2) ADOLESCENTS & YOUNG ADULTS: Primary infection results in “adult type” upper lobe cavitary disease. Puberty influences tendency to apical cavitation soon after initial infection. 23% of those infected betw/15-19, 13% of those infected betw/ 20-24, and 4% of those infected from 25-29 develop cavitary disease. Only 2% of patients over 30 develop cavitary disease w/primary infection.
(MIDII #13)


Discuss AIDS Nosocomial Outbreaks of Tuberculosis
AIDS nosocomial outbreaks of TB occur in AIDS wards, homeless shelters and prisons. Occurs when an undiagnosed patient w/active pulmonary TB is hospitalized in an AIDS ward or a shelter for AIDS patients. All patients have CD4 < 50. The index patient coughs and infects the other AIDS patients. AIDS patients with no cellular immune function can't mobilize CD4 and MΦs to contain or kill the bacillus. Results in rapid dissemination or death if untreated. Blood cultures will be positive for M. tuberculosis. (Multi-drug resistant TB outbreaks killed many patients, resistance wasn't monitored.)
(MIDII #14)


In what percentage of infected individuals does reactivation occur? When does it occur and why?
Reactivation occurs in 10-15% of all infected with MTB. Involves persistence of viable organisms following containment of the initial infection. Disease occurs years after infection when the cellular immune response is no longer able to contain MTB: (a) IATROGENIC => in transplant patients, immunosuppressive therapy for connective tissue disorders (autoimmune disorders); (b) IMMUNOCOMPROMISING DISEASES => AIDS, malignancies, end stage renal disease, cirrhosis; © MALNUTRITION; (d) OLD AGE => hypersensitivity and cellular immunity wane w/age; (e) UNKNOWN CAUSES => hormonal, stress (immigrants).
(MIDII #15)


Where does reactivation occur and what happens?
Pulmonary location is the most frequent site of reactivation of TB, occurs in 85% of patients. The posterior aspect of the upper lobe is the focus where reactivation begins. [Due to increased O2 in the apices and MTB's aerophilia. Also possible that deficient lymphatic flow at the apices results in retention of bacillary antigen and a hypersensitivity rxn leads to necrosis.] There is a localized pneumonitis => the inflammatory response produces fibrin rich exudates into the alveoli, caseating necrosis, liquefaction. There is drainage into the bronchial tree with cavity formation. Cavities favor bacillary multiplication in HUGE amts, 5-6 logs > than # of organisms in non-cavitary lesions. There are 10^9 or 10^10 organisms per gram of tissue. Cavitary disease is the MOST contagious as the cough aerosolizes hundreds of thousands of organisms.
(MIDII #16)


Discuss extrapulmonary tuberculosis
Viable organisms remain alive and dormant for years in all sites to which the bacilli disseminated during the primary infection. (1) LYMPH NODES => scrofula is the most frequent form of extrapulmonary TB. Usually affects the cervical or supraclavicular chain. Biopsy and culture are essential (fine needle aspirate usually smear and culture negative). (2) MENINGES => rupture of subependymal tubercle into subarachnoid space. Meningitis is most severe at the base of the brain causing a thick gelatinous exudate. Affects the cranial nerves as they exit. CSF exam is essential to make the diagnosis: low glucose, elevated protein, lymphocytic pleocytosis. (3) BONES => 1/3of cases of extrapulmonary TB involve the spine (Pott's disease): hematogenous spread from contiguous disease, lymphatic spread from pleural disease. Early focus is the anterior part of the vertebral body. Spreads to the disk and then to adjacent vertebra. X-ray shows anterior wedging of 2 adjacent vertebral bodies and destruction of the disk. A tender spine prominence on exam is called a GIBBUS.
(MIDII #17)

(1)SYSTEMIC symptoms are non-specific: fever, fatigue, night sweats, weight loss.

(2)PULMONARY symptoms include cough, productive or dry. Most patients have the cough but may ignore it for weeks.

(3)HEMOPTYSIS: (a) Mild-moderate w/chronic blood streaking results from caseous sloughing or endobronchial erosion and is seen in advanced disease. (b) Rasmussen's Aneurysm: sudden massive hemoptysis results from erosion of the pulmonary artery and is the only TB emergency
(MIDII #18)

MYCOBACTERIUM TUBERCULOSIS: Diagnostic Procedures => SPUTUM staining and cultures: (1)Acid Fast Stain and (2) Culture
**SMEAR POSITIVITY means AT LEAST 10,000 ORGANISMS/mL of SPUTUM** (1) ACID FAST STAIN: Acid fast implies mycobacterial species (note: nocardia is weakly acid fast) but many other species besides M. tuberculosis complex will be AFB positive (including Mycobacterium avium, kansasii, abscessus, chelonae). (a) ZIEHL-NEELSEN STAIN: fixed smear covered with carbol-fuchsin, heated, rinsed, decolorized with acid alcohol. Kinyoun stain is similar but does not require heating. (b) FLUOROCHROME STAIN with phenol-auramine or auramine-rhodamine; modified acid alcohol step and potassium permanganate counterstaining. Fluorescent mycobacteria are visible with 20 or 40x magnification. (2) CULTURE is the GOLD STANDARD. (a) Solid media can detect 10-100 organisms, takes 3-8 wks. Lowenstein Jensen is egg-based. Middlebrook 7H11 is agar based. Can detect colony morphology & mixed infections. (b) Liquid broth takes 1-3 wks to detect organisms. Middlebrook 7H12 and BACTEC systems. **CULTURE IS NECESSARY TO DETERMINE DRUG SUSCEPTIBILITIES**
(MIDII #19)

MYCOBACTERIUM TUBERCULOSIS: Diagnostic Procedures => SPUTUM molecular diagnostics:
(1)Nucleic Acid Amplification and (2) DNA Fingerprinting
(1)Nucleic Acid Amplification can detect M. tuberculosis in fresh sputum. (Developed world technology, too costly for resource poor countries). Sensitivity is intermediate between acid fast smear and culture. If the AFB smear is negative, nucleic acid amplification is 40-77% sensitive. If the AFB smear is positive, nucleic acid amplification is 95% sensitive and nearly 100% specific. (2) DNA Fingerprinting: RFLP is also developed world technology. A restriction endonuclease produces DNA fragments; we separate the fragments by electrophoresis; apply a probe to a repetitive DNA sequence (insertion sequence 6110). There are numerous copies of IS6110 present in the M. tuberculosis chromosome at highly variable locations. Useful for: (a) identifying transmission from person-to-person; (b) distinguishing endogenous reactivation from exogenous reinfection in recurrent TB; (c) lab cross-contamination
(MIDII #20)

MYCOBACTERIUM TUBERCULOSIS: Diagnostic Procedures => Chest X-Ray
Luxury of developed world technology. CXR patterns of patients w/TB include: (1) Upper lobe infiltrate w/ or w/o cavitations (apical or sub-apical). This pattern is most common in reactivation disease in an intact immune system. The radiologic extent of disease reflects tissue damage, which reflects the host's ability to have a hypersensitivity rxn. (2) Hilar adenopathy w/or w/o infiltrates. Pattern is most common in AIDS patients, reflects minimal cellular immune response. (3) Pleural effusion: ALWAYS exudative. (a) Seen in post-primary infection, w/ scant organisms (this is a tissue hypersensitivity rxn). The smear will NEVER be positive. Culture is positive in 25% of cases. (b) Can also be seen as a complication of reactivation pulmonary TB, in which case organisms are more likely. Smear positive 50%; culture positive 60%. (4) Miliary pattern: (millet seed pattern). CXR shows 0.5-1.0 mm nodules. Follows childhood infection and disseminated infection. Also occurs in immunocompromised patients suffering from alcoholism, cirrhosis, & rheumatologic diseases, or patients being treated w/immunosuppressive agents. Dx can be tough – might be multiple organ involvement w/millet seed granulomas in tissues. Transbronchial biopsy is the highest yield diagnostic location. (5) Can also see atypical infiltrates.
(MIDII #21)

MYCOBACTERIUM TUBERCULOSIS: General Principles of Treatment of TB
Always use at least 2 drugs; usually begin with 3 or 4 pending sensitivities. There is a natural incidence of spontaneous drug resistance (1 in 10^5 organisms are resistant to each drug). Bacilli resistant to 1 drug will be killed with the other drug. Natural resistance to 2 drugs occurs spontaneously in 1 in 10^10 or 10^11. A prolonged treatment of 6-9 months is necessary if the organism is pan-sensitive. Directly observed therapy is advised for all patients. No one is 100% compliant regardless of age, sex, race, or education. Given daily treatment for the first 2 months; intermittent w/adjusted doses for continuation phase of 4-7 months depending on the regimen.
(MIDII #22)

ALL GIVEN ONCE DAILY TOGETHER: NEVER DIVIDE DOSES. (1) ISONIAZID (INH): bactericidal against dividing organisms. Causes hepatitis. Chemical = 20%, Clinical (age-related: <35=0.3%; >65=4%). (2) RIFAMPIN (RMP): bactericidal. Enables short course treatment (6-9 months vs. 18-24 months with non-RMP regimens). Causes drug-drug interactions. RMP is a potent INDUCER of hepatic microsomal enzymes (cytochrome P450). (3) PYRAZINAMIDE (PZA): enables shortening of the regimen from 9 months to 6 months. (4) ETHAMBUTOL (EMB): used in drug resistance and in situations where INH or RMP cannot be used (INH: hepatotoxicity; RMP: drug-drug interactions).
(MIDII #23)

MYCOBACTERIUM TUBERCULOSIS: Who needs a PPD test? What is a positive PPD test? What is “the Booster phenomenon”? When do you treat a patient with a positive PPD?
(I) Targeted testing: PPD is NOT a general screen. Only use PPD for patients at high risk of developing active TB: (1) Immunocompromised: HIV infected, chemotherapy patients, patients undergoing organ transplant, patients on immunosuppressive Rx for autoimmune diseases, rheumatoid arthritis. (2) Close contacts of infectious case (household or close working quarters). (3) Previously untreated patients with chest x-ray evidence of old fibrotic changes (not just calcified granuloma). (4) Recent immigrants (in US <5 yrs) from endemic areas. (5) Ppl who work in institutions where TB exposure is likely: hospitals, nursing homes, homeless shelters, prisons. (II) Definition of a positive PPD (purified protein derivative): 5 mm => HIV infected, close contacts of infectious case, chest x-ray evidence of old disease (fibrotic scarring, not just calcified granuloma); 10 mm => patients from endemic areas of TB. (III) The Booster Phenomenon: 2 step testing is essential for all those >55 whose exposure/infection was in the distant past and for those with BCG (the vaccine). (IV) Treat only those at high risk of reactivation, with INH for 9 months.
(MIDII #24)

MYCOBACTERIUM TUBERCULOSIS: How do you determine of a positive PPD reactivity represents TB infection, not BCG?
Enzyme-linked immunospot (ELISPOT) is a T-cell based assay from the blood. M. tuberculosis genes not present in M. bovis BCG produce an antigen to which T cells react. 1 tube of blood is needed for the test, which is not feasible in resource poor settings. This is useful in outbreaks for contact investigations.
(MIDII #25)

(1)What is it? This is an M. Bovis strain attenuated through serial passage. There is no standardized strain or procedure to make one.
(2) Does it work? The largest study performed in India suggests that there is no protection from TB infection. Other studies, done in England, suggest there is protection from TB infection. Prevalence of non-TB mycobacteria in a given region may interfere. The background prevalence of TB determines the utility of the vaccine.
(3) Who uses it? All agree that it is highly effective for infants and small children in preventing dissemination and meningitis when infected by M. tuberculosis. Newborns are/should be vaccinated in all high prevalence areas of the world.
(MIDII #26)

FUNGI: While fungal virulence is not common, fungi are not casual pathogens. Most humans have a strong natural immunity to the fungi, but when this immunity is breached the consequences can be severe and dramatic. As modern medicin
Key take home point on fungi: **EVERY FUNGAL INFECTION IS “OPPORTUNISTIC”** Fungi are taking advantage in some way, eluding the patient's ordinarily competent defenses against them. The ways in which they do this vary.
(MIDII #27)

Definition: FUNGUS
A non-motile eukaryotic protist characterized by the absence of chlorophyll and the presence of a rigid cell wall.
(MIDII #28)

Definition: YEAST
A fungus growing as a round or oval unicellular organism, reproduced by budding or fission.
(MIDII #29)

Definition: MOLD
A fungus growing in hyphal form, reproduced by sexual or asexual spore formation.

[According to Wikipedia, a hypha (plural hyphae) is a long, branching filament. A typical hypha consists of a tubular wall, usually made of chitin, which surrounds, supports, and protects the cells that compose a hypha. For most fungi, a cell within a hyphal filament is separated from other cells by internal cross-walls called septa (singular septum).]
(MIDII #30)

Definition: HYPHA (pl. hyphae)
A branching, tubular filament.
(MIDII #31)

Definition: MYCELIUM (pl. mycelia)
A mass of hyphae.
(MIDII #32)

A filament (or mass of filaments) composed of many elongated yeast cells adherent to each other end to end.
(MIDII #33)

Definition: DIMORPHIC
Having the ability to transform between yeast and mold stages in response to specific changes in the environment.
(MIDII #34)

Definition: SPORE
The sexual or asexual reproductive unit of a mycelium
(MIDII #35)

Definition: MYCOSIS (pl. mycoses)
A human disease caused by a fungus. The term mycosis refers to conditions in which fungi pass the resistance barriers of the human body and establish infections. Mycoses are classified according to the tissue levels initially colonized: (1) Superficial mycoses are limited to the outermost layers of skin & hair; (2) Cutaneous mycoses extend deeper into the epidermis, as well as invasive hair & nail diseases. Host immune responses may be evoked, resulting in pathologic changes expressed in the deeper layers of skin. Dermatophytes cause these diseases, called ringworm (tinea). Cutaneous mycoses are caused by Microsporum, Trichophyton, and Epidermophyton fungi which comprise 41 species. (3) Subcutaneous mycoses involve the dermis, subcutaneous tissues, muscle, & fascia. Chronic, can be initiated by piercing trauma to the skin which allows the fungi to enter. Difficult to treat, may require surgical debridement. (4) Systemic mycoses due to primary pathogens originate primarily in the lungs and may spread to many organ systems. Organisms are inherently virulent, generally dimorphic. (5) Systemic mycoses due to opportunistic pathogens: infections of patients with immune deficiencies who would otherwise not be infected. Examples of immunocompromised conditions include AIDS, alteration of normal flora by antibiotics, immunosuppressive therapy, and metastatic cancer. Examples of opportunistic mycoses include Candidiasis, Cryptococcosis, and Aspergillosis.
(MIDII #36)

Distinctions between Bacteria & Fungi

(1)Size; (2) Nuclear Structure; (3) Cell Membrane Composition; (4) Cell Wall Composition; (5) Respiration; (6) Metabolism; (7) Preferred Temperature; (8) Toxin Production; (9) La
(1) SIZE: (F) diameter 2-10 microns, volume (yeasts) 20-50 microns; (B) diameter 1 micron, volume 1-5 microns. (2) NUCLEAR STRUCTURE: (F) eukaryotic; (B) prokaryotic. (3) CELL MEMBRANE COMPOSITION: (F) sterols present (ergosterol); (B) no sterols present. (4) CELL WALL COMPOSITION: (F) chitin, glucans, mannans; (B) peptidoglycans, lipopolysaccharides, lipoproteins. (5) RESPIRATION: (F) aerobes or facultative anaerobes. No strict anaerobes; (B) aerobic or anaerobic. (6) METABOLISM: (F) heterotrophic; (B) heterotrophic or autotrophic. (7) PREFERRED TEMPERATURE: (F) usually 25-35 deg C; (B) usually 37 deg C. (8) TOXIN PRODUCTION: (F) rarely; always irrelevant in human infection; (B) occasionally; may be impt in human infection. (9) LAB STAIN OF CHOICE: (F) KOH preparation of smears, methenamine silver or PAS stains of tissue. Gram stain is unreliable; (B) Gram stain. (10) IMPT IMMUNITY: (F) cellular immunity is most important, with the role of Abs still under investigation; (B) humoral and cellular immunity are both important.
(MIDII #37)

Discuss Clinical Categories of Fungal Infections:
(1)Superficial/Cutaneous: infection of the outer layer of skin by lipophilic or keratinolytic fungi (e.g. Pityriasis, Dermatophytosis); (2) Subcutaneous: infection of subcutaneous tissues from the traumatic implantation of the fungus into the skin (e.g. Sporotrichosis); (3) “The True Pathogenic Fungi” can cause disease even in immunocompetent hosts (e.g. Histoplasmosis, Coccidioidomycosis, Blastomycosis); (4) “The Opportunistic Fungi” generally cause disease only in immunocompromised hosts (e.g. Cryptococcus, Candidiasis, Aspergillosis, Mucormycosis)
(MIDII #38)

Distinguish between pathogenic and opportunistic fungi with regard to the following categories: (1) Habitat; (2) Morphology; (3) Route of Infection; (4) Host Infected; (5) Host Response; (6) Immunity; (7) Prognosis
(1)HABITAT: (Pathogens) Generally confined to endemic areas; (Opportunists) Omnipresent; (2) MORPHOLOGY: (P) Dimorphic; (O) No true dimorphs; (3) ROUTE OF INFECTION: Usually pulmonary; (O) varies; (4) HOST INFECTED: (P) Often 'normal'; (O) Usually immunocompromised; (5) Host Response: (P) Pyogenic or granulomatous; (O) Necrosis to pyogenic to granulomatous, depending on degree of host impairment; (6) IMMUNITY: (P) Resolution imparts strong specific immunity; (O) No specific resistance to reinfection; (7) PROGNOSIS: (P) Most resolve spontaneously; (O) Depends on degree of host impairment.
(MIDII #39)

Name the primary host defense against the following fungi: (1) Dermatophytes, (2) Sporothrix, (3) Histoplasma, (4) Blastomyces, (5) Coccidioides, (6) Cryptococcus, (7) Candida, (8) Aspergillus, (9) Mucorales
(1) Dermatophytes: intact skin barrier; (2) Sporothrix: intact skin barrier; (3) Histoplasma: lymphocyte function; (4) Blastomyces: lymphocyte function; (5) Coccidioides: lymphocyte function; (6) Cryptococcus: lymphocyte function, possibly humoral immunity; (7) Candida: cutaneous infection => intact skin barrier; mucocutaneous infection => lymphocyte function; disseminated infection: PMN function, intact skin and mucosal barrier; (8) Aspergillus: invasive infection => PMN function; (9) Mucorales: invasive infection => PMN function
(MIDII #40)

Superficial Fungal Infections: DERMATOPHYTOSIS
(ORGANISM): Caused by Dermatophytes (members of the general Trichophyton, Microsporum, Epidermophyton). (MYCOLOGY): Related species of mold all possessing keratinases. (EPIDEMIOLOGY): Omnipresent. (PATHOGENESIS): Spores on shed skin or hairs adhere to the stratum corneum, germinate, and invade. Individual susceptibility varies. Organisms are confined to the stratum corneum, with surrounding inflammation penetrating deeper layers of skin. (CLINICAL): Tinea corporis (ringworm), Tinia pedis (athlete's foot), Tinea cruris (jock itch), Tinea capitis (scalp and hair), onychomycosis (nails). (DIAGNOSIS): characteristic appearance, KOH preps or Wood's light exam, smear and culture of specimen. (TREATMENT): topical (azoles), systemic (griseofulvin, azoles, allylamines)
(MIDII #41)

Superficial Fungal Infections: PITYRIASIS VERSICOLOR
(ORGANISM): Malassezia furfur. (MYCOLOGY): A lipophilic yeast, part of the normal skin flora. (EPIDEMIOLOGY): Omnipresent; the disease is more prevalent in the tropics. (PATHOGENESIS): Yeasts proliferate in settings of lipids and sweat. Rare in kids, appears in adolescence. (CLINICAL): Pityriasis = tinea versicolor. Seborrheic dermatitis. Infusion of lipid-containing nutritional solutions is rarely associated with M. furfur bacteremia. (DIAGNOSIS): Characteristic appearance; UV light fluorescence. (TREATMENT): Topical (selenium sulfide or azoles), catheter removal for fungemia
(MIDII #42)

Subcutaneous Fungal Infections: SPOROTRICHOSIS
(ORGANISM): Sporothrix schencki. (MYCOLOGY): Dimorphic. Grows as mycelium in culture; as yeast w/variably sized and shaped cells in the host. (EPIDEMIOLOGY): Worldwide distribution in soil, especially Mexico and South America. (PATHOGENESIS): Thorny plants (roses) or splinters inoculate fungus into subcutaneous tissues. Infection spreads slowly along draining lymphatics. Tissue reaction is mixed pyogenic and granulomatous. Fungi are often difficult to identify in lesions. (CLINICAL): Small hard nodule at the primary inoculation site discolors and ulcerates. There are multiple similar nodules along lymphatics, which themselves become hard and cordlike. Rare bone and joint involvement; rarer dissemination. Likelihood of severe disease appears to be increased with poor nutrition or repeated exposure. (DIAGNOSIS): Culture of tissue or drainage (fungi are rarely seen on smear or section). Skin test determines exposure but not disease. (TREATMENT): ketoconazole, itraconazole, amphotericin B.
(MIDII #43)

The “Pathogenic” Fungi: HISTOPLASMOSIS

Organism, Mycology, Epidemiology, Pathogenesis
(ORGANISM): Histoplasma capsulatum. (MYCOLOGY): Dimorphic fungus: grows as mycelium in culture; as small budding yeasts in host. (EPIDEMIOLOGY): Worldwide distribution in soils w/high nitrogen content – guano of birds, poultry, bats. Caves. Starling habitats. Endemic in the Ohio-Mississippi Valley, Central & South America, Puerto Rico. (PATHOGENESIS): Spores are inhaled from the soil, transform to yeast in the lungs, where they are phagocytosed by MΦs with intracellular multiplication, dissemination, granulomatous host response as in TB. May resolve with scarring, remain active in the lungs, or disseminate in settings of immunocompromise. Moves through reticuloendothelial organs (liver, spleen, lymph nodes). Like TB, may reactivate years after the initial exposure (in the context of AIDS, etc.)
(MIDII #44)

The “Pathogenic” Fungi: HISTOPLASMOSIS (II)

Clinical presentation, Diagnosis, Treatment
(CLINICAL) 1) Self-limited disease => asymptomatic (95%) or flu-like syndrome. Noninfectious complications of healing may include mediastinal fibrosis or pericarditis. 2) Chronic pulmonary disease mimics TB; most common in settings of prior lung damage. 3) Disseminated disease (0.01%): fever, wasting, variable hepatosplenomegaly, ulcerations in mouth, GI tract, adrenal insufficiency, pancytopenia. (DIAGNOSTIC TESTS): Skin test confirms exposure, but not infection. It may artificially boost subsequent serologic testing. Antibody titers are often unreliable, especially in settings of immunocompromise. Antigen detection in serum and urine is sensitive but not commercially available. (DIAGNOSIS) 1) Pulmonary: exposure + compatible illness + compatible x-ray + Ab titer +/- sputum culture. 2) Disseminated disease: organisms seen intracellularly in blood/bone marrow/liver/urine/tissue and/or grown in culture. (TREATMENT) Mild pulmonary: none. Severe pulmonary or disseminated: Amphotericin B
(MIDII #45)

The “Pathogenic” Fungi: COCCIDIOIDOMYCOSIS (“Cocci”)
(ORGANISM): Coccidioides immitis; (MYCOLOGY): Dimorphic => grows as mycelium in culture; as spherules producing endospores in host; (EPIDEMIOLOGY): Endemic in arid, hot areas of North, Central, South America ('the lower Sonoran life zone.') Creosote bushes and rodent burrows. Soil organisms aerosolize late summer-fall. Note outbreak of cocci in Southern California immediately following LA earthquake of fall 1994. (PATHOGENESIS): Inhaled spores swell into spherules in the lung, burst releasing hundreds of endospores. Spherules evoke mononuclear response, endospores are neutrophilic. Pulmonary pathology: consolidation, caseation, necrosis, cavity, and nodule formation. Hematogenous dissemination of endospores to meninges, skin, bone, liver, spleen with lesions ranging from pyogenic abscesses to granulomas. Reactivation years after exposure may occur.
(MIDII #46)

The “Pathogenic” Fungi: COCCIDIOIDOMYCOSIS (“Cocci”)
(CLINICAL): 60% asymptomatic or mild pulmonary infection. 40% significant pulmonary infection: fever, cough, malaise, sputum production, erythema multiforme (“Valley Fever”). 5% progressive pulmonary disease, residual pulmonary nodule or cavity. 0.5% disseminated disease (more common in men, pregnancy, dark-skinned races, immunocompromise). Skin: fungating masses or ulcers; bone: osteolytic and/or blastic lesions; meninges (25%): chronic meningitis.(DIAGNOSIS): Coccidioidin skin test positive 1-3 wks after infection but not reliable: anergy common in disseminated disease. Complement fixation antibody may be (+) in disseminated disease, but not helpful in immunocompromised hosts. Best: smear and culture of pus, tissue. (TREATMENT): For severe primary disease, for primary disease in persons at risk of dissemination, for disseminated disease. Amphotericin B; fluconazole; itraconazole
(MIDII #47)

The “Pathogenic” Fungi: BLASTOMYCOSIS
(ORGANISM): Blastomyces dermatitidis. (MYCOLOGY): Dimorphic fungus. Grows as mycelium in culture, as large yeasts with refractile cell walls and broad-based buds in host. (EPIDEMIOLOGY): Primarily in humid, wooded areas of North America. Prefers decaying organic matter and wood to soil. Outbreaks associated with log cabin building, peanut harvesting, camping, beaver dam viewing. (PATHOGENESIS): Conidia inhaled, convert to yeast form in lungs. Evoke neutrophilic and granulomatous response. Dissemination via infected MΦs to skin, bone, urinary tract. In skin, pseudoepitheliomatous hyperplasia can simulate squamous cell cancer. (CLINICAL): Frequency of asymptomatic infection unknown. Pulmonary infection mimics histoplasma, TB. Disseminated disease: Skin involvement is most common: ulcerating, heaped-up pigmented nodules face/hand/legs/mucosa +/- sinus tracts. 25-50% bone disease: well-demarcated osteolysis long bones/skull. Urogenital disease 5-20%: epididymitis/prostatitis/scrotal ulcers. Disease of liver, spleen, GI tract rare. (DIAGNOSIS): No adequate skin test, Ab or Ag assays exist. Best option is smear or culture of affected site. (TREATMENT): Amphotericin B, ketoconazole, itraconazole
(MIDII #48)

The “Opportunistic” Fungi: CRYPTOCOCCOSIS
(FUNGUS): Cryptococcus neoformans. (MYCOLOGY): A yeast without a clinically relevant mold phase. Characterized by unique thick polysaccharide capsule. (EPIDEMIOLOGY): The pigeon and its excreta, worldwide. Some species colonize eucalyptus trees instead. (PATHOGENESIS): Aerosolized yeasts inhaled. Pulmonary disease (nodular, miliary) may occur. Hematogenous dissemination to CNS, where symptomatic human disease most common. Result: edematous, mucoid, cloudy meninges, usually with a poor inflammatory response. Intracerebral masses of yeasts may form (cryptococcomas). (CLINICAL): Acute or chronic meningitis, w/headache, fever, elevated intracerebral pressure. Often associated w/AIDS or other immunocompromise, although cases in apparently normal hosts do occur. Cell-mediated immunity of paramount importance. Ab is formed against capsule and may play a role in host defense. (DIAGNOSIS): India ink preparation, culture of CSF (or other body site). Polysaccharide capsule allows for easy cryptococcal Ag assay in serum or CSF -> sensitive and specific when titers are high. (TREATMENT): Amphotericin B +/- 5-flucytosine; fluconazole.
(MIDII #49)

The “Opportunistic” Fungi: CANDIDIASIS
ORGANISM: Candida albicans; MYCOLOGY: Yeast cells, pseudohyphae & hyphae in both culture & host. Organism stains Gram (+); EPIDEMIOLOGY: Normal inhabitant of the alimentary tract & mucosa (mouth, vagina, anus). NOT on normal skin, although damage, moisture, or any biochemical change leads to rapid colonization. Also predisposing to colonization: infancy, debilitation, pregnancy, antibiotics.
(PATHOGENESIS): Primary host defenses consist of intact skin and mucosa w/normal pH & flora (oral, GI tract, vaginal). Antibiotics, pregnancy, skin maceration leaves even healthy ppl susceptible to superficial Candida infection. Dissemination occurs via defect in mucosa, or formation of biofilm on a catheter/foreign body. Once organism gets thru dermis or into blood, PMNs are 1st line of defense: phagocytize & kill blastopores, damage psuedohyphae. Monocytes & Eos can ingest & kill the organism. Phagocytes w/o myeloperoxidase, or w/o ability to generate superoxide ion can't kill effectively. Serum or plasma alone (with Abs & complement) CANNOT kill Candida. Complement IS necessary for opsonization; IgG also opsonizes. Patients w/disseminated Candidiasis can have a significant Ab response to infection. Lymphocytes protect against proliferation on the skin or mucosa, so HIV infection predisposes primarily to oral/vaginal candidiasis. NK cells w/ anti-candidal activity have been ID'd. Lymphocyte cytokines are necessary to recruit phagocytes in disseminated disease.
(MIDII #50)

The “Opportunistic” Fungi: CANDIDIASIS

Discuss Virulence Factors of Candida albicans!!
There are NO hypervirulent species of candida. People are infected by their own previously nonpathogenic flora, NOT someone else's “monster strain.” Virulence factors include: (a) ENVIRONMENTAL TOLERANCE => survive acid pH (stomach), bloodstream, mucosal surfaces. (b) SECRETED HYDROLASES => produce extracellular proteases, phospholipases, and other enzymes that destroy CT and kill host cells. (c)ADHERENCE => to host cells as well as dentures, catheters, and prosthetic devices. (d) Ability to grow in HYPHAL form => induced by presence of serum and long associated w/invasive disease in tissue sections. Hyphal form protects yeast after phagocytosis, allowing escape from the phagocytic cell and progression of infection. In the mouse model, mutant Candida deficient in hyphal formation was avirulent. NOTE: gene INT1 encodes surface protein Int1p which enables the organism to grow esp in filamentous form and seems linked to virulence. In vitro, superantigen-like effects (T cells activated; cascade of cytokines ---> sepsis syndrome) result when Int1p is cleaved by heparin (a common clinical anticoagulant).
(MIDII #51)

The “Opportunistic” Fungi: CANDIDIASIS

Discuss the Clinical Presentation of Infection w/Candida albicans, Diagnosis & Treatment
Mucocutaneous: thrush (oral candidiasis); vaginitis, balanitis, onychomycosis; esophageal candidiasis. UT: dysuria, fever. Disseminated: fever, chills, renal dysfunction, endophthalmitis, skin lesions. Cellular immunity is most impt in protecting against mucosal infection. PMNs protect against invasion.

Dx: (1) Pathognomonic appearance (thrush); (2) Smear/culture of usually sterile site (distinction between colonization and infection can be tricky); (3) Pathologic confirmation: organism in tissue.

Rx: (1) Remove the breach of host defense! (2) Local (nystatin, clotrimazole); systemic (amphotericin B, fluconazole, voriconazole, caspofungin)
(MIDII #53)

The “Opportunistic” Fungi: MUCORMYCOSIS (phycomycosis, zygomycosis)

(Organisms): Species of the order Mucorales, esp rhizopus. (Mycology): Molds w/o a yeast phase. Broad, nonseptate hyphae, haphazardly branching at r
(Pathogenesis): Spores are inhaled into the alveoli & the nasal turbinates. If unchecked, grow w/blood vessel invasion. Erode thru sinuses, nerves, bone. Thrombosis, infarction, tissue necrosis, surrounded by neutrophilic cellular infiltrate. (Clinical): Most fulminant fungal infection known! Disease almost uniformly confined to patients w/acidosis (diabetes, diarrhea, uremia, salicylate ingestion) or leukemia, occasionally other forms of immunocompromise. PMNs are the primary defense, so neutropenia is a major risk. Rare in AIDS but reported. (a) Rhinocerebral: facial pain, headache, sinusitis, facial swelling, orbital cellulitis, cranial nerve signs. (b) Pulmonary: cough, hemoptysis, rapidly progressive pulmonary insufficiency. (Dx): Stain and culture of biopsy specimen. (Rx): Correct hyperglycemia, acidosis, immunocompromise, Amphotericin B
(MIDII #54)

Targets of Antifungal Agents: Discuss POLYENES (amphotericin B, nystatin)

Amphotericin B is produced by Streptomyces nodosus (aerobic actinomycete). (MOA): Focuses on fungal cytoplasmic membrane and binding to sterols, s
(Spectrum): Amphotericin is the most broad spectrum antifungal and is considered the GOLD STANDARD for Rx of a variety of fungal infections including Candida, Cryptococcus neoformans, Blastomyces dermatitidis, Histoplasma capsulatum, Sporothrix schenkii, coccidiodomycosis, paracoccidiodes, Aspergillus, Penicillium, mucormycosis. (Pharmacokinetics): Available only as an injection for IV use. Oral suspension may be compounded for RX of oropharygeal & esophageal candidal infections. When given orally it has negligible GI absorption and is not reliable for treatment of systemic infections. After IV administration, most drug leaves circulation quickly. Stored in liver and other organs. Reenters circulation slowly. Extensively bound to tissues. Half-life is prolonged (~15 days) and can be detected in the body up to 7 wks after discontinuing therapy. 2-5% of each dose is excreted unchanged in the urine. Most drug is degraded in the body. No dose adjustment is needed in patients w/renal and/or hepatic dysfunction. Amphotericin achieves its highest concentrations in the liver & spleen w/less in the kidneys & lungs. It doesn't appear to penetrate CSF, brain, pancreas, muscles, bone, vitreous humor, normal amniotic fluid, & bronchial secretions well.
(MIDII #55)

Targets of Antifungal Agents: Discuss POLYENES (amphotericin B, nystatin)

Discuss FORMULATIONS of Amphotericin!
Amphotericin B deoxycholate: amphotericin is insoluble in water & is formulated as a complex w/the bile salt deoxycholate. This is “conventional” amphotericin B. Lipid formulation of amphotericin are advantageous w/respect to toxicities. Nephrotoxic but cause less nephrotoxicity than conventional amphotericin. 30-50 times more expensive so their use is cost limited. No difference in efficacy. (a) Amphotericin B colloidal dispersion (ABCD, Amphotec) is a colloidal dispersion containing equal amts of amphotericin B and cholesteryl sulfate. ABCD particles are disk shaped. (b) Amphotericin B lipid complex (ABLC, Abelcet) contains dimyristoyl phophatidylcholine and dimyristoyl phosphatidylglycert in a 7:3 mixture w/35 mol % amphotericin in a ribbon-like sheet structure. © Liposomal amphotericin B (Ambisome) is a unilamellar liposome (one molecule ampho B per 9 molecules lipid). Lipid contains phosphatidylcholine, cholesterol, and distearoylphosphatidylglycerol in a 10:5:4 molar ratio.
(MIDII #56)

Targets of Antifungal Agents: Discuss POLYENES (amphotericin B, nystatin)

(1)Nephrotoxicity => All amphotericin products cause some degree of nephrotoxicity. Manifests as a dose-dependent decrease in glomerular filtration rate (GFR) via direct vasoconstrictive effect on afferent renal arterioles. Also: potassium, magnesium, and bicarbonate wasting, and decrease in erythropoietin production. Permanent renal failure is related to the total dose and is due to destruction of renal tubular cells, disruption of the tubular basement membrane, and loss of functioning nephron units. Risk factors for nephrotoxicity include concomitant nephrotoxic drugs (cyclosporine, aminoglycosides), hypotension, intravascular volume depletion, renal transplant, and other pre-existing renal conditions. Nephrotoxicity may be reduced by saline loading w/each dose. May not be possible if patients can't tolerate fluids (heart failure, renal failure, other fluid overload states). Lipid products are less nephrotoxic, but can still cause nephrotoxicity.
(MIDII #57)

Discuss TOXICITIES of Amphotericin: (2) Electrolyte Abnormalities & (3) Infusion Related Reactions
(2) Electrolyte Abnormalities => Renal tubular acidosis and renal wasting of potassium, magnesium, and phosphate is seen during treatment w/ amphotericin and for several wks after treatment is discontinued. Close monitoring (daily) and electrolyte supplementation are necessary. (3) Infusion Related Rxns (IRRs) => Common early in course. Rxns are most severe w/1st 3-5 doses. Usually subside w/cont'd use & premedications. Chills, fever, tachypnea may occur during infusion. Premedicating w/diphenhydramine & acetominophen for all doses can diminish rxns. Is recommended. If severe shaking chills (rigors) occur, infusion are temporarily discontinued and meperidine administered to shorten the rigor. These rxns are expected. Should NOT be mistaken for anaphylaxis. Allergic rxns can occur but are extremely rare. (4) Other Rxns => Nausea and vomiting are common. Thrombophlebitis may occur w/peripheral administration. A normocytic, normochromic anemia may occur and is associated w/decreased epoetin levels.
(MIDII #58)

Discuss CLINICAL USES of Amphotericin
(1)Candidiasis (azole resistant strains, too), Cryptococcal meningitis, mucormycosis, invasive aspergillosis; (2) Empiric therapy in patients with febrile neutropenia; (3) Intrathecally for coccidiodal meningitis; (4) Intraocular injections for fungal endopthlamitis; (5) Bladder irrigation for fungal cystitis; (6) Oral suspension for oropharyngeal and esophageal candidiasis.
(MIDII #59)

FLUCYTOSINE => Flucytosine is a fluorinated pyrimidine related to fluorouracil. It is deaminated to 5-fluorocytosine. (MOA): Flucytosine is deaminated to 5-FC, then converted to 5-fluorodeoxyuridylic acid monophosphate which is
(Sp): Flucytosine has useful activity (in combo w/other agents) against Cryptococcus neoformans, Candida, & chromomycoses. (Ph): Available for oral admin only. Rapid, complete absorption from GI tract. Excreted RENALLY w/90% excreted unchanged in urine. Req's dose adjustment in pts w/renal disease. Penetrates well into CSF (75% of serum levels), aqueous humor, joints, bronchial secretions, peritoneal fluid, brain, bile, & bone. Usual half-life is ~3-6 hrs, but may be increased to ~200 hrs in pts w/renal failure. Serum [Cx] monitoring is recommended. (Tox's): (1) Bone marrow suppression = most common & severe toxicity. Dose-related; associated w/concentrations >100-125mcg/mL. Leukopenia & thrombocytopenia are usual manifestations. Risk factors: underlying hematologic disorders & concomitant BM suppressive drugs. (2) Nausea, vomiting, diarrhea (GI disturbances) are also common. (Clin): Clinically useful against cryptococcal, candidal, & chromomycoses, but NOT a drug of choice. Not as efficacious as other agents. Associated w/development of resistance. Usually used in combo w/other agents (commonly amphotericin).
(MIDII #60)

Discuss AZOLES as a class of antifungal agents.
(MIDII #60)

Discuss AZOLES as a class of antifungal agents.
(MIDII #61)


Ketoconazole is a synthetic imidazole antifungal. Rarely used. (Spectrum): Used to treat mucocutaneous candidiasis, coccidiodomycosis, histoplasmosis, paracoccidiodomycosis, and blastomycos
(TOXICITIES): GI disturbances include dose-dependent nausea, vomiting, and anorexia. Endocrinopathies: Ketoconazole inhibits steroid biosynthesis in humans (as it does in fungi) and may result in endocrinopathies. Causes a dose-dependent decrease in testosterone synthesis and can suppress androgen production. Gynecomastia and oligospermia in men and menstrual irregularities in women have been seen. Hepatotoxicity may occur but is less common. Majority of cases occur in the first 3 months of therapy. (CLINICAL USES): Ketoconazole use is very limited for fungal infections and has largely been replaced with itraconazole. Topical ketoconazole shampoo is the most commonly used formulation of ketoconazole.
(MIDII #62)

Discuss ITRACONAZOLE, an AZOLE => Spectrum, Pharmacokinetics
Itraconazole is a triazole antifungal w/broader spectrum of activity, a more desirable pharmacokinetic profile, & less toxicity than ketoconazole. Still limited by pharmacokinetics, drug interactions, & toxicities. (SPECTRUM): Blastomycosis, histoplasmosis, coccidiodomycosis, paracoccidiodomycosis, sporotrichosis, ringworm (including onychomycosis), tinea versicolor, & aspergillosis. (PHARMACOKINETICS): Until recently, itraconazole was only available in an oral formulation. Use of this agent has been limited due to variability in Cx's achieved following oral dosing and association w/treatment failures. It is water insoluble. Available in oral capsules and an oral soln. The soln is coformulated w/cyclodextrin to improve H2O solubility and absorption (30% greater than capsules). Oral absorption of capsules is improved w/food. Soln is best absorbed on an empty stomach. Tissue, pus, and bronchial secretion Cx's are higher than plasma levels. CSF and ocular levels are low. Primarily metabolized in the liver by CYP3A4 and also inhibits this enzyme system. Subject to drug interactions. T ½ = 30-40 hrs, will be prolonged in pts w/severe liver disease.
(MIDII #63)

Discuss ITRACONAZOLE, an AZOLE => Toxicities and Clinical Uses
(TOXICITIES): (a) GI disturbances: dose-related nausea, diarrhea, & abdominal discomfort. (b) Hepatotoxicity. (c) Thrombophlebitis => venous inflammation with thrombus formation. Significant thrombophlebitis is associated w/the IV formulation when it is administered peripherally. Increased fluid dilation volume is often necessary. (d) Negative inotrope: IV administration in dogs and humans has resulted in neg inotropic effects. Should not be administered to pts w/congestive heart failure or pts w/ventricular dysfunction for the treatment of onychomycosis. (CLINICAL USES): (a) Treatment of pulmonary & extrapulmonary blastomycosis. (b) Histoplasmosis, including chronic cavitary pulmonary disease & disseminated, nonmeningeal histoplasmosis. (c) Pulmonary & extrapulmonary aspergillosis, usually in pts who are resistant to treatment (refractory) with amphotericin or who are intolerant to amphotericin. (d) Empiric fungal therapy in pts w/neutropenic fever.
(MIDII #64)

Discuss FLUCONAZOLE, an AZOLE => Spectrum and Pharmacokinetics
Fluconazole is the best tolerated and most widely used antifungal agent. (Spectrum): Candidiasis, Cryptococcus neoformans, Coccidiodomycosis. SOME activity against sporotrichosis, ringworm, histoplasmosis, and blastomycosis, but NOT as efficacious as itraconazole. NO activity against Aspergillus or mucormycosis. (Pharmacokinetics): Available in both oral and IV forms. Oral formulation is very well absorbed from the GI tract. Bioavailability is >80%. Oral absorption is not affected by pH. Fluconazole is mainly excreted renally w/60-75% of the drug appearing unchanged in the urine. Dose adjustment is necessary in pts w/renal disease. Penetrates well into saliva, sputum, urine, and other body fluids including CSF (~70% of plasma levels) and brain. Subject to a limited number of drug interactions including interactions w/phenytoin, cyclosporine, warfarin, rifampin, rifabutin, sulfonylureas, and tacrolimus.
(MIDII #65)

Discuss FLUCONAZOLE, an AZOLE => Toxicities & Clinical Uses
(Toxicities): Adverse effects are uncommon. Nausea, vomiting, anorexia may occur at higher doses. Rare hepatotoxicity has occurred (with high, prolonged doses).

(Clinical Uses): Commonly used for infections caused by susceptible Candida sp. (systemic, cutaneous, oropharyngeal, esophageal, intra-abdominal, vaginal, etc.). Has been used as initial treatment and maintenance of cryptococcal meningitis in HIV patients. Fungal prophylaxis in neutropenic patients. Coccidiodomycosis (meningitis and disseminated).
(MIDII #66)

Discuss VORICONAZOLE, an AZOLE => Spectrum & Pharmacokinetics
Voriconazole is the newest triazole antifungal. Structurally related to fluconazole. Expands on fluconazole's clinical activity to include fluconazole-resistant Candida sp., Aspergillus sp., and rare molds (Scedosporium sp., Fusarium sp.) Appears fungistatic against yeast but fungicidal against molds. Available orally w/good bioavailability. Attractive option for long term maintenance therapy for mold infections in immunosuppressed pts. Pharmacokinetic profile is significantly improved over itraconazole but drug interactions and toxicity may still be problematic.

(Spectrum): Candida sp. (incl. most fluconazole-resistant strains), Aspergillus sp., Blastomyces dermatitidis, Coccidiodes immitis, Histoplasma capsulatum. Some strains of Pseudoallescheria boydii, Scedosporium apiospermum, Fusarium sp., Paecilomyces sp., Bipolaris sp., & Alternaria sp. Voriconazole is less active against Sporothrix schenkii. (Pharmacokinetics): Available in IV & ORAL formulations. Oral is best absorbed on an empty stomach w/>90% bioavailability. Exhibits nonlinear pharmacokinetics in adults due to saturation of metabolism. Significant degree of interpatient variability in serum Cx's. In children, elimination is linear; higher doses are req'd to attain similar Cx's as adults. Large Vd & distributes well into CSF (50% of plasma Cx). Metabolized in the liver by CYP2C9, CYP3A4, CYP2C19. Drug interactions are of major importance in drug safety; should be carefully evaluated. Usual t ½ is 6 hrs. Dosage adjustment necessary in pts w/liver dysfunction. The IV form is formulated w/SBECD to increase solubility of voriconazole. SBECD is eliminated via kidneys & accumulates in pts w/renal disease. Use is NOT recommended in pts w/CrCL < 50 ml/min w/o risk/benefit evaluation.
(MIDII #67)

Discuss VORICONAZOLE, an AZOLE => Toxicities and Clinical Uses
(Toxicities): (a) Visual Disturbances occur in 30% of pts but rarely results in discontinuation of therapy. Symptoms occur early in therapy w/1st few doses, begin within 30 min of a dose, and last for 30 min. Visual effects include altered color discrimination, blurred vision, appearance of bright spots, & photophobia. Associated w/changes in electroretinogram tracings, which normalize upon discontinuation of treatment. (b) Hepatotoxicity: elevations in hepatic enzyme levels may occur w/ voriconazole. Most patients have asymptomatic elevations, but life threatening hepatitis has been described. Dose-related effects. Resolve w/discontinuation of therapy. (c) Skin Rashes: Skin rxns have been associated w/voriconazole. Most have been characterized as a photosensitivity rxn. (Clinical Uses): (1) Primary or salvage therapy of invasive aspergillosis. (b) Infections due to fluconaole-resistant Candida sp. (c) Treatment of Psuedoallescheria/Scedosporium and Fusarium sp. Infections. (d) Empiric antifungal therapy in pts w/neutropenic fever. (e) Voriconazole may be useful in cryptococcus, blastomycosis, coccidiodomycosis, & histoplasmosis. Clinical data currently lacking to support routine use for these infections.
(MIDII #68)

Discuss the ECHINOCANDINS => Caspofungin, Micafungin, Anidulafungin

How do the echinocandins differ from other currently available antifungals?
These drugs, unlike all other currently available antifungals, target the fungal CELL WALL rather than the fungal CELL MEMBRANE. Echinocandins are noncompetitive inhibitors of the β(1,3)-D-glucan synthase enyme which blocks the synthesis of β(1,3)-D-glucan, an essential component of the fungal cell wall. The chains of β(1,3)-D-glucan form a solid 3D matrix which gives the wall its shape and mechanical strength. Mechanism is both fungistatic and fungicidal. Fungistatic effect correlates to decreased cell wall synthesis and reduction in fungal growth. Fungicidal effect results from changes to the integrity of the cell wall. Mammalian cells do not contain β(1,3)-D-glucan so it should be selective against fungal cells.
(MIDII #69)

Discuss the ECHINOCANDINS => Caspofungin, Micafungin, Anidulafungin => Mechanisms of Resistance and Spectrum of Activity
(MOR): Little is known about resistance to echinocandins. Potential mechanisms include: (a) mutations in FKS gene coding for the catalytic subunit (Fks p) of β(1,3)-D-glucan synthase which binds intracellular UDP-glucose and a regulatory subunit Rho 1 p (binds intracellular GTP); and (b) mutations in the Rho 01 gene coding for the reg subunit of β(1,3)-D-glucan synthase.

(SPECTRUM): Active primarily against Candida sp. (fungicidal), including azole resistant or amphotericin resistant strains, and Aspergillus sp. (fungistatic). Not active against Cryptococcus neoformans. Echinocandin activity against filamentous fungi varies. In vitro, echinocandins are active against Paecilomyces variotii, but not Paecilomyces lilacinus and are active against Scedosporium apiospermum but not Scedosporium prolificans. Echinocandins are NOT active against Fusarium sp and mucormycosis. Active against Blastomyces dermatitidis and Histoplasma capsulatum, but Sporothrix schenckii is less susceptible.
(MIDII #70)

Discuss the ECHINOCANDINS => Caspofungin, Micafungin, Anidulafungin => Pharmacokinetics, Toxicities, and Clinical Uses
(Pharm): Echinocandins are only available as IV formulations. High levels in kidneys, liver, spleen & lungs. Lower levels in brain. Mainly eliminated in urine & feces as metabolites. Metabolism is independent of the CYP450 system. Do NOT inhibit CYP450 enzymes. Plasma clearance is det'd by rate of distribution into tissues. T ½ = 9-15 hrs for caspofungin/micafungin & 40-50 hrs for anidulafungin. (Toxicities): Extremely well tolerated w/little to no adverse effects. Fever, flushing, nausea, headache, vomiting, & infusion-related phlebitis were the most frequent adverse events seen in clinical trials. Elevations in serum transaminases may rarely occur. (Clinical Uses): (a) Treatment of infections due to Candida sp., especially azole & amphotericin resistant strains. (b) Approved for use in aspergillosis as salvage therapy (caspofungin). (c) Empiric antifungal therapy in pts w/neutropenic fever (caspofungin). (d) Prophylaxis of Candida sp. infections in stem cell transplant recipients (micafungin). (e) Less or limited efficacy has been shown for other pathogens. (f) Clinically useful in combo w/ amphotericin or voriconaole for synergy in Rx of severe, invasive fungal infections (e.g. Invasive aspergillosis).
(MIDII #71)

Discuss the Anti-Tuberculosis Agents => Primary goals of anti-TB therapy, 1st line agents, 2nd line agents, MOA
(1)Primary goals of anti-TB therapy is to kill tubercle bacilli rapidly, prevent emergence of drug resistance, and eliminate persistent bacilli from host to prevent relapse. (2) Drugs can be divided into 1st and 2nd line. 1st line agents combine greatest efficacy w/an acceptable degree of toxicity. 2nd line agents are associated w/less efficacy, greater toxicity, or both. (3) Anti-TB drugs differ in MOAs and in their delivery to TB lesions. All first line agents (except ethambutol) are bactericidal. 4 of the 1st line agents (INH, rifampin, streptomycin, and ethambutol) are active against large populations of tubercle bacilli in cavities. Streptomycin, other aminoglycosides, & capreomycin penetrate cells poorly and are inactive at acidic pH. Pyrazinamide is active ONLY in acidic environments. Slowly replicating organisms in necrotic foci are killed by RIFAMPIN and less by INH.
(MIDII #72)

Discuss ISONIAZID (INH) => Pharmacology
1st line anti-TB drug. Isoniazid is indicated for all forms of TB. (MOA): Inhibits mycolic acid synthesis, an impt component of mycobacterial cell wall. Bactericidal against actively growing MTB; bacteriostatic against nonreplicating organisms. (MOR): Low level resistance is associated w/point mutations or short deletions in the catalase-peroxidase gene (katG). High level resistance is associated w/major deletions within the gene with most enzymatic activity. Resistance may develop in a 2nd gene involved in mycolic acid synthesis (inhA).

(Pharm): Well absorbed orally or IM. CSF levels: 20% of plasma levels & higher w/inflamed meninges. Metabolized in the liver by N-acetyltransferase. Consider reducing the dose in “slow” acetylators with severe hepatic dysfunction, who exhibit higher levels of INH. (Adverse Effects): (1) Increase serum transaminases (AST, ALT) in 12-15% , hepatotoxicity (1%) => Watch for RUQ pain, dark urine, jaundice. Discourage alcohol use. (2) Peripheral neuropathy => burning, tingling, numbness. Coadminister vit B6. Alcoholics, children, diabetics, & malnourished at ↑ risk. (Drug Interactions): (a) Phenytoin => watch for mental status changes, nystagmus, and ataxia. (b) Rifampin => ↑ risk of hepatotoxicity (20-30%).
(MIDII #73)

Discuss RIFAMPIN => Pharmacology
1st line anti-TB drug. Rifampin is indicated for all forms of TB. (MOA): Inhibits DNA-dependent RNA polymerase. Bactericidal. (MOR): Resistance results from a point mutation or deletion within the region encoding the β subunit of RNA polymerase.

(PHARM): Well absorbed orally. CSF levels: 50% of plasma levels w/inflamed meninges. Deacetylated in the liver to an active form which undergoes biliary excretion & enterohepatic recirculation. Severe liver dysfunction req's a dosage reduction. (Adverse Effects): (1) ↑ in hepatic enzymes (AST, ALT, bilirubin, alkaline phosphatase) 10-15%. Hepatotoxicity (1%) => Watch for RUQ pain, dark urine, jaundice. Alcoholics at ↑ risk. (2) Rash, GI disturbances. (3) Orange discoloration of body fluids => urine, tears, sweat, soft contact lenses, etc. (4) Flu-like syndrome => fever, joint pain, muscle cramps. (Drug Interactions): Interacts w/ >100 drugs => always think drug interactions. DO NOT COADMINISTER protease inhibitors. Rifampin ↓ Cx's of these drugs when coadministered: fluconazole, itraconazole, warfarin, digoxin, metoprolol, quinidine, verapamil, oral contraceptives, cyclosporine, diazepam, haloperidol, methadone, phenytoin, sulfonylureas.
(MIDII #74)

Discuss PYRAZINAMIDE (PZA) => Pharmacology
1st line anti-TB drug. PZA is a req'd component of a 6-month short course x 2 months. Unknown MOA. Bactericidal w/optimal activity against semidormant organisms in an acidic environment. (MOR): Resistance results from mutations in the gene encoding pyrazinamidase (pncA) which converts PZA to the active form of pyrazinoic acid.

(Pharm): Well absorbed orally. Achieves adequate levels in CSF. Metabolized in the liver but metabolites are excreted by the kidneys. Dosage modification in renal dysfunction is necessary. (Adverse Effects): (1) Nausea/vomiting. (2) Hepatotoxicity (↑ liver enzymes) => watch for RUQ pain, dark urine, jaundice. Preexisting liver disease and higher doses ↑ risk. (3) Hyperuricemia => potential for exacerbation of gout.
(MIDII #75)

Discuss ETHAMBUTOL => Pharmacology
1st line anti-TB drug. Ethambutol is commonly used as the 4th drug in RIPE and often in regimens w/isolates resistant to INH or rifampin. (MOA): Inhibits arabinosyl transferase enzymes involved w/synthesis of arabinogalactan and lipoarabinomannan within the cell wall. Bacteriostatic. (MOR): Resistance results from point mutations in the arabinosyl transferase enzyme EmbB, which is coded by the embB gene.

(Pharm): Well absorbed (75-80%). Distributes into CSF. Predominantly excreted via kidneys. Dosage reduction necessary in renal dysfunction. (Adverse Effects): (1) Neuropathy (optic neuritis) => Complaints of blurry vision or inability to see the color green (more common at higher doses). (2) Hyperuricemia => potential for exacerbation of gout. (3) Rash
Discuss STREPTOMYCIN => Pharmacology

1st line anti-TB drug. Streptomycin is used as the 4th drug in patients at significant risk of drug resistance or in regimens with known drug resistance. (MOA): Inhibits protein synthesis. Bactericidal.
(PHARM): Administered IV or IM. Does not reach reliable Cx's in the CSF. Eliminated via the kidneys. Dosage reduction necessary in pts w/renal dysfunction as well as those w/low body wt or age >50 yrs. T ½ = 1.5-3.5 hrs in pts w/normal renal function. (Adverse Effects): (1) Vestibular toxicity => complaints of tinnitis, ↓ hearing, problems w/balance. (2) Renal toxicity (<1%) => ↓ urine output, ↑ serum creatinine levels. (3) Pain at injection site.
(MIDII #77)

Discuss RIFABUTIN => Pharmacology

2nd line anti-TB drug. Rifabutin is as effective as rifampin. Used in pts w/HIV receiving protease inhibitors b/c it has less effect on their metabolism. (MOA): Inhibits RNA polymeras
(PHARM): Well absorbed. Long plasma half-life and good distribution into tissues. (Adverse Effects): (1) Uveitis => complaints of ocular pain & blurred vision. (2) Polymyalgia syndrome (pseudojaundice) => yellowish-tan discoloration of the skin. (3) Hepatitis (↑ liver enzymes) => watch for RUQ pain, dark urine, jaundice. (4) Rash, GI. (5) May discolor body fluids orange => includes urine, sweat, tears, soft contact lenses, etc. (Drug Interactions): (a) Rifabutin ↓ plasma Cx's of these agents (to a lesser degree than Rifampin): verapamil, methadone, digoxin, cyclosporine, corticosteroids, oral anticoagulants (warfarin), theophylline, barbiturates, ketoconazole, oral contraceptives, quinidine, protease inh, NNRTIs. (b) Indinavir, ritonavir, nelfinavir ↑ Cx of Rifabutin (↓ dose of Rifabutin). (c) DO NOT COADMINISTER rifabutin w/ delavirdine as it ↓ concentrations of delavirdine.
(MIDII #78)

Discuss QUINOLONES => Pharmacology

2nd Line anti-TB agents. Quinolones (levofloxacin, moxifloxacin) are incorporated into regiments for multi-drug resistant TB. (MOA): Inhibits DNA gyrase. Some bactericidal. (MOR): Re
(PHARM): Available IV and orally. Well absorbed orally. Distribute well throughout body w/variable penetration into the CSF. Most quinolones are excreted by the kidneys (except moxifloxacin) and require dosage modification in renal dysfunction. (Adverse Effects): (1) GI => nausea, vomiting, diarrhea, abdominal pain. (2) CNS effects => dizziness, insomnia, irritability, anxiety, seizures. (3) Rashes and photosensitivity. (Drug Interactions): (a) Antacids (Al, Ca, Mg-containing), iron result in a significant ↓ in absorption of the quinolones. Separate oral administration times by 2 hrs. (b) Quinolones may inhibit metabolism of warfarin. Watch for ↑ INR, bruising, bleeding. (c) ↓ theophylline metabolism.
(MIDII #79)

2nd line anti-TB agent. Capreomycin is used in regimens w/MDR-TB resistant to streptomycin. (MOA): Inhibits cell wall synthesis. (MOR): unknown. (PHARM): IM injection. Excreted via the kidneys. (Adverse Effects): (1) hearing loss, tinnitus. (2) renal dysfunction => ↓ urine output, ↑ serum creatinine levels.
(MIDII #80)

2nd line anti-TB agents. Amikacin is used as an alternative to streptomycin and capreomycin in regimens for MDR-TB. (MOA): Inhibits protein synthesis. (MOR): Resistance results from an A to G change at base pair 1408 of the 16-S ribosomal RNA gene. (Pharm): Administered IV or IM. Does not reach reliable Cx's in the CSF. Eliminated via the kidneys. Dosage reduction necessary in pts w/renal dysfunction. (Adverse Effects): (1) Renal toxicity => ↓ urine output, ↑ serum creatinine levels. (2) Hearing loss, tinnitus.
(MIDII #81)

Discuss Para-aminosalicylic Acid (PAS)
2nd line anti-TB drug. Use is limited to MDR-TB. (MOA): Inhibits folate synthesis. Bacteriostatic. (Pharm): Incompletely absorbed orally. Distributes well except for CSF (10-50%). Primarily eliminated via kidneys. Dosage adjustment necessary in renal dysfunction. (Adverse Effects): (1) GI => often results in poor compliance. (2) Hepatotoxicity. (3) Hypothyroidism => thyroid hormone replacement may be necessary.
(MIDII #82)

2nd line anti-TB drug. Alternative in a regimen for MDR-TB. (MOA): Inhibits cell wall synthesis. Bacteriostatic. (MOR): Resistance sometimes mediated by reduced uptake into the cell. (Pharm): Well absorbed orally. Distributes throughout the body including the CSF. Primarily excreted via the kidneys. (Adverse Effects): (1) Peripheral neuropathy. (2) CNS dysfunction => confusion, irritability, somnolence, headache, nervousness, vertigo, seizures.
(MIDII #83)

2nd line anti-TB drug. Alternative in a regimen for MDR-TB. (MOA): inhibits mycolic acid synthesis. Bacteriostatic. (MOR): Unknown. (PHARM): Well absorbed orally. Widely distributed including the CSF. Metabolized in the liver w/metabolites excreted renally. (Adverse Effects): (1) Nausea, vomiting. (2) Peripheral neuropathy => burning, tingling, numbness. (3) Psychiatric disturbances. (4) Hepatotoxicity w/increased liver enzymes => watch for RUQ pain, dark urine, jaundice. (5) Poor glycemic control =>↑ glucose, ↑ need for insulin in diabetics.
(MIDII #84)

An infectious agent composed of nucleic acid (RNA or DNA), a protein shell (capsid), and, in some cases, a lipid envelope. Virions have full capacity for replication when a susceptible target cell is encountered.
(MIDII #85)

The protein coat that surrounds the viral nucleic acid is called a capsid. This is composed of repeating protein subunits called capsomeres. Generally, capsids have either helical or icosahedral symmetry.
(MIDII #86)

The complete protein-nucleic acid complex of a virus is called a nucleocapsid.
(MIDII #87)

Viruses which require a second virus (helper virus) for replication. Hepatitis delta virus is the major human pathogen example. It requires the presence of hepatitis B virus to complete its replication cycle.
(MIDII #88)

Definition: VIROIDS
Viroids are the smallest known autonomously replicating molecules. They consist of single-stranded, circular RNA, 240-375 residues in length. They are plant pathogens.
(MIDII #89)

Definition: PRIONS
Prions are not viruses but are often discussed within this microbiologic category. Prions are infectious protein molecules that contain no definable nucleic acid and are responsible for the transmissible and familial spongiform encephalopathies: Creutzfeldt-Jakob disease, kuru, fatal familial insomnia, Gerstmann-Straussler-Sheinker syndrome, and bovine spongiform encephalopathy (“mad cow disease”). The pathogenic prion protein (PrP^Sc) is formed from a normal human protein (PrP^C) through post-translational processing.
(MIDII #90)

Discuss viral classification => Modern classification of viruses is based upon what 3 characteristics of viruses?
(1) Type of viral nucleic acid (RNA or DNA, single-stranded or double-stranded) and its replication strategy.
(2) Capsid symmetry (icosahedral or helical).
(3) Presence or absence of a lipid envelope.
(MIDII #91)

Discuss Pathogenesis of Viral Diseases => Outcome of a particular virus w/human host is dependent on pathogen and host factors. Viral strains within a genus may have different cell tropisms, replication capacities, and cytopatho
Key elements: (1) Viral strain; (2) Inoculum size; (3) Route of exposure; (4) Susceptibility of host (i.e., is there preexistent immunity from past exposure or vaccination?); (5) Immune status and age of host.

Net result of virus-host interaction: (1) No infection; (2) Abortive infection w/limited viral replication; (3) Asymptomatic infection; (4) Symptomatic infection; (5) Depending upon the agent and the immune status of the host, persistent/latent or self-limited infection.
(MIDII #92)

Pathogenic Steps in Human Infection: Provide a generalized schema of viral infection leading to disease in the human host.
(1)Virus enters thru skin, mucous membranes, respiratory tract, GI tract, via a transfusion or transplanted organ or via maternal-fetal transmission. (2) Local replication occurs at site of inoculation. Certain agents exhibit pathology at the skin or mucous membrane surface (e.g., HSV, HPV). (3) For some neurotropic viruses there may be spread along peripheral nerve routes to ganglia (e.g., HSV) or the CNS (e.g. Rabies virus). For other neurotropic agents, the CNS is seeded following viremia. (4) For many agents there is replication in regional lymph nodes w/subsequent viremia and spread to target organs. Some viruses travel in the bloodstream free in the plasma (e.g. Picornaviruses); others are cell-associated (e.g., cytomegalovirus). (5) Replication in target organs may lead to local damage and further rounds of viremia. (6) Non-specific and specific host immune responses come into play to try to control and downregulate the viral replicative process.
(MIDII #93)

Immune Responses to Viral Infections => Non-specific Immunity, Specific Immunity, and Intense Immunologic Rxns
(1)Non-specific immunity => elements of the immune system that can clear virus or virally infected cells immediately upon or shortly after viral exposure and which are NOT dependent on immunologic memory. Non-specific immunity may include: (a) Phagocytic cells like PMNs, Mfs and monocytes, (b) Cytokines (IFNs) and Chemokines, (c) NK cells, (d) Poorly defined antiviral factors may exist in blood or body fluids. (2) Specific immunity => antigen specific B and T cell responses that lead to development of Abs, cytotoxic T cells and Ab-dependent cellular cytotoxicity (ADCC). (3) sometimes an intense immunologic rxn to a viral agent can result in immunopathology and a serious clinical syndrome. Example: dengue hemorrhagic fever which is likely due to Ab-dependent enhancement and T cell activation on re-exposure to the dengue virus.
(MIDII #94)

Mechanisms of Viral Persistence => Chronic Persistent Infection and Latent Infection
(1)Viruses may cause chronic, persistent infection w/continuous viral replication in face of an immune response. Examples: HIV, hep B and C viruses. Some viruses may demonstrate persistent infection in immune compromised hosts, including herpesviruses, human papillomavirus, and rubella virus. (2) Latent infection is characterized by a quiescent or minimally transcriptionally active viral genome w/periods of reactivation. Examples: herpesviruses (cytomegalovirus, Epstein-Barr virus, herpes simplex virus, varicella-zoster virus), human papillomavirus, human retroviruses. Recurrent herpes labialis or genital herpes due to HSV or herpes zoster due to varicella zoster virus are examples of latency & reactivation. Viruses which exhibit latency may exhibit chronic, persistent replication in the setting of immune compromised host.
(MIDII #95)

Mechanisms of Viral Persistence => How do viruses go about producing chronic infections? What mechanisms of persistence do they employ? What are sites of persistent viral infection?
Mechanisms of persistence include antigenic variation to escape Ab or cytotoxic T cell responses, downregulation of Class I MHC resulting in diminished recognition by cytotoxic T cells and modulation of apoptosis. Viruses which establish latent infection escape recognition by the immune system thru decreased viral antigen expression and presentation.

Sites of persistence include the nervous system (herpes simplex virus, varicella zoster virus, measles virus, poliovirus, JC virus), the liver (hepatitis B virus, hepatitis C virus), and leukocytes (HIV, cytomegalovirus, Epstein-Barr virus).
(MIDII #96)

Discuss the association of certain viruses with oncogenesis.
Several viruses are associated with human cancers, including: (a) Epstein-Barr Virus w/lymphoma, nasopharyngeal carcinoma, & leiomyosarcoma; (b) herpesvirus 8 w/ Kaposi's sarcoma and body cavity B-cell lymphoma; (c) hepatitis B and C viruses w/hepatocellular carcinoma; (d) human papillomavirus w/cervical cancer and anogenital carcinoma. Mechanisms of oncogenesis include transformation (EBV and herpesvirus 8) and binding of tumor suppressor proteins (HPV).
(MIDII #97)

Diagnosis of Viral Infections => Methods to diagnose viral infections.

(A) Dx of viral infection relies 1st on recognition of a distinct clinical syndrome (e.g. Herpes zoster infection) or a consideration of the viral
(1)Isolation of virus in tissue culture, animals, embryonated eggs. Most diagnostic labs only use tissue culture to isolate viruses. A specific cytopathic effect or induction of a characteristic function (e.g., hemagglutination) can indicate growth of viruses in tissue culture. Can be confirmed w/virus-specific antisera applied to the tissue monolayer linked to a tissue stain or to neutralize the cytopathic effect or hemagglutination. (2) Ag detection in body fluids (e.g. Respiratory tract for respiratory viruses) or blood (e.g., cytomegalovirus) or lesion scrapings (for HSV or varicella-zoster virus) w/specific immune sera linked to fluorescence or enzyme immunoassay detection. (3) PCR amplification and/or nucleic acid probes to detect viral nucleic acid in body fluids or tissues. (4) Ab detection. IgM Ab detection can assist w/acute diagnosis. 4X rises in IgG specific Ab or conversion from seronegative status to seropositive status can secure a Dx. Not helpful in an acute setting. (5) Examination of tissue samples by light microscopy for viral induced cytopathology and Ag detection by immunohistochemical staining. (6) Examination of body fluids or tissues by electron microscopy. Not efficient. Depends on LOTS of virions present to permit detection.
(MIDII #98)

Prevention and Therapy for Viral Infections
(A) Vaccines => Eradication of smallpox occurred thru a vaccine. Effective vaccines exist for polio, mumps, measles, rubella, influenza, hepatitis A, hepatitis B, varicella-zoster, rabies, adenovirus, Japanese B encephalitis and yellow fever. (B) Immune globulin can prevent or ameliorate clinical disease due to certain viral agents. Examples: varicella-zoster immune globulin for exposure in immune compromised hosts, rabies immune globulin (administered w/rabies vaccine) following an exposure, CMV immune globulin for transplant recipients, respiratory syncytial virus immune globulin and immune serum globulin for hepatitis A. (C) Screening of blood for prevention of transmission of HIV, hep B and C, and CMV in certain transplant situations. (D) Safe sexual practices for prevention of HIV, Hep B and HPV infection. (E) Advances in specific antiviral therapy over the past 30 yrs have been marked. Effective therapy exists for: HSV, varicella-zoster virus, cytomegalovirus (CMV), HIV, influenza virus, RSV (respiratory syncytial virus), hepatitis B and hepatitis C.
(MIDII #99)

This is the first step in viral replication. Surface proteins of the virus interact w/specific receptors on the target cell surface. These may be specialized proteins w/limited distribution or molecules that are more widely distributed on tissues throughout the body. Presence of a virus-specific receptor is necessary but not sufficient for viruses to infect cells and complete the replicative cycle.
(MIDII #100)

Enveloped viruses (e.g., HIV, influenza virus) penetrate cells through fusion of the viral envelope w/the host cell membrane. Non-enveloped viruses penetrate cells by translocation of the virion across the host cell membrane or receptor mediated endocytosis of the virion in clathrin coated pits w/accumulation of viruses in cytoplasmic vesicles.
(MIDII #101)

STEPS IN VIRAL REPLICATION: Step 3 => Uncoating (disassembly)
A complex process which differs by taxonomic class and is not fully understood for many agents. This process makes the nucleic acid available for transcription to permit multiplication of the virus.
(MIDII #102)

STEPS IN VIRAL REPLICATION: Step 4 => Transcription and Translation
The key to understanding the genomic expression of viruses is noting the fact that viruses must use HOST cellular machinery to replicate and make functional and structural proteins.
(MIDII #103)

STEPS IN VIRAL REPLICATION: Step 5 => Assembly and Release
The process of virion assembly involves bringing together newly formed viral nucleic acid and the structural proteins to form the nucleocapsid of the virus. There are basically 3 strategies that viruses employ: (1) Non-enveloped viruses exhibit full maturation in the cytoplasm (e.g. Picornaviruses) or the nucleus (e.g. Adenoviruses) w/disintegration of the cell & release of virions. (2) For enveloped viruses, including (-) strand RNA viruses, the (+) strand togaviruses, & retroviruses, final maturation of the virion takes place as the virion exits the cell. Viral proteins are inserted into the host cell membrane. Nucleocapsids bind to the regions of the host cell membranes w/tehse inserted proteins and bud into the extracellular space. Further cleavage & maturation of proteins may occur after viral extrusion to impart full infectivity on the virion. Viruses differ in their degree of cytolytic activity. (3) Herpesviruses (which are enveloped) assemble their nucleocapsids in the nuclei of infected cells and mature at the inner lamella of the nuclear membrane. Virions accumulate in this region, in the ER and in vesicles protected from the cytoplasm. Release of virions from the cell surface is associated w/cytolysis.
(MIDII #104)

Strategies of Genomic Expression => Positive (+) Strand RNA Viruses Coding for One Genome-Sized mRNA [e.g., poliovirus, flaviviruses including hep C virus]
(1)Genomic RNA binds to ribosomes and is translated into a polyprotein. (2) Polyprotein is cleaved. (3) Genomic RNA's serve as templates for synthesis of complementary full length (-) strand RNA's by a viral polymerase. (4) (-) strand RNA serves as a template for (+) strand RNAs. These (+) strands can serve to produce more polyprotein, more (-) strand RNAs or as part of new virions which are forming.
(MIDII #105)

Strategies of Genomic Expression => Positive (+) Strand RNA Viruses Coding for Subgenomic RNA's (e.g. Togaviruses)
(1)Genomic RNA binds to ribosomes but only a portion of the 5' end is translated into non-structural proteins.
(2) (-) strand RNA is synthesized. Different size classes of (+) RNAs are produced. One is translated into a polyprotein which is cleaved to form structural proteins. Another is full length and serves as genomic RNA for new virions which are forming.
(MIDII #106)

Strategies of Genomic Expression => Single Strand RNA Viruses with 2 Identical Strands (e.g., retroviruses)
(1) Genomic RNA serves as a template for production of DNA copy. This is produced by an RNA-dependent DNA polymerase (reverse transcriptase) contained in the virion. (2) Digestion of genomic RNA and synthesis of second, complementary DNA strands then proceeds. (3) Double-stranded DNA migrates to the nucleus and integrates into the host cell genome. (4) Integrated DNA may remain silent or be transcribed into genomic, full-length RNA or shorter, spliced RNA's. The latter code for accessory and structural proteins. Full-length RNA transcripts are packaged into forming virions.
(MIDII #107)

Strategies of Genomic Expression => Negative (-) Strand Nonsegmented RNA Viruses (e.g. Paramyxoviruses, rhabdoviruses, filoviruses)
(1)Transcription of (-) strand occurs after entry and is mediated by virion packaged transcriptase.
(2) (+) strand RNAs are produced; proteins are synthesized.
(3) Full-length (-) strand RNA's are produced and packaged into newly forming virions.
(4) Transcription and translation take place entirely in the cytoplasm of infected cells.
(MIDII #108)

Strategies of Genomic Expression => Negative (-) Strand Segmented RNA Viruses (e.g.bunyaviruses and orthomyxoviruses)
(1) mRNAs are synthesized from each segment. (2) Viral proteins are synthesized. (3) (+) strand RNAs are synthesized and serve as templates for (-) strand genomic RNAs.
(MIDII #109)

Strategies of Genomic Expression => Double Stranded RNA Viruses (e.g., reoviruses)
(1)Genome is transcribed by virion packaged polymerase. Messenger RNAs (mRNAs) are translated to viral proteins or transcribed to complementary strands to yield double stranded RNA genomes for new virion formation.
(MIDII #110)

Strategies of Genomic Expression => Double Stranded DNA Viruses Which Replicate in Nucleus (e.g., papovaviruses, papillomaviruses, adenoviruses, herpesviruses).
(1)Sequential, ordered rounds of mRNA and protein production regulate replication.
(2) Structural proteins produced during last cycle of transcription.
(MIDII #111)

Strategies of Genomic Expression => Single-Stranded DNA Viruses (e.g., parvoviruses)
(1)Complementary DNA strand synthesized in nucleus.
(2) Transcription of the genome ensues.
(MIDII #112)

Strategies of Genomic Expression => Partially Double Stranded DNA Viruses (hepadnaviruses)
(1) Genome of hepatitis B is circular and partially double stranded; it is replicated in the nucleus.
(2) Genome is converted to a closed circular molecule by a DNA polymerase which is virion packaged.
(3) 2 classes of RNA species are produced: one that codes for viral proteins and one that serves to produce genomic DNA by a virally encoded reverse transcriptase.
(MIDII #113)

What three requirements must be satisfied to ensure successful viral infection in an individual host?
(1)Sufficient virus must be available to initiate infection. (2) Cells at the site of infection must be accessible, susceptible, and permissive for the virus. (3) Local host anti-viral defense systems must be absent or initially ineffective.

To infect its host, a virus must first enter cells at a body surface. Common sites of entry include the mucosal linings of the respiratory, alimentary, and urogenital tracts, the outer surface of the eye (conjunctival membranes or cornea) and the skin.
(MIDII #114)

Discuss viral entry through the Respiratory Tract (RT)
Respiratory tract (RT) => most common route of viral entry. 140 m^2 of absorptive area in human lung. 6 L of air per min are exchanged, introducing large #'s of foreign particles & aerosolized droplets into the lungs w/each breath. Many of these particles/droplets contain viruses. Mechanical barriers play significant role in anti-viral defense. Tract is lined w/mucociliary blanket: ciliated cells, mucous-secreting goblet cells, sub-epithelial mucous-secreting glands. Foreign particles deposited in the nasal cavity or upper RT are trapped in mucus, carried to the back of the throat & swallowed. Lower RT: particles trapped in mucus are brought up from the lungs to the throat by ciliary action. Lowest portions of the RT (alveoli) lack cilia or mucus but MΦs lining the alveoli ingest & destroy particles. Other cellular & humoral immune responses also intervene. Viruses enter the RT as aerosolized droplets expelled by an infected individual by coughing/sneezing, or through contact w/saliva from an infected individual. Larger virus-containing droplets are deposited in the nose, while smaller droplets find their way into the airways or the alveoli. To successfully infect the respiratory tract, viruses must NOT be swept away by mucus, neutralized by Ab, or destroyed by alveolar MΦs.
(MIDII #115)

Discuss viral entry through the Alimentary Tract (AT):
The alimentary tract (AT) is a common route of infection & dispersal. Eating, drinking & kissing/sex place viruses in the AT, which is designed to mix, digest & absorb food. Good opportunity for viruses to encounter susceptible cells & interact w/cells of the circulatory, lymphatic, & immune systems. Extremely hostile environment. Stomach is acidic, intestine is alkaline, digestive enzymes & bile detergents abound, mucus lines the epithelium, lumenal surfaces of intestines contain Abs & phagocytic cells. Viruses that infect via intestinal rte must be resistant to extremes of pH, proteases, & bile detergents. Viruses lacking those features are destroyed when exposed to the alimentary tract – must infect at other sites.

The entire intestinal surface is covered w/columnar villous epithelial cells w/apical surfaces densely packed w/microvilli. Brush border, together w/surface coat of glycoproteins & glycolipids and the overlying mucous layer, is permeable to electrolytes & nutrients but presents a formidable barrier to microorganisms. However, enteric adenoviruses & Norwalk virus (a calicivirus) replicate extensively in intestinal epithelial cells. Mechanisms by which they bypass physical barriers & enter susceptible cells are not well understood. Scattered throughout intestinal mucosa are lymphoid follicles covered on the lumenal side w/follicle-associated epithelium consisting of columnar absorptive cells and M cells. M-cell transcytosis provides mechanism by which some enteric viruses enter deeper tissues of the host from the intestinal lumen.
(MIDII #116)

How does the hostile environment of the Alimentary Tract facilitate infection by some viruses?
Reovirus particles are converted by host proteases in the intestinal lumen into infectious subviral particles which subsequently infect intestinal cells. Most enveloped viruses do not initiate infection in the alimentary tract b/c viral envelopes are susceptible to dissociation by detergents (such as bile salts). Enteric coronaviruses are notable exceptions => not known why these enveloped viruses can withstand the harsh conditions of the alimentary tract.
(MIDII #117)

Discuss viral entry through the Urogenital Tract.
Some viruses enter the urogenital tract as the result of sexual activities. UG tract is well-protected by physical barriers including mucus & low pH (vagina). Normal sexual activity can result in minute tears or abrasions in the vaginal epithelium or urethra, allowing viruses to enter. Some viruses infect the epithelium and produce local lesions (HPV => genital warts). Other viruses access cells in the underlying tissues and infect cells of the immune system (HIV type 1) or sensory and autonomic neurons (herpes simplex virus).
(MIDII #118)

Discuss viral entry through the eyes.
The epithelium covering the exposed part of the sclera & conjunctivae is the route of entry for several viruses. Every few seconds the eyelid passes over the sclera, bathing it in secretions that wash away foreign particles. Little opportunity for viral infection of the eye, unless it is injured by abrasion. Direct inoculation into the eye may occur during ophthalmologic procedures or from environmental contamination (e.g. improperly sanitized swimming pools). Usually replication is localized and results in inflammation of the conjunctiva (conjunctivitis). Systemic spread of the virus from the eye is rare but does occur. Herpesviruses can infect the cornea at the site of a scratch or other injury, which can lead to immune destruction of the cornea & blindness.
Discuss viral entry through the skin.
Skin is an effective barrier against viral infections as the dead outer layer cannot support viral growth. Entry occurs when skin's integrity is breached through breaks or punctures. Replication is limited to the site of entry b/c the epidermis is devoid of blood or lymphatic vessels that could provide pathways for further spread. Other viruses gain entry to vascularized dermis through bites of arthropod vectors such as mosquitoes, mites, ticks, & sandflies. Deeper inoculation into the tissue & muscle below the dermis can occur by hypodermic needle punctures, body piercing or tattooing, animal bites, or sexual contact when body fluids are mingled through skin abrasions or ulcerations. Viruses that initiate infection in dermal or sub-dermal tissue can reach nearby blood vessels, lymphatic tissues, and cells of the nervous system. So they can spread to other sites in the body.
(MIDII #120)

Discuss viral spread => What are disseminated infections? Systemic?
After replication at site of entry, virus particles can remain localized or can spread to other tissues. Local spread of infection in the epithelium occurs when newly released virus infects adjacent cells. Infections are contained by the physical constraints of the tissue and brought under control by intrinsic & immune defenses. Infection that spreads beyond the primary site of infection is called DISSEMINATED. If many organs become infected, the infection is described as systemic. For an infection to spread beyond the primary site, physical and immune barriers must be breached. After crossing the epithelium, virus particles reach the basement membrane (bm). Integrity of the bm may be compromised by epithelial cell destruction & inflammation. Below the bm are sub-epithelial tissues where the virus encounters tissue fluids, the lymphatic system, & phagocytes. These play an impt role in clearing foreign particles but may disseminate infectious virus from the primary site of infection.
(MIDII #121)

Discuss the directional release of viral particles from polarized cells at the mucosal surface, an important mechanism for avoiding local host defenses and facilitating spread within the body.
Virions can be released from the apical surface, from the basolateral surface, or from both. After replication, virus released from the apical surface is outside the host. Such directional release facilitates the dispersal of many newly replicated enteric viruses in the feces (e.g., poliovirus). Virus particles released from the basolateral surfaces of polarized epithelial cells have been moved away from the defenses of the lumenal surface. Directional release is therefore a major determinant of the infection pattern. Generally, viruses released at apical membranes establish a localized or limited infection, while release of viruses at the basal membrane provides access to the underlying tissues and may facilitate systemic spread.
(MIDII #122)

Discuss Hematogenous Spread of Viruses
Viruses that escape from local defenses often do so by entering the bloodstream. Virus particles may enter the blood directly thru capillaries, by replicating in endothelial cells, or thru inoculation by a vector bite. Once in the blood, viruses may access almost every tissue in the host. Hematogenous spread begins when newly replicated particles produced at the entry site are released into the extracellular fluids which can be taken up by the local lymphatics. Lymphatic capillaries are more permeable than circulatory system capillaries, facilitating viral entry. Since lymphatic vessels ultimately join the venous system, virus particles in the lymph have free access to the bloodstream. Virions pass through lymph nodes in the lymphatics, where they encounter migratory cells of the immune system. Viral pathogenesis resulting from direct infection of immune cells (HIV, measles) initiates this way. Some viruses replicate in infected lymphoid cells. Progeny are released into the blood plasma. Infected lymphoid cells may migrate away from the local lymph node to distant parts of the circulatory system.
(MIDII #123)

What is Viremia? Active Viremia? Passive Viremia? Primary Viremia? Secondary Viremia?
(1)VIREMIA refers to presence of infectious virus particles in the blood. May be free in the blood or contained within infected cells (lymphocytes). (2) ACTIVE VIREMIA is produced by virus replication, while (3) PASSIVE VIREMIA results when viral particles are introduced into the blood w/o viral replication at the site of entry (injection of a viral suspension into a vein). (4) Progeny virions released into the blood after initial replication at the site of entry constitute PRIMARY VIREMIA. Cx of virus particles during primary viremia is low. (5) Subsequent disseminated infections that result are often extensive, releasing considerably more virus particles. Delayed appearance of a high Cx of infectious virus in the blood is termed SECONDARY VIREMIA.
(MIDII #124)

How is viremia helpful in a diagnostic sense? What practical problems can ensue from viremia?
Viremias are of diagnostic value and can be used to monitor the course of an infection, but they also present practical problems. Infections can be spread inadvertently in the population when pooled blood from thousands of individuals is used directly for therapeutic purposes (transfusions) or as a source of therapeutic proteins (e.g., gamma globulin or blood-clotting factors).
(MIDII #125)

Discuss the neural spread of viruses.
Many viruses spread from the primary site of infection by entering local nerve endings. For some virsues (rabies virus, alpha herpesviruses), neuronal spread is the definitive characteristic of their pathogenesis. For other viruses (poliovirus & reovirus), invasion of the nervous system is a less frequent diversion from their site of replication & destination. Some viruses (mumps, HIV, measles virus) may replicate in the brain but spread by a hematogenous route. Because protein synthesis does not occur in the extended processes of neuronal cells, virus particles must be transported over relatively long distances to the site of viral replication. All evidence indicates that viruses are carried in the infected neuron by cellular systems, but viral proteins may facilitate the direction of spread.
(MIDII #126)

Discuss Viral Spread to the CNS: Neural, Olfactory, & Hematogenous
(1)NEURAL: Poliovirus, yellow fever virus, mouse hepatitis virus, Venezuelan encephalitis virus, rabies virus, reovirus, herpes simplex virus (HSV) types 1 & 2, pseudorabies virus
(2)OLFACTORY: Poliovirus (experimental), HSV, coronavirus
(3)HEMATOGENOUS: Poliovirus, coxsackievirus, arenavirus, mumps virus, measles virus, HSV, cytomegalovirus
(MIDII #127)

What is a NEUROTROPIC virus? NEUROINVASIVE virus? NEUROVIRULENT virus? Give examples.
(a) A NEUROTROPIC virus can infect neural cells. Infection may occur by neural or hematogenous spread initiating from a peripheral site.
(b) A NEUROINVASIVE virus can enter the CNS (spinal cord & brain) after infection of a peripheral site.
(c) A NEUROVIRULENT virus can cause disease of nervous tissue, manifested by neurological symptoms and often death.

Examples:(1) Herpes simplex virus has low neuroinvasiveness of the CNS but high neurovirulence. Always enters the peripheral nervous system but rarely enters the CNS. When it does, the consequences are almost always severe if not fatal. (2) Mumps has high neuroinvasiveness but low neurovirulence. Most infections lead to invasion of the CNS but neurological disease is mild. (3) Rabies virus has high neuroinvasiveness and high neurovirulence. Readily infects the peripheral nervous system and spreads to the CNS with 100% lethality unless antiviral therapy is administered shortly after infection.
(MIDII #128)

ORGAN INVASION => Once virions enter the blood and are dispersed from the primary site, any subsequent replication requires invasion of new cells & tissues. 3 main types of blood vessel-tissue junctions provide routes of ti
3 types of blood vessel-tissue junctions are: capillary, venule, & sinusoid.
(MIDII #129)

In some systemic viral infections, rashes occur when virions leave blood vessels, producing different types of skin lesions: macules & papules when inflammation occurs in the dermis, w/inflammation confined in/near the vascular bed. Vesicles & pustules occur when viruses spread from capillaries to superficial layers of skin. Destruction of cells by virus replication is the primary cause of lesions.

Examples: (a) Coxsackievirus A 16 => hand-foot-and-mouth disease w/maculopapular rash; (b) Echoviruses 4, 6, 9, 16; Coxsackieviruses A9, 16, 23 => maculopapular rash; (c) Measles virus => maculopapular rash; (d) Parvovirus => erythema infectiosum w/maculopapular rash; (e) Rubella virus => German measles w/maculopapular rash; (f) Varicella-zoster virus => chickenpox, zoster (shingles) w/vesicular rash.
(MIDII #130)

ORGAN INVASION => Liver, Spleen, Bone Marrow & Adrenal Glands
These tissues are characterized by presence of sinusoids lined with MФs (reticuloendothelial system) to filter the blood & remove foreign particles. Invade a portal of entry into various tissues. Viruses that infect the liver enter from the blood, leading to infection of Kupffer cells ( MФs) that line the liver sinusoids. Virions may be transcytosed across Kupffer and endothelial cells w/o replication to reach the underlying hepatic cells. Alternately, viruses may multiply in Kupffer and endothelial cells and then infect underlying hepatocytes. Either mechanism may induce inflammation and necrosis of liver tissue, a condition called HEPATITIS.
(MIDII #131)

ORGAN INVASION => CNS, Connective Tissue, Skeletal & Cardiac Muscle
Capillary endothelial cells in these tissues are usually not fenestrated; are backed w/ a dense basement membrane. In several well-defined parts of the brain, the capillary epithelium is fenestrated & the basement membrane is sparse. These highly vascularized sites include the choroid plexus. Some viruses (mumps virus, togaviruses) pass thru capillary endothelium & enter stroma of the choroid plexus, where they cross the epithelium into the CSF by transcytosis, or replication & directed release. Once in the CSF these viruses infect the ependymal cells lining the ventricles & invade the underlying brain tissue. Other virsues may directly infect or be transported across the capillary endothelium (picornaviruses & togaviruses). Some viruses cross the endothelium within infected monocytes or lymphocytes (HIV & measles virus). Increased local permeability of the capillary endothelium caused by certain hormones also permit viral entry into brain & spinal cord.
(MIDII #132)

ORGAN INVASION => The Renal Glomerulus, Pancreas, Ileum & Colon
To enter tissues that lack sinusoids, viruses must first adhere to endothelial cells lining capillaries or venules where blood flow is slowest and walls are thinnest. Once blood-borne viruses have adhered to the vessel wall, they can invade the renal glomerulus, pancreas, ileum or colon b/c the endothelial cells that make up the capillaries are fenestrated, permitting the virus or virus-infected cells to cross into the underlying tissues (poliovirus). Some viruses (herpes simplex, yellow fever, measles viruses) cross the endothelium while being carried by infected monocytes or lymphocytes via diapedesis.
(MIDII #133)

In a pregnant female, viremia may lead to infection of the developing fetus. The basement membrane is less well developed in the fetus. Infection can occur by invasion of the placental tissues followed by fetal tissue. Infected circulating cells (monocytes) may enter the fetal bloodstream directly. Virus may be transmitted to the baby during delivery or breast-feeding.

Examples: (a) Fetal death and abortion => smallpox virus, parvovirus; (b) Congenital defects => cytomegaloviruses, rubella; (c) Immunodeficiency => HIV 1; (d) Inapparent (lifelong carrier) => Lymphocytic choriomeningitis virus.
(MIDII #134)

Most viruses do not infect all the cells of a host but are restricted to specific cell types of certain organs. The spectrum of tissues infected by virus is called TROPISM. ENTEROTROPIC virus replicates in the gut. A NEUROTROPIC virus replicates in cells of the nervous system. Some viruses are PANTROPIC, infecting and replicating in many cell types and tissues.

Tropism is governed by 4 parameters: (1) Distribution of receptors for entry (SUSCEPTIBILITY); (2) Requirement of the virus for differentially expressed intracellular gene products to complete the infection (PERMISSIVITY); (3) Even if a cell is permissive and susceptible, infection may not occur b/c virus is physically prevented from interacting w/the tissue (ACCESSIBILITY); (4) An infection may not occur even when tissue is accessible & cells are permissive and susceptible b/c of local intrinsic and innate immune defenses.
(MIDII #135)

Discuss CELLULAR PROTEASES and their role in viral infections.
Many viruses require cellular proteases to cleave viral proteins for form the mature infectious virus particle. A cellular protease cleaves influenza virus HA precursor into 2 subunits so that fusion of the viral envelope and cell membrane can proceed. In mammals, the replication of flu virus is restricted to epithelial cells of the upper and lower respiratory tract. Tropism of this virus is influenced by limited expression of the protease that processes HA, called TRYPTASE CLARA, is secreted by non-ciliated CLARA cells of the bronchial and bronchiolar epithelia.
(MIDII #136)

Viral virulence refers to the capacity of a virus to cause disease in an infected host. It is a quantitative statement of the degree or extent of pathogenesis. A virulent virus causes significant disease, whereas an avirulent virus causes no disease and an attenuated virus causes reduced disease.
(MIDII #137)

How is viral virulence measured?
(1)Determine Cx of virus that causes death or disease in 50% of infected animals: the 50% lethal dose (LD50), the 50% paralytic dose (PD50), or the 50% infectious dose (ID50), depending on the parameter measured. (2) Mean time to death or appearance of symptoms, and measurement of fever or weight loss. (3) Virus-induced tissue damage can be measured directly by examining histological sections or blood. (4) Safety of live attenuated poliovirus vaccine is det'd by assessing extent of pathological lesions in the CNS. (5) Reduction in blood Cx of CD4+ lymphocytes caused by HIV type 1 is a measurement of virulence. (6) Indirect measurements of virulence include assays for liver enzymes (alanine or aspartate amino-transferases) that are released into the blood as a result of virus-induced liver damage.
(MIDII #138)

General Determinants of Virulence (1 of 4) => “Gene products that alter the ability of the virus to replicate”
Genes that encode proteins affecting viral replication & virulence can be placed in 1 of 2 subclasses. (a) Viral mutants of subclass exhibit reduced or no replication in the animal host and in many cultured cell types. Reduced virulence results from failure to produce sufficient numbers of virus particles to cause disease, due to mutations in any viral gene. (b) Mutants of 2nd subclass exhibit impaired virulence in animals, but no replication defects in cells in culture.
(MIDII #139)

General Determinants of Virulence (2 of 4) => “Gene products that modify the host's defense mechanisms”
The study of viral virulence genes has ID'd a diverse array of viral proteins that sabotage the body's intrinsic defenses and innate & adaptive systems. Some of these viral proteins are called VIROKINES (secreted proteins that mimic cytokines, growth factors, or extra-cellular immune regulators) or VIROCEPTORS (homologs of host receptors). Mutations in genes encoding either class of protein affect virulence, but these genes are NOT required for growth in cell culture. Most virokines and viroreceptors have been discovered in the genomes of large DNA viruses.
(MIDII #140)

Genetic Determinants of Virulence (3 of 4) => “Genes that enable the virus to spread in the host”
Mutation of some viral genes disrupts spread from peripheral sites of inoculation to the organ where disease occurs. After intramuscular inoculation in mice, reovirus type 1 spreads to the CNS thru the blood while type 3 spreads by neural routes. The gene encoding the viral outer capsid protein S1 (which recognizes the cell receptor), determines route of spread.
(MIDII #141)

Genetic Determinants of Virulence (4 of 4) => “Toxic viral proteins”
Some viral gene products cause cell injury directly, and alterations in these genes reduces viral virulence. Evidence of intrinsic activity is obtained by adding purified proteins to cultured cells, or by synthesis of proteins from plasmids or viral vectors. The most convincing example of a viral protein w/intrinsic toxicity relevant to the viral disease is the NSP4 protein of rotaviruses which causes gastroenteritis and diarrhea. NSP4 is a non-structural glycoprotein, participates in formation of a transient envelope as particles bud into the ER. When protein is fed to young mice it causes diarrhea by potentiating chloride secretion. Acts as a viral enterotoxin and triggers a signal transduction pathway in the intestinal mucosa.
(MIDII #142)

Injury Caused By Viral Infections => Do the clinical symptoms of viral disease result from the host response to infection or from the virus itself?
Clinical symptoms of viral disease in the host (fever, tissue damage, aches, pains, nausea) result primarily from the host response to infection which is initiated by cell injury caused by viral replication. Cell injury can result from the direct effects of viral replication on the cell, or from the consequences of the host's intrinsic, innate, and adaptive immune responses.
(MIDII #143)

Injury Caused By Viral Infections => Direct Effects of Primary Infection: Cytopathic Effect
(1)Infection of cultured cells can result in visible changes in the cells called CYTOPATHIC EFFECT. Direct alteration of the cell can clearly account for some of the damage observed during infections in an animal host. Poliovirus induces cytopathic effects in cultured neuronal cells. Virus-induced killing of neurons in the CNS can account for paralytic symptoms characteristic of poliomylitis. Apoptosis of virally infected cells leads to visible cell damage. (2) Viral infection may also cause cessation of essential host processes such as translation, DNA & RNA synthesis, & vesicular transport; ↑ permeability of cell membranes. Lysosomal contents diffuse into the cytoplasm resulting in autolytic digestion of the cell. (3) Host genome can be directly damaged by viral infection. Retrovirus replication cycle requires insertion of a proviral DNA copy into random locations in the cell genome. Insertion affects expression or integrity of a cellular gene, a process known as INSERTIONAL MUTAGENESIS.
(MIDII #144)

How is immunopathology “too much of a good thing”?
Most symptoms and many diseases caused by viral infection are a consequence of the immune response. Damage caused by the immune system is called immunopathology – price paid by the host to eliminate a viral infection. For non-cytolytic viruses the immune response is the sole cause of disease.
(MIDII #145)

Immunopathological Lesions => Lesions caused by cytotoxic T lymphocytes (CTLs)
Examples: (1) Myocarditis (inflammation of the heart muscle) caused by coxsackievirus B infection of mice requires presence of CTLs. Perforin is a major determinant of endocarditis. Mice lacking the perforin gene develop a mild form of heart disease but are still able to clear the infection. (2) The acute and fatal respiratory disease caused by hantaviruses is characterized by prominent infiltration of CTLs into the lung.
(MIDII #146)

Immunopathological Lesions => Lesions caused by CD4+ T cells
CD4+ T cells elaborate more cytokines than CTLs and recruit and activate many non-specific effector cells => Delayed type hypersensitivity responses. Most recruited cells are PMNs and mononuclear cells which are protective and cause tissue damage. Immunopathology results from release of proteolytic enzymes, reactive free radicals such as H2O2 and nitric oxide (NO), and cytokines (TNF-alpha).
(MIDII #147)

Immunopathological Lesions => Lesions Caused by CD4+ Th1 Cells: Herpes Stromal Keratitis
(MIDII #147)

Immunopathological Lesions => Lesions Caused by CD4+ Th1 Cells: Herpes Stromal Keratitis
(MIDII #148)

Immunopathological Lesions => Lesions Caused by CD4+ Th2 Cells: Respiratory Syncytial Virus
RSV may be mediated by CD4+ Th2 cells. Non-cytopathic virus. Impt cause of lower respiratory tract disease in infants and the elderly. Lesions are minor in immunosuppressed mice, but become severe after transfer of viral antigen-specific T cells, particularly CD4+ Th2 cells. Lesions in the respiratory tract contain many eos, which may be responsible for pathology. Th2 cells secrete cytokines that recruit eos.
(MIDII #149)

Immunopathologic Lesions => Lesions Caused By B Cells: Viral-Antibody Complexes
Virus-Ab complexes accumulate in high Cx's when lots of viral replication occurs at sites inaccessible to the immune system or continues in the presence of an inadequate immune response. These complexes aren't efficiently cleared by the reticuloendothelial system and continue to circulate in the blood. They become deposited in the smallest capillaries and cause lesions that are exacerbated when complement is activated. Deposition of these immune complexes in blood vessels, kidney and brain may result in vasculitis, glomerulonephritis, and mental confusion, respectively.
(MIDII #150)

How can Abs enhance viral infection in Dengue Hemorrhagic Fever?
Disease is transmitted by mosquitoes. Endemic in the Caribbean, Central & South America, Africa & Southeast Asia where billions of ppl are at risk. Primary infection is usually asymptomatic, may result in a self-limiting acute febrile illness with severe headache, back & limb pain, and a rash. There are 4 viral serotypes and Abs to any one of these does not protect against infection by the other. After infection by another serotype of dengue virus, non-protective Abs bind virus particles and facilitate uptake into normally non-susceptible peripheral blood monocytes carrying Fc-receptors. The infected monocytes produce pro-inflammatory cytokines which stimulate T cells to produce more cytokines, resulting in high Cx's of cytokines and chemical mediators that trigger plasma leakage & hemorrhage characteristic of dengue hemorrhagic fever. Lots of internal bleeding => fatal dengue shock syndrome can result. Dengue hemorrhagic fever occurs in 1 in 14,000 primary infections. After infection w/dengue virus of another serotype, incidence of hemorrhagic fever increases to 1 in 90, and shock syndrome is seen in as many as 1 in 50.
(MIDII #151)

Discuss Viral Infection Injury Mediated By Free Radicals.
NO is produced in virus-infected tissues during inflammation as part of the innate immune response. Inhibits replication of viruses in cultured cells and animal models. Low Cx's of NO have a protective effect. High Cx's or prolonged production can contribute to tissue damage. Treating infected animals w/ inhibitors of NO synthase prevents tissue damage. NO is relatively inert but rapidly reacts with O2 to form peroxynitrite (ONOO-) which is much more reactive and may be responsible for cytotoxic effects on cells.
(MIDII #152)

What is the basis for establishing a diagnosis of HIV in established (non-acute) HIV infection?
Initial test: ELISA that can be HIV-1 specific or may screen for HIV-1 and HIV-2 (frequently seen in blood banks). A positive ELISA Ab must be confirmed by a 2nd test, typically a western blot which detects serum Abs to specific HIV proteins (antigens) that are separated on a gel. Recently, rapid HIV antibody tests that can give results in 30 minutes to a few hours have been approved by the FDA. [Remember, an HIV test cannot be legally performed on any person w/o written permission. Pre-test and post-test counseling must be provided to all individuals willing to be tested.]
(MIDII #153)

What testing should be done on a patient presenting with acute (or primary) HIV infection?
Patient may present to a health care provider prior to full seroconversion. HIV Ab should be done but the ELISA may be negative or the ELISA may be positive with a negative or indeterminate Western blot. A plasma HIV-1 RNA test should be done as the viral load (RNA concentration in plasma) is typically very high during the acute phase of HIV infection. A follow-up Ab test should be done to confirm that full seroconversion (positive ELISA and Western blot) has occurred.
(MIDII #154)

Discuss the structure of the HIV virus.
HIV is a retrovirus. It is a single-stranded RNA virus w/an icosahedral nucleocapsid and a lipid envelope. Virion has 2 identical copies of RNA and carries a unique viral enzyme, the reverse transcriptase. Virus replication scheme is as follows: (1) Binding & infection; (2) Reverse transcription & integration of viral DNA into the host genome; (3) Transcription & translation of HIV proteins; (4) Modification and assembly; (5) Budding and final assembly.
(MIDII #155)

The HIV genome is 10 kB in length and consists of 3 major (Gag, Pol, Env) and 6 accessory genes. Discuss these:
(1) Gag codes for internal structural proteins. (2) Pol codes for the viruses major enzymes – reverse transcriptase, protease and integrase. (3) Env codes for the gp120 envelope glycoprotein and the gp41 transmembrane protein, which mediate attachment and entry of the virus into the host cell. (4) Tat, Rev, Nef, Vif, Vpr and Vpu are accessory proteins which are involved in amplification of virus replication, infectivity, and pathogenesis.
(MIDII #156)

What are the pathogenic steps for primary (acute) HIV infection?
(1)Viruses must interact with Dcs – first encounter the virus has following deposition on the mucosal surface. Primary infection is w/R5 (MΦ-[M]-tropic) viral strains. DC-SIGN is an impt HIV receptor on the surface of DC's. (2) Virus is delivered to lymph nodes where very active replication takes place. DC's act as transporters of HIV and do not primarily support HIV replication. (3) High levels of viremia and viral dissemination occur. (4) Downregulation of virus replication by immune response occurs in the absence of treatment. Mediated by CD8+ cytotoxic T cells. Neutralizing Abs are formed but virus titers fall before these Abs are fully developed. (5) A viral 'set point' is reached after approximately 6 months. Viral set point is predictive of the rate of subsequent disease progression.
(MIDII #157)

Does HIV viral replication occur during the long latency period between the time of infection and the development of clinical AIDS? How is clinical AIDS defined? What are rapid progressors and long-term non-progressors?
Active viral replication is present throughout the course of disease despite clinical latency period. Clinical AIDS is defined as a CD4+ count <200/mm3, or the development of an HIV-related opportunistic infection or malignancy). Clinical illness develops as the CD4 count falls below 200/mm^3. Risk is progressive as CD4 count falls below this level. Avg time from infection to clinical AIDS is 8-10 yrs. There are rapid progressors who develop AIDS within 2 yrs and long-term non-progressors (LTNPs) who maintain normal CD4 counts and very low viral loads in the absence of treatment for >10-15 yrs. These form a very small percentage of overall HIV infected population but provide insights into how an HIV vaccine might be created.
(MIDII #158)

HIV virus is measured in peripheral blood. Are there reservoirs of infection outside the blood compartment?
Yes! Include lymphoreticular tissues (the major “factory” of HIV in lymph nodes, spleen & GI tract), the CNS, and the genital tract.
(MIDII #159)

Does HIV exist in the host as one species or multiple quasispecies?
The virus exists as multiple quasispecies or swarms of viruses. Mixtures of viruses w/differential phenotypic & genotypic characteristics may coexist in the same body compartment or across body compartments. Viruses with different cell tropisms or drug resistance patterns may (and do) coexist in an infected person.
(MIDII #160)

How many virions are produced and destroyed each day in the life of an infected individual? What is the half life of HIV in plasma?
10 x 10^9 virions are produced and destroyed each day in the life of an infected individual. Turnover of HIV in the body is enormous with the half life in plasma estimated at <6 hrs and as short as 30 min. Outcome of infection is a balance between virulence of the pathogen and host factors. Strength of innate and acquired immune responses (especially CD8+ cytotoxic T cells), chemokine receptor status and HLA type of the infected person are impt codeterminants of the outcome.
(MIDII #161)

Gastrointestinal viruses are major causes of morbidity and mortality throughout the world. All of these viruses enter through the GI tract but the diseases associated with them are not confined to the GI tract. Among enteroviruses
Enteroviruses produce a variety of disease syndromes. Rotaviruses and calciviruses are mainly confined to GI illness.
(MIDII #162)

Discuss Enteroviruses as a family.
Members of the Picornaviridae family of very small RNA viruses which also includes aphthoviruses, cardioviruses and rhinoviruses. Enteroviruses have been divided into subgroups consisting of polioviruses, coxsacieviruses, echoviruses, and the so-called 'newer enteroviruses.' They are small, non-enveloped single stranded positive sense RNA viruses.
(MIDII #163)

Discuss molecular biology and pathogenesis of infection by enteroviruses.
Enter the body thru GI tract, usually from fecally contaminated material. Virus multiplies in submucosal lymphatic tissues of the gut and passes to regional lymph nodes and the reticuloendothelial system. Usually contained in the immune system resulting in subclinical infection. However, sometimes heavy sustained replication occurs and the virus is shed into the bloodstream causing disease at many distant sites. Virus penetrates host cells, uncoats and releases its RNA into the cytoplasm within minutes. Inside the cell, RNA is translated into a polyprotein which is subsequently cleaved to form: (a) 4 viral capsid proteins which make up the icosohedral covering of the virus protecting the viral RNA and allowing the virus to attach to its host cell. (b) 8 non-structural proteins whose functions include: RNA replication, protease activity, and inhibition of host cell protein synthesis.
(MIDII #164)

At what point in enterovirus infection are inflammatory or necrotic lesions noted in body tissues? Are such lesions found in the gut or lymphoreticular system where the virus originally replicates?
While virus originally replicates in gut or lymphoreticular system, few inflammatory or necrotic lesions are noted in these tissues. However, targets of the viremic stage usually show significant inflammation and necrosis which correlates with the titer of virus present. Produces much of the morbidity associated with the infection.
(MIDII #165)

POLIOVIRUS => Discuss Pathogenesis of the Virus.
Polioviruses cause poliomyelitis, a systemic infectious disease of varying severity. Usually affects the CNS and can result in paralysis. Humans are the only natural host and reservoir of polioviruses. Discuss the characteristic histopathology of poliovirus, with regard to the distribution of lesions.

Polio enters thru the gut, replicates in the submucosal lymphoid tissue, & spreads to the reticuloendothelial system. In susceptible hosts, virus spreads thru blood to CNS where it causes extensive necrosis of neurons in the gray matter of the spinal cord & brain. Polio primarily infects autonomic and motor neurons. Destruction of these neurons is accompanied by an inflammatory infiltrate of PMNs, lymphocytes and MΦs. Main sites of attack are the gray matter of the anterior horn of the spinal cord and motor nuclei of the pons and medulla.
(MIDII #166)

Discuss the Epidemiology of Poliomyelitis.
Before 1900 polioviruses were ubiquitous, resulted in mostly inapparent early childhood infection. With rising hygienic standards, infection was delayed until later in childhood, creating a pool of susceptible hosts and conditions for an epidemic. Increase in paralytic disease in the 1950s was due to the fact that (while infected infants are protected by maternal Ab) older children have no natural immunity. Introduction of the 1st polio vaccine in 1955 led to dramatic reductions in polio cases in the US and other developed countries. Last case of WT polio in the US occurred in 1979. Polio has been eradicated from the western hemisphere and Europe. Polio infections still occur in developing countries.
(MIDII #167)

Clinical Features of Polio Infection
The incubation period of polio (from presumed contact until onset of the prodrome) is 9-12 days. Manifestations of infection by poliovirus range from inapparent illness to severe paralysis and death. At least 95% of infections are asymptomatic. Symptomatic disease can take a number of increasingly severe forms, including: (1) Abortive poliomyelitis, (2) Nonparalytic poliomyelitis, (3) Spinal paralytic poliomyelitis, (4) Bulbar paralytic poliomyelitis, (5) Polioencephalitis.
(MIDII #168)

Clinical Features of Polio Infection => Abortive Poliomyelitis and Nonparalytic Poliomyelitis
ABORTIVE POLIOMYELITIS: Occurs in 4-8% of infections. Characterized by fever, headache, sore throat, listlessness, anorexia, vomiting, and abdominal pain. Neurological exam is normal and illness lasts only a few days.

NONPARALYTIC POLIOMYELITIS: Differs from abortive polio b/c there are signs of meningeal irritation. Systemic symptoms are generally more severe. Clinically indistinguishable from other enterovirus-associated meningitides and full recovery is the norm.
(MIDII #169)

Clinical Features of Polio Infection => Spinal Paralytic Poliomyelitis
SPINAL PARALYTIC POLIOMYELITIS occurs in 0.1% of polio infections. In kids there is a biphasic course with 'minor' and 'major' illnesses. The minor illness corresponds with viremia and symptoms mimic those of abortive polio. Patient recovers after 1-3 days of mild illness, remains well for 2-5 days but then abruptly becomes ill w/headache, fever, vomiting, & neck stiffness. Characteristic of onset of this major illness is muscle pain that is relieved by motion. This phase lasts 1-2 days before frank weakness and flaccid paralysis ensues. Severity is variable, ranging from a single portion of one muscle to quadriplegia. Paralysis is asymmetric, w/proximal muscles more affected than distal muscles. Continues to evolve until fever dissipates in 2-3 days. Sensory loss is very rare in polio and suggests an alternate diagnosis.
(MIDII #170)

Clinical Features of Polio Infection => Bulbar Paralytic Poliomyelitis
BULBAR PARALYTIC POLIOMYELITIS is characterized by paralysis of the muscles innervated by the cranial nerves. Results in dysphagia, nasal speech, and dyspnea with cranial nerves 9 and 10 (vagus) most commonly affected. Involvement of vasomotor and respiratory centers is less common, but results in a rapid pulse, hypoxia, and elevated BP followed by a circulatory collapse which can result in death.
(MIDII #171)

Clinical Features of Polio Infection => Polioencephalitis
POLIOENCEPHALITIS is manifested by confusion and changes in mental status. It is uncommon and occurs primarily in infants. Paralysis, if it occurs, is spastic, suggesting upper motor neuron disease.
(MIDII #172)

How is poliovirus infection diagnosed?
Poliovirus can be diagnosed from throat secretions in the 1st wk of illness, and from feces for several wks. Unlike other enteroviruses, RARELY isolated from the CSF. Diagnosis can also be made by testing paired acute and convalescent sera for a rise in Ab titer.
(MIDII #173)

Discuss prevention of poliovirus infection by vaccines.
Poliovirus vaccines have been used successfully for over 30 yrs. Wiped out WT poliovirus infections from the western hemisphere. 2 vaccine formulations are currently available: (1) the oral polio vaccine (OPV) and (2) the inactivated polio vaccine (IPV). OPV is a live attenuated vaccine and was the mainstay of vaccination compaigns worldwide for many yrs. Given orally, more immunogenic than the original IPV, and since it is excreted in the feces of vaccinated individuals, it allows for spread of vaccine virus to unimmunized individuals. As cases of naturally occuring poliovirus infection fell in the developed world, the risk of the very rare (but real) paralytic disease resulting from the OPV virus itself exceeded that of naturally occurring polio and the US has since recommended that we only use IPV. The original Salk IPV has been modified to make it more immunogenic (at least equal to OPV). Excellent safety record and can be safely given to immunocompromised individuals as it contains no live virus.
(MIDII #174)

Central Nervous System Infections Caused by Enteroviruses: => Aseptic Meningitis
Aseptic meningitis is characterized by signs & symptoms of meningeal irritation in the absence of bacteria or fungi. Most community acquired cases of aseptic meningitis are caused by viruses. 90% are caused by group B coxsackieviruses & echoviruses. Infants less than 3 months of age have the highest rates of clinically recognized aseptic meningitis. In older kids and adults severity of disease varies widely. Typical patient has prodrome of fever & chills followed by headache, stiff neck, & symptoms of an upper RT infection. These infections are usually self-limited & uncomplicated. Dx depends on examination of CSF which shows clear fluid with 10-500 white cells. White cells may be predominantly PMNs initially but the differential will shift to a lymphocyte predominance over 1st 1-2 days of illness. CSF glucose and protein Cx's are normal or very slightly elevated. Enteroviruses can be detected in the CSF by PCR or cell culture. Differential Dx includes partially treated bacterial meningitis, other viral meningitis (arbovirus, LCMV, HIV-associated meningitis), Lyme disease, leptospirosis.
(MIDII #175)

CNS infections caused by Enteroviruses => Aseptic Meningtis

How do we treat aseptic meningitis caused by enteroviruses?
Treatment consists of asymptomatic relief. Pleconaril, an orally administered picornaviral capsid-stabilizing drug, decreases the duration of symptoms but has been disappointing in clinical trials of severely ill patients.
(MIDII #176)

CNS infections caused by Enteroviruses => ENCEPHALITIS
Enterovirus caused encephalitis is a rare manifestation of echo virus and coxsackievirus CNS infection. Enteroviruses (including polio) account for 11-22% of viral encephalitis cases. Kids and young adults are most commonly affected w/clinical manifestations ranging from lethargy and drowsiness to seizures and coma. The prognosis (except in infants) is excellent.
(MIDII #177)

CNS infections caused by Enteroviruses => Chronic Meningoencephalitis
Seen in pts w/acquired or hereditary defects in B-lymphocyte function. Mostly children with X-linked agammaglobulinemia. Mostly caused by echoviruses. Nervous system manifestations range from mild nuchal rigidity & headache to seizures & ataxia. Enterovirus can be recovered from the CSF for months to years. In many (if not most) affected persons the disease ends in death. Prophylactic use of intravenous immune globulin is currently used to prevent enteroviral infection in pts w/B cell defects. Used less successfully to treat those w/chronic meningoencephalitis.
(MIDII #178)

CNS infections caused by Enteroviruses => Paralysis
Paralysis is associated w/infection by coxsackie and enterovirus infection. Coxsackievirus A7 and enterovirus 71 have been associated w/outbreaks of flaccid paralysis. Paralytic disease caused by nonpolio enteroviruses is less severe than poliovirus associated paralysis. Paresis (partial or incomplete paralysis) is not permanent. Guillain-Barre syndrome, transverse myelitis, and Reye's syndrome have all been associated w/enteroviruses.
(MIDII #179)

Enterovirus associated exanthems or rashes => MORBILLIFORM EXANTHEMS
[NOTE: Exanthems or Rashes are common features of enterovirus infections. Note that these rashes can take many forms and (with the exception of hand-foot-and-mouth disease) are not distinctive enough o make a definitive diagnosis.]

Morbilliform Exanthems are fine, erythematous, maculopapular (both flat and raised) rashes that are common manifestations of echovirus infections especially during summer outbreaks. Most common serotype associated w/this rash is echovirus 9. Rash appears simultaneously w/fever and starts on the face spreading to the chest and extremities.
(MIDII #180)

Enterovirus associated exanthems or rashes => ROSEOLIFORM EXANTHEMS
Discrete, nonpruritic (not itchy), salmon-pink macules and papules which appear on the face and upper chest. Usually associated with a prodrome of fever and pharyngitis. Rash doesn't appear until after the patient has defervesced. Generally lasts 1-5 days. Infections are quite contagious, usually occur in young children. Echovirus 16 is most commonly associated w/this syndrome.
(MIDII #181)

Enterovirus associated exanthems or rashes => Hand-Foot-and-Mouth Disease
Distinctive vesicular eruption caused by coxsackie virus A16 or enterovirus 71. Common in children under age 10. Characterized by fever and vesicles in the oral cavity, hands and feet. Palms and soles may be affected. Lesions are tender – mixed papules and clear vesicles with surrounding erythema. The Differential Dx is chickenpox. However, while patients with HFM disease invariably have oral mucosal lesions, oral lesions are less common in chickenpox. Pts with chickenpox appear more ill than those with HFM disease. It can be tough to tell them apart, however, and these may account for 'repeat' cases of chickenpox.
(MIDII #182)

Enterovirus associated exanthems or rashes => Generalized vesicular eruptions
Coxsackievirus A9 and echovirus 11 cause generalized vesicular eruptions. Similar to the lesions of HFM disease but occur in crops on the head, trunk, and extremities. Unlike chickenpox, they do not evolve into pustules and scabs.
(MIDII #183)

Enterovirus associated exanthems or rashes => Herpangina
Well-characterized vesicular rash involving the pharynx and soft palate accompanied by fever, sore throat, and pain on swallowing. Usually seen in summer outbreaks. Group A coxsackieviruses are commonly associated. Illness begins suddenly w/fever, vomiting, myalgia and headache which resolve quickly. Sore throat and pain on swallowing precede development of oral lesions. Prompt recovery over a week occurs in all cases.
(MIDII #184)

Enterovirus Associated Respiratory Disease => COLDS
Many enteroviruses cause undifferentiated febrile illness with sore throat, cough, and corya (runny nose). Account for a large number of viruses recovered from children with summer colds. Coxsackieviruses A21 and A24 produce common cold symptoms indistinguishable from rhinovirus infections except for a higher percentage of fever. Echovirus 11 is the most common echovirus cause of cold-like syndromes.
(MIDII #185)

Enterovirus Associated Respiratory Diseases => EPIDEMIC PLEURODYNIA
Acute disease characterized by fever and sharp, spasmodic pain in the chest or upper abdomen. Group B coxsackieviruses are the most common etiologic agents, can cause major epidemics. Pleurodynia is a disease of muscle (not pleura) and tenderness mimicking the pain of infection can be elicited by placing pressure on the affected muscle. Characterized by abrupt onset of spasmodic pain with fever which peak one hr after each paroxysm and subside as pain decreases. Most patients are ill for 4-6 days and adults are more severely affected than children. Disease may relapse and can occur over a several month period, but all patients eventually recover completely.
(MIDII #186)

Discuss Enterovirus Associated Myopericarditis => Pathogenesis
Myopericarditis is inflammation of the myocardium and pericardium. Enteroviruses are the most common viral etiologic agents accounting for 50% of all cases of acute myopericarditis. All group B coxsackieviruses, group A types 4 and 16 and echoviruses types 9 and 22 have been definitively linked to myopericarditis. Virus reaches the heart during the viremia that follows replication in the retinculoendothelial system. Virus replication occurs in the myofibers and results in scattered myofiber necrosis followed by focal inflammatory infiltrates. Healing is accompanied by a variable degree of interstitial fibrosis and evidence of myocyte loss.
(MIDII #187)

Discuss Enterovirus Associated Myopericarditis => Clinical Manifestations and Sequelae
Variable clinical manifestations. Occurs in all ages. Special predilection for physically active adolescents and young adults. Incidence in males is 2x that in females. In >60% of cases, upper respiratory illness precedes onset of cardiac symptoms by 7-14 days. Cardiac involvement is heralded by dyspnea, chest pain, fever & malaise. Chest pain is in the precordial area, usually dull and relieved by sitting up and forward. EKG abnormalities are almost always present. Echocardiograms may show dilatation and dysfunction of the heart muscles. Serum levels of myocardium enzymes are elevated. Frank congestive heart failure is present in 20% of cases. Chronic dilated cardiomyopathy leading to chronic congestive heart failure is the most dreaded complication of enteroviral myopericarditis. 1/3 of pts will have some permanent sequelae including EKG changes, cardiomegaly & chronic constrictive pericarditis. Management is mainly supportive. IV IG (pooled immunoglobulin from multiple blood donors) has shown some value.
(MIDII #188)

Discuss Enteroviral Infection of Newborn Infants => How do they catch infection?
Neonates are uniquely susceptible to enterovirus infections. Group B coxsackieviruses serotypes 2-5 and echovirus 11 are the most common culprits. Most neonatal enteroviral infections are acquired directly from the mother but nosocomial outbreaks have been reported. Most neonates are infected in the perinatal period. 60-70% of women transmitting the infection will have fever in the wk prior to delivery. Once newborn is infected, enterovirus spreads systemically through the bloodstream. Has particular tropism for the heart & liver. Neonates can form neutralizing Ab to enteroviral infections but lack the MΦ activity needed to control infection completely.
(MIDII #189)

Discuss the Clinical Manifestations of Enteroviral Infection of Neonates.
Symptoms develop between 3-7 days of life. Generally mild and nonspecific. 1/3 have a biphasic illness w/ a period of 1-7 days of well-being between initial symptoms and more serious manifestations. Generalized enterovirus disease in the newborn usually occurs in 1 of 2 characteristic clinical syndromes: myocarditis or hepatitis. Myocarditis, which is often accompanied by encephalitis, is usually caused by Group B coxsackieviruses. Fulminant hepatitis is characterized by hypotension, profuse bleeding, jaundice, and multiple organ failure and results from echovirus 11 infection. Dx of neonatal enterovirus infection is rapidly made w/PCR or viral culture. Virus can be detected in urine, feces, blood, CSF, and oropharyngeal secretions. Management is supportive. IVIG may be considered (as may pleconaril but results with this drug have been disappointing to date.)
(MIDII #190)

Discuss Acute Hemorrhagic Conjunctivitis associated with Enterovirus 70 Infection.
Contagious ocular infection characterized by pain, swelling of the eyelids, and subconjunctival hemorrhage that generally resolves spontaneously within a wk. Enterovirus 70 causes this infection. Has caused epidemics in Africa, Asia, and Europe. Acute hemorrhagic conjunctivitis is spread from fingers directly to the eye. Highly contagious and spreads rapidly. Begins abruptly with burning, pain, photophobia and watery discharge in one eye followed a few hours later by symptoms in the other eye. Conjunctival hemorrhage can be pinpoint or occupy the entire conjunctiva. Most cases resolve spontaneously but a few may be followed by motor paralysis mimicking poliomyelitis.
(MIDII #191)

ROTAVIRUSES => General Description
Rotaviruses are large, non-enveloped RNA viruses that are responsible for 10-20% of all diarrhea related deaths in children worldwide and account for up to 120,000 hospitalizations in the US each year. They are members of the Reovirus family. Wheel-like appearance. Genome is segmented dsRNA. Segmented genome allows for reassortment of strands when 2 different strains coinfect a single host cell. Allows the virus to adapt to combat host defenses, although major antigenic shifts like those in influenza A don't occur. Since the genome RNA is double-stranded it can't function directly as mRNA so these viruses package their own RNA polymerase to make mRNA.
(MIDII #192)

ROTAVIRUSES => Describe the replicative cycle of rotaviruses.
(1) Viral entry via phagosome; (2) Release from phagosome; (3) Uncoating, release of RNA, and transcription into mRNA; (4) Production of viral proteins; (5) Viral RNA synthesis; (6) Movement of viral proteins; (7) Movement of core to ER; (8) Assembly of viral particle; (9) Release of viral particle.
(MIDII #193)

Discuss Pathogenesis of Infection with Rotaviruses.
There are 7 (A-G) antigenic groups of Rotaviruses. Only groups A-C cause disease in humans with group A viruses responsible for most human disease worldwide. Rotaviruses are spread via the oral-fecal route. Since they lack an envelope they are quite resistant to gastric acid (and disinfectants). Highly infectious, infectious dose as low as 1pfu. Replicate in mature villus epithelial cells of the small intestine. Infection of the villus epithelium results in loss of those cells, resulting in decrease in absorptive area of the intestine. Can result in lactase deficiency, a common sequela of viral and bacterial gastroenteritis. Cause vomiting and diarrhea. NSP4 protein of rotavirus acts in a toxinlike manner to promote Ca2+ ion influx into enterocytes, release of neuronal activators, and a neuronal alteration in water absorption.
(MIDII #194)

Discuss the Epidemiology of Infection with Rotaviruses.
Occur worldwide. Almost everyone over age 2 has been infected w/at least 1 strain of rotavirus. Infections in temperate climates are seasonal, occurring over a 3-4 month period in the winter. In North America infections begin in late autumn in Mexico and spread north and east ending up in eastern Canada by the spring. No cases in the summer months.
(MIDII #195)

ROTAVIRUSES => Clinical Features
Range from asymptomatic to severe gastroenteritis. First infection is the most severe. Rotavirus infection involves both vomiting and diarrhea accompanied in severe cases by nausea and a high fever. Diarrhea is watery w/o mucous or blood. Fecal leukocytes might be present in severe rotavirus infections (not in viral diarrheas). Dehydration and electrolyte imbalance are the most common reasons for hospitalization and death from rotavirus infection.
(MIDII #196)

ROTAVIRUSES => Diagnosis and Treatment
(Dx) Diagnosis can be suspected clinically in a febrile young infant with both vomiting and diarrhea in the winter. Most clinical labs use ELISAs to detect rotavirus antigens in stool samples or PCR. Electron microscopy of stool will show characteristic wheel-shaped viral particles (but this doesn't happen in most hospital labs). (Rx) Treatment is supportive w/replacement of fluid and electrolytes either orally or IV. Early feeding is encouraged, promotes enterocyte regeneration and decreases intestinal permeability. Antidiarrheal medications are not recommended (body is trying to get rid of the virus for a reason and we don't want to interfere with that attempt).
(MIDII #197)

ROTAVIRUSES => Prevention
Good hygiene (washing hands!!) and chemical disinfection are the best ways to prevent rotavirus spread in communities and households. Almost everyone is infected and mounts an immune response to infection by age 2, but immunity to rotavirus is NOT complete and infections do occur in older children and adults. Usually these infections are milder than in young infants. A vaccine called Rotashield was licensed for use in the US in 1998 and was highly effective but after 10 months on the market there were 10 reported cases of intussusception (telescoping of the bowel on itself – a nasty complication that requires surgical repair and can be fatal) were reported in young vaccinated infants. In July of 1999 the FDA halted vaccination and the vaccine was pulled from the market. Few children die from Rotavirus infection in the US, so this is reasonable here, but in the developing world death rates are much higher to the decision to pull the vaccine is more controversial.
(MIDII #198)

CALICIVIRUSES (“The Love Boat Bugs”) => General Description
Single stranded, positive sense RNA viruses that are responsible for many outbreaks of gastrointestinal illness across the world. They get their name from the 'cuplike' indentations on their surfaces. Caliciviruses are simple, non-enveloped viruses whose genome encodes 4 polypeptide products: (1) HELICASE which unwinds double helical regions in RNA during recombination, replication, transcription and splicing. (2) PROTEASE which is responsible for cleaving the single polypeptide into its functional parts. (3) RNA POLYMERASE which is responsible for replicating RNA. (4) CAPSIDE which covers the RNA genome.
(MIDII #199)

CALICIVIRUSES => Pathogenesis
Caliciviruses can't grow in cell culture. Spread thru oral-fecal route. Since they are non-enveloped they can survive gastric acid and pass into the small bowel where they cause disease. Acute calicivirus infection results in a reversible lesion in the jejunum characterized by blunting of the villi which appears within 24 hrs of infection. Mucosa remains intact. PMNs are seen in the lamina propria. Diarrhea is associated w/ transient malabsorption of D-xylose and fat; and decreased activity of brush-border enzymes. Infection is NOT associated w/ detectable toxin production. Exact mechanism behind diarrhea & vomiting is not known. Virus shedding in stool is highest in the first 24-48 hrs after illness; rarely detected beyond 72 hrs after illness onset. Patients are most infectious while symptomatic.
(MIDII #200)

CALICIVIRUSES => Epidemiology
Caliciviruses are widespread and common. Disease can occur throughout the year and affects all age groups. Almost any type of food that has contact with contaminated water (shellfish) may serve as a vehicle for outbreaks of calicivirus gastroenteritis. Contaminated drinking water and swimming pools and lakes in which infected ppl have swum can be vehicles for outbreaks. Viruses are hardy, can withstand chlorination, heat inactivation (cooking doesn't eliminate risk of transmission). Responsible for recent large outbreaks of diarrheal illness reported on cruise ships.
(MIDII #201)

CALICIVIRUSES => Clinical Manifestations
Caliciviruses were first ID'd in point-source outbreaks of gastroenteritis and remain impt causes of such outbreaks. Features: short-lived illness (2-3 days duration), with vomiting as predominant symptom, incubation period of 24-48 hrs, high secondary attack rates (spread well!). Both vomiting and diarrhea generally occur; myalgias, malaise, and fever are common. Disease manifestations generally last 48-72 hrs and remit without sequelae.
(MIDII #202)

CALICIVIRUSES => Diagnosis, Treatment and Prevention
(Dx) Diagnosis can be suspected on clinical and epidemiological grounds and by the absence of any other pathogen. Lab tests are NOT useful. Electron microscopy of stool samples can reveal characteristic virus particles but is not used outside of research setting. An ELISA test for calicivirus antigens is in the works but is not yet widely available. (Rx and Prevention): Treatment is supportive only. Oral fluid replacement is all that is necessary. Some will need IV rehydration. No vaccine is available. Prevention is primarily through judicious food and water handling.
(MIDII #203)

INFLUENZA => General Description
Influenza viruses cause acute, self-limited febrile illnesses most often in the winter months. Belong to orthomyxoviridae family. 3 types: flu A, B, and C. Influenza A and B cause human disease w/significant morbidity & mortality. Flu C infections are generally subclinical. Influenza viruses are enveloped viruses w/segmented, neg sense RNA genomes. Flu A and B have 8 segments in their genome while Flu C has 7 (lacks a neuraminidase protein).
(MIDII #204)

INFLUENZA => Discuss the important Viral Proteins
(1)PB I, PB2, and PA => viral polymerase proteins, allow the virus to replicate its RNA. (2) NA (neuraminidase protein) => protrudes thru the viral envelope, catalyzes removal of sialic acid residues, allowing the virus to escape from its host cell and move thru mucus. (3) HA (hemagglutinin protein) => protrudes thru the viral envelope, binds to sialic acid residues, major attachment protein for the virus. Mediates fusion between the viral envelope and the endosome, which is how the virus gains entry into cells. NA and HA are the 2 major antigenic proteins against which neutralizing Abs are made to influenza. (4) NP is the nucleocapsid protein, covers viral genome. (5) M protein(s) located inside the viral envelope. M1 is found in Flu A&B and provides stability to the virion. M2 (only in flu A) acts as an ion channel within the endosome. (6) NS => nonstructural proteins, function is not clear.
(MIDII #205)

INFLUENZA => Immune Evasion

Influenza is constantly changing in order to avoid immune detection. Accounts for annual flu seasons and periodic pandemics. Mechanisms that the virus uses to change its antigenic sites ar
(1: Antigenic Drift): HA &NA proteins of influenza viruses are major sites for Ab recognition on the virus. To help the virus evade Ab detection, the RNA segments that encode NA & HA mutate, keeping their functions intact but making them less well recognized by Abs. Ongoing mutation = antigenic drift: occurs in influenza A & B. Drift is responsible for year-to-year variation in the influenza viruses; reason we must keep changing the makeup of the flu vaccine. (2: Antigenic Shift): Since RNA genomes of influenza are segmented they undergo reassortment (mixing up) if 2 different influenza viruses infect the same cell. Impt in understanding how influenza causes pandemics. When circulating human influenza virus infects an animal or bird host already infected w/ its own virus, segments of the 2 viruses can be mixed up & packaged together. When a foreign HA and/or NA ends up in a virus w/human segments encoding other proteins, we get a virus that can replicate in human cells but is not recognized at all by any human Abs. Pandemics occur when human populations are faced w/influenza viruses to which they have no immunity. Antigenic shift only occurs w/flu A, b/c these viruses infect both humans & animals.
(MIDII #206)

INFLUENZA => Discuss the History of Influenza Pandemics and fears about future pandemics caused by Avian flu: A/H5N1.
1918 Spanish flu killed 20-40 million ppl => result of antigenic shift. Most recent shift occurred in 1968 (Hong Kong flu), and drifted variants of this flu (A/H3N2) are predominant strains circulating today. Flu shifts occur every 30 yrs. Avian flu A/H5N1 is the next big worry, killed 5 people and millions of birds in Hong Kong in the late 1990's. H5N1 has been circulating in birds for some time but this strain of H5N1 killed the birds. Humans could be infected; mortality rate in humans was over 50%. This flu strain has several virulence factors including a highly cleavable hemagglutinin that can be activated by multiple cellular proteases, a specific substitution in the polymerase basic protein 2 that enhances replication, and a substitution in nonstructural protein 1 that confers increased resistance to inhibition by IFNs and TNF-α in vitro and prolonged replication in swine, as well as greater elaboration of cytokines, particularly TNF-α, in human MФs exposed to the virus. Humans infected have exaggerated symptoms of the flu and die of primary influenza pneumonia. For the virus to make the jump to pandemic status it will need to be efficiently transmitted from person-to-person (not from bird-to-person) and it has not done this yet.
(MIDII #207)

INFLUENZA => Clinical Manifestations
Nasty, self-limited infection. Classic presentations require presence of fever above 101 along with at least one systemic symptom (myalgias, chills, malaise) and at least one respiratory symptom (cough, nasal discharge). The onset of symptoms is abrupt and occurs 1-2 days after acquisition of the virus. Systemic symptoms usually dissipate after 3-4 days but other symptoms can persist for up to 2 wks. GI symptoms other than anorexia are rare (no such thing as the stomach flu). Complications are quite common and can be deadly.
(MIDII #208)

INFLUENZA => Pneumonia
Most common complication of influenza infection. Can be primary viral pneumonia or secondary bacterial pneumonia. Primary influenza pneumonia virus directly infects the lower RT causing rapidly progressive bilateral pneumonia which is very often fatal. Influenza pneumonia is responsible for the overwhelming majority of deaths in pandemics, especially in otherwise healthy young adults. In older adults and those w/chronic medical conditions, secondary bacterial pneumonia is the major cause of mortality. Influenza infection allows pathogenic bacteria to secondarily infect the lung with a resulting bacterial pneumonia. Strep pneumo and Staph aureus are common post-influenza pulmonary pathogens.
(MIDII #209)

INFLUENZA => Myositis, Neurologic Complications, Reye's Syndrome
(1)MYOSITIS: Inflammation of the muscles is seen in children following some influenza B infections. Muscles of the legs are particularly involved. (2) Neurologic complications include a post-infectious encephalitis and Guillain-Barre syndrome. (3) Reye's syndrome involves changes in mental status and liver dysfunction. Seen in children with influenza and other viruses who are given aspirin. Mortality from increased intracranial pressure is high and aspirin is no longer recommended as an antipyretic in children.
(MIDII #210)

INFLUENZA => Diagnosis
Self-diagnosis on the basis of clinical symptoms is adequate for most people. Gold standard for lab diagnosis is virus culture, but this is time and labor consuming and is used primarily by state labs to monitor outbreaks. Rapid antigen tests are the current diagnostic tests of choice for influenza. These test are performed directly on pt samples and can be highly sensitive and specific. PCR is now available for influenza although it is not yet widespread.
(MIDII #211)

INFLUENZA => Treatment
Most ppl require only rest and fluids. Some will benefit from specific antiviral agents. (a) Amantidine and rimantadine are primary symmetric amines which interfere with viral uncoating by blocking the action if influenza A's M2 protein. If given within 48 hrs of onset of symptoms, these drugs can reduce duration of symptoms by 1-2 days. Very effective at preventing infection if given during an influenza outbreak. Unfortunately, many viruses are resistant to amantidine & rimantidine. Side effects (esp CNS effects) are a problem with these agents. Dose adjustments have to be made in elderly pts and in ppl w/renal insufficiency. (b) The neuraminidase inhibitors are preferable. Decrease duration of symptoms of influenza and prevent infections. Block neuraminidase activity. Effective against both flu A & B. Side effects are milder than amantidine & rimantidine and they work against viruses resistant to those drugs. However, NONE of these agents prevents the complications of influenza infection.
(MIDII #212)

INFLUENZA => Prevention
Trivalent inactivated influenza vaccine is the mainstay of prevention. Vaccine is made up of inactivated viruses thought to circulate in the coming flu season. Since the virus is inactivated it is IMPOSSIBLE to get the flu from the flu vaccine. Efficacy of the vaccine is 50-80% and is lower in elderly & immunosuppressed populations. Vaccine reduces hospitalizations by 70% and death by 80% in these groups so it's definitely worth giving. Recommended that all individuals >50 receive the vaccine along w/anyone w/cardiac, pulmonary (asthma included), or renal disease, & ppl w/diabetes, hemoglobinopathies, immunosuppressive illnesses. Residents of nursing homes should be vaccinated as should caregivers of any of the above groups. Takes 2 wks for protective Ab to develop and Ab titers decline over several months. Optimal time to get the flu vaccine is from October to mid-November in the US. Yearly vaccination is req'd for cont'd protection. 5% of vaccinees will experience low grade fever and mild systemic symptoms. 30% will notice some tenderness at the vaccination site.
(MIDII #213)

INFLUENZA => Discuss the Live Attenuated Nasally Administered Vaccine
A live attenuated nasally administered vaccine was approved 2 yrs ago for prevention of influenza in healthy individuals aged 5-49. Will soon be approved for infants as young as 6 months and for older adults. It is highly efficacious. Shedding and some limited transmission of the vaccine virus has been seen; however, it remained attenuated and no-one got sick.
(MIDII #214)

RSV is a member of paramyxoviridae family (which includes the parainfluenza viruses, mumps, and measles viruses). Most common cause of bronchiolitis in infants. Major contributor to morbidity & mortality especially in premature infants and the elderly. RSV is an enveloped, single-stranded, negative sense RNA virus. Genome encodes 10 viral proteins: (a) the glycosylated surface proteins F, G, and SH which mediate attachment of the virus to its host cell and fusion of viral w/host membranes. (b) proteins N, L, and P are associated w/the nucleocapsid. (c) M and M2 are non-glycosylated matrix proteins. (d) NS 1 and NS 2 are highly conserved non-structural proteins that may play a role in RNA replications.
(MIDII #216)

RSV => Epidemiology
Ubiquitous virus. Outbreaks occur every year: seasonality in winter and early spring. Begin in Nov, peak in Jan, continue 'til Apr. Virtually all kids are infected by age 2. Serious illness is more common in young infants. Boys & kids from lower socioeconomic backgrounds are more at risk for serious disease. Particularly at risk: preemies, esp w/bronchopulmonary dysplasia, kids w/congenital heart disease, kids w/pulmonary disease. RSV is impt pathogen in the elderly w/hospitalizations for complications of RSV disease being as common in this group as hospitalizations for influenza.
(MIDII #217)

RSV => Clinical Manifestations
Primary infection w/RSV is usually symptomatic. Lower RT involvement is most often seen in primary RSV infections. Pneumonia & bronchiolitis are seen most commonly in infants w/primary infection. Symptoms start w/nasal congestion, sore throat & fever. Cough develops in 1st few days, becomes deeper & more prominent as infection proceeds. Increased resp rate; retraction of lower intercostal muscles. Duration of symptoms: 7-21 days. Hospitalization rates due to lower RT involvement ~ 40% in infants <6 mo. Infection in adults & older kids is rarely asymptomatic despite previous immunity. Nasal congestion & cough mimicking common cold are the most common manifestations. Older adults at greater risk for serious infection than younger adults. Mortality from RSV pneumonia almost 20% in older adults. Immunocompromised individs at particular risk. Kids w/SCID & adults w/transplants & hematologic malignancies are at particular risk of fatal lower RT infections. Many infections are acquired nosocomially as RSV spreads very efficiently in hospitals.
(MIDII #218)

RSV => Treatment
Supportive care is mainstay of therapy in management of seriously ill infants. Supplemental oxygen should be given to hypoxic infants. Efficacy of bronchodilators is questionable although they are routinely used. Ribavirin: broad spectrum antiviral, works by interfering with viral RNA polymerase activity & inhibiting inosine 5'-monophosphate dehydrogenase which depletes intracellular nucleotide pools). Currently approved for Rx of lower tract RSV disease. Usually administered by aerosol and is generally reserved for high-risk pts.
(MIDII #220)

Rhinoviruses => Molecular Biology/General Description
Rhinoviruses are most frequently associated w/common cold symptoms: 30% of upper RT infections. Small, non-enveloped single-stranded RNA viruses. They are members of picornaviridae family which include the enteroviruses (polioviruses, coxsackieviruses, echoviruses, & other enteroviruses) and hep A virus. 110 rhinovirus serotypes, enormous diversity makes vaccine development impossible. Optimal growth at 33 deg. C, corresponds to temp of the nose and large airways. Majority of rhinovirus serotypes uses a single cellular receptor-intercellular adhesion molecule (ICAM-1), which is found on many cell type surfaces. Cell surface ligand for LFA-1, impt role in immunologic and inflammatory rxns. Smaller group utilizes members of the low-density lipoprotein receptor family.
(MIDII #221)

RHINOVIRUSES => Pathogenesis
Enter nasopharynx thru nasal or ophthalmic mucosal (by touching nose or eyes). Viral replication occurs in nonciliated lymphoepithelial cells of the nasopharynx. Primary infection involves adenoidal tissues. Small number of epithelial cells are infected. Viremia does not occur. Rhinoviruses enter thru ICAM-1. Found on luminal surface of nonciliated lymphoepithelial cells in the nasopharynx, in the endothelial cells of the microvasculature, & in the germinal centers of the pharyngeal lymph nodes. Virus is deposited in nares & conjunctiva of the host, transported to the nasopharynx via mucociliary action, encounters ICAM-1 rich adenoidal crypts. Infection spread anteriorly along the nasal passages. Infection of the lower airways by rhinovirus may be more common in children. Fatal rhinovirus pneumonia and histologic evidence of rhinovirus in alveolar cells have been described in infants.
(MIDII #222)

It used to be assumed that, like influenza and adenovirus, the symptoms associated with rhinovirus resulted from direct cytopathic effects on infected epithelial cells. Is this the case?
In situ hybridization of the nasal mucosa biopsy specimens suggest that only a few cells are infected and cytopathology is conspicuously absent. Host inflamm rxn, NOT direct viral damage, is responsible for symptoms of the common cold. Elevated levels of pro-inflamm cytokines (IL-6, IL-8), TNF-alpha and GM-CSF have been documented during rhinovirus infection. IL-8 induces proinflamm changes associated w/infection. IL-8 Cx increases proportionally w/severity of rhinorrhea & nasal obstruction. Rhinovirus interacts w/its receptor, resulting in release of IL-8.
(MIDII #223)

Rhinovirus => Epidemiology
Most adults experience between 1-3 acute respiratory illnesses per yr. Infants <1 yr have highest rates of illness, w/avg of 6.1 infections per yr. Rates decline steadily w/increased age except for a slight increase in the 20-29 yr old age group. Due to presence of children in the home, supported by higher rates of illness in women in this age group who are primary caregivers for their kids. Multiple serotypes simultaneously present in the community, antigenic drift, and reinfection possibility permit multiple infections within a single individual; contribute to frequency of rhinovirus infections. 80% of infections are associated w/clinical illness. Illness is mild. Infected ppl don't seek medical attn.
(MIDII #224)

Rhinoviruses => Seasonal Pattern

Do rhinoviruses display a seasonal pattern of infection?
Well-established seasonal pattern in temperate climates. Peaks are seen in early fall & spring. Rhinovirus activity is low in the winter w/coronaviruses & other agents are responsible for more debilitating winter colds. In tropical climates rhinovirus activity is greatest during the rainy season. Not known why. Seasonal changes in living conditions account for outbreaks. Environmental temperatures do not affect infection rates or severity of illness, despite popular belief exposure to cold or rainy weather does not affect host resistance to rhinovirus infection.
(MIDII #225)

Rhinoviruses => Transmission
Spread from person-to-person throughout communities via virally contaminated respiratory secretions. In homes or schools (relatively closed communities) spread is very efficient; secondary attack rates of 25-70%. Spread of rhinovirus colds among ppl at work is less common. Direct contact w/infected secretions & aerosols are efficient means of rhinovirus spread. Rhinoviruses can survive on environmental surfaces for several hrs. Porous materials such as tissues and cotton handkerchiefs do NOT allow virus survival and are NOT efficient modes of virus transmission. Decontamination of environmental surfaces w/virucidal disinfectants like Lysol decreases rate of transmission.
(MIDII #215)

RSV => Pathogenesis

Is Cell-Mediated Immunity or Humoral Immunity involved in viral pathogenesis? Which combination of these immunities contributes to infection? Which type of immunity is important for protection fro
Presence of RSV is detected by characteristic formation of syncytia (cells which have fused to form multinucleated giant cells). RSV is inoculated thru eyes or nose. Infection is confined to the RT. May involve upper RT or may spread to involve entire lower RT. In lower RT involvement, pathology shows a lymphocytic peribronchiolar infiltrate w/edema of the bronchial walls. Later, proliferation & necrosis of the bronchioles develops. Collections of sloughed epithelium lead to obstruction of small bronchioles and subsequent air trapping. Reabsorption of trapped air leads to atelectasis (collapse of parts of the lung) esp in young children. Viral infection in alveolar spaces can lead to frank viral pneumonia w/syncytia formation.

Infections occur when Abs are high and cellular immunity is low (during infancy or late adulthood). Infants given an RSV vaccine had MORE serious illness than UNvaccinated infants so immunologic mechanisms probably play a role in RSV pathogenesis (non-protective Abs are not helpful). Immunity to RSV is incomplete so reinfections are common. Cell-mediated immunity is impt & necessary to protect against severe disease.
(MIDII #236)

HIV and AIDS => Natural Hx of HIV Infection
(1)Does HIV have a period of clinical latency? Microbial latency?
(2)When HIV is difficult to culture, is the HIV virus still replicating?
(3)When are levels of viremia
(1) HIV is a chronic, progressive process w/a variable period of clinical latency but NO microbial latency. (2) Virtually all pts have evidence of active viral replication at all times. (3) Levels of viremia are highest right after infection and then actively suppressed by a host cellular immune response after a few months. (4) A set point is established for concentration of HIV RNA in plasma by 6 months which is predictive for the subsequent course of HIV disease.
(MIDII #237)

HIV and AIDS => Natural Hx of HIV Infection
(1)What is the hallmark of the progressive immunodeficiency of HIV disease?
(2)What is the outcome of untreated HIV disease? How do treatments ameliorate disease course?
(1)CD4+ T cell depletion is the hallmark of HIV. There is a series of CD4 levels below which risk of specific opportunistic infections rise greatly; valuable in targeting diagnostic evaluations and using specific prophylactic mechanisms. (2) Ultimate outcome of untreated disease is progression to AIDS and death in nearly all pts. Better clinical care w/prophylaxis of opportunistic infections extends survival. Combination anti-retroviral therapy can reverse even severe immunodeficiency, reducing risk of opportunistic infections or death dramatically.
(MIDII #238)

HIV and AIDS => Natural Hx of HIV Infection

(1) What accounts for variability in speed of HIV progression? (2) What distinguishes long-term progressors from other patients?
There is substantial variability in HIV's course. (1) Viral load setpoint accounts for most variability seen in the speed of HIV progression. (2) Long-term non-progressors are a small subgroup (5-10%) who have relatively normal CD4 counts (>500) and no HIV-related disease for 10 yrs or more without anti-retroviral therapy. Maintain much lower HIV viral loads than pts starting at same time after infection w/same CD4 count who have more rapidly progressing disease.
(MIDII #239)

HIV and AIDS => Acute Retroviral Syndrome

(1) Do all HIV infected pts display an “acute retroviral syndrome”? (2) What impact does the severity of clinical symptoms associated with this syndrome have on disease p
(1) Only 20% of HIV pts seek care for a clinical illness coinciding w/appearance of HIV Abs several wks to a few months after HIV infection. Usually mild, self-limited, and non-specific. Clinical features include fever, fatigue, sore throat, lymphadenopathy, and a macular erythematous rash. (2) Severity of these initial symptoms predicts more rapid progression of HIV infection than in pts with few or no symptoms. (3) Identifying pts at onset of infection may have a significant therapeutic benefit => antiretroviral therapy at this early stage can extend the time to disease progression. Under investigation whether viral load set point may reset to a lower level when pts are treated for approximately 1 yr after acute infection and then stop HIV therapy.
(MIDII #240)

HIV and AIDS => Asymptomatic Phase of HIV Infection

(1)Are CD4 cells depleted during the variable asymptomatic period (during which there is ongoing viral replication but no symptoms of infection or its complication
(1)Progressive CD4 cell depletion characterizes the variable asymptomatic period. Average pt has a viral load setpoint of 30,000 copies of HIV-1 RNA per ml of plasma and loses 50 CD4+ T cells per year. There are NO symptoms in most pts during this time. (2) There are compelling reasons to ID HIV positive asymptomatic patients: (a) Behavior changes can be made to lower or eliminate risk of further transmission of HIV; (b) Prophylactic regimens to prevent life-threatening opportunistic infections can be utilized based on CD4 cell risk staging; (c) If antiretroviral treatment is initiated before the late stages of HIV disease immune deterioration can be halted or reversed before complications develop.
(MIDII #241)

HIV and AIDS => Early Manifestions of HIV Disease

Discuss some of the clinical manifestations which occur more often in HIV patients as prodromal events (although they are not HIV-specific).
These illnesses include bacterial pneumonia esp due to S. pneumoniae, herpes zoster, new onset or major flares of psoriasis and sebhorreic dermatitis, salmonella septicemia; and increasingly frequent or severe recurrences of ano-genital Herpes simplex. Most of these events are associated with moderately advanced immunodeficiency and are quickly followed by an AIDS-defining event if HIV infection is not recognized and prophylaxis initiated. There are 2 exceptions: Herpes zoster may precede AIDS by years, as well as tuberculosis. Any of these manifestations in an individual with hx of HIV risk behavior or otherwise healthy young adults should raise possibility of HIV infection.
(MIDII #242)

HIV and AIDS => Prognostic Markers for the Course of HIV Disease
Absolute value of the CD4 count is the best surrogate marker to predict time to AIDS, risk of specific opportunistic infections, or death, especially once counts have fallen from the normal range of 800-1200 to 300 or less. The CD4% by itself is less accurate prognostically. Immune activation markers (neopterin, beta-2 microglobulin) may add to precision of CD4. Combining HIV-1 viral load measurement w/CD4 count provides an accurate prediction of risk of developing AIDS 5 yrs in the future, even w/pts with nearly normal CD4 counts at baseline.
(MIDII #243)

HIV and AIDS => Clinical Features: Spotlight on PNEUMOCYSTIS PNEUMONIA (PCP)
(2)Risk factors
(1: Pathogen) Pneumocystis is a fungus. Most ppl become exposed during childhood; fungus is widely distributed. Caused by pneumocystis jirovecii. (2: Risk Factors) Prototypic opportunistic pathogen, initially implicated in nursery outbreaks among malnourished infants. Prior to AIDS, encountered in US in congenitally immunodeficient or iatrogenically immunosuppressed transplant & cancer pts. Most AIDS pts w/PCP have CD4 counts <200. Pts w/higher counts (>350) and symptoms like oral thrush, fever, & wt loss are at high risk as well. Reinfection rather than reactivation accounts for some cases but secondary cases/outbreaks have not been well-documented. (3: Pathogenesis) Proliferation in alveoli leading to exudative response produces typical disease. Hematogenous dissemination occurs in some cases. Extrapulmonary involvement at numerous sites has been encountered.
(MIDII #244)

HIV and AIDS => Clinical Features: Spotlight on Pneumocystis Pneumonia

(1)Clinical Features
(1: Clinical Features) Fever & dry cough w/slowly progressive dyspnea (over 4 wks) are common. Chest X-ray may show diffuse interstial infiltrate or various localized abnormalities. Severe disease is defined by an A-a gradient >35 or pO2 <70. (2: Dx) No culture, antigen detection, or serologic diagnostic procedure is available. Dx rests on histological ID of cysts or trophozoites. Lung biopsy is definitive. Alveolar contents obtained by bronchoscopic lavage has excellent yield. (3: Rx) Trimethoprim-sufamethoxazole or pentamidine isethionate have comparable efficacy. High rate of drug intolerance so they've looked for other agents. Atovaquone is approved only for mild-moderate disease w/limited bioavailability & efficacy. Dapsone w/trimethoprim, or clindamycin w/primaquine are other alternatives. Pts progressing to respiratory failure have a high mortality rate. No salvage regimen has been found. Early use of systemic corticosteroids in pts w/severe disease can lower mortality by 50%. (4: Prevention) TMP-SMZ orally is the most effective agent (failure rate <5%). Dapsone or aerosolized pentamidine (given by inhalation monthly) are good but far less effective alternatives.
(MIDII #245)

HIV and AIDS => Clinical Features: Spotlight on TOXOPLASMOSIS

(2)Risk Factors
(4)Clinical Manifestations
(1)Toxoplasma gondii is a protooon parasite of members of the cat family. Affects other animals (incl humans) who ingest fecally excreted oocysts which survive in the environment or who ingest organisms encysted in skeletal muscle of domestic animals (undercooked meat). (2: Risk Factors) Prior infection as indicated by serum Abs is a reliable way to establish risk. Some cases have been reported in sero-negatives. CD4 counts <100 when toxoplasmosis develops. IgG Abs are reliable indicator of prior infection and population studies show dramatically different rates between populations. (3: Pathogenesis) Reactivation of a dormant cyst. Most disease is recognized in the brain, although cysts are widely distributed throughout skeletal and smooth muscle. (4: Clinical) Cases present as focal CNS events consistent with an expanding mass lesion. Seizures, motor defects or a “stroke” are most common.
(MIDII #246)

HIV and AIDS => Clinical Features: Spotlight on TOXOPLASMOSIS

(1: Diagnosis) => Typical clinical presentation plus focal lesions on head CT or MRI in an HIV positive pt w/T gondii Abs is sufficient to begin therapy. Characteristically, symptomatic & radiologic improvement is evident after 1-2 wks treatment. Diagnostic brain biopsy needed if no response. (2: Treatment) => A combo of pyrimethamine and sulfadiazine is standard and reliably effective but not tolerated for long-term use. Clindamycin can substitute for sulfa drugs. Long-term suppression with one of these combos is needed. (3: Prevention) => Trimethoprim-sulfamethoxazole lowers risk of toxoplasmosis compared to dapsone or aerosolized pentamidine. Individuals w/ T gondii Abs and CD4 counts less than 100 should have pyrimethamine added to dapsone or pentamidine PCP prophylaxis if they are unable to tolerate TMP-SMZ.
(MIDII #247)

HIV and AIDS => Clinical Features: Spotlight on TB

(1)Risk Factors
(3)Clinical Manifestations
(1: Risk Factors) Globally, TB is greatest cause of HIV-associated mortality. Past infection or recent infection are impt contributors to high rate of TB in HIV pts. (2: Pathogenesis) HIV is most powerful accelerant to promote activation of latent TB infection. Reinfection w/new strains occurs in AIDS pts; also increased susceptibility to primary infection. Lack effective cell-mediated immunity so pts w/advanced HIV disease experience reactivation of dormant TB infection at a high rate, but don't develop immune responses producing characteristic lung cavities & productive sputum (that you'd see in non-HIV pts). PPD rxn may be absent making Dx difficult. (3: Clinical) TB can occur in HIV pts at any CD4 count but frequency/severity rise & manifestations change as CD4 count drops. Pulmonary disease most common. Typical pattern of apical cavity on x-ray, productive cough, AFB (+) sputum smear is less common at low CD4 counts. Hilar adenopathy & lower lobe infiltrates in these pts may reflect atypical reactivation, primary infection, or reinfection. Proportion of TB cases w/disseminated or extrapulmonary disease is ↑ in advanced HIV infection.
(MIDII #248)

HIV and AIDS => Clinical Features: Spotlight on TB

(1: Dx) TB is defined by reactive PPD (>/= 5 mm) in an HIV infected pt. Many pts w/advanced HIV are anergic. Typical cases of active pulmonary disease are easy to detect, but many outbreaks start when atypical presentations are missed. Sputum AFB smears are only (+) in 50% of cases. An aggressive, invasive approach may be needed to detect extrapulmonary sites. Culture takes several wks. Rapid tests (PCR) are helpful if (+) but false (-) results may occur. (2: Rx) Standard therapy works well for sensitive isolates. 6-9 months of Rx is recommended. Drug resistant TB epidemics have been centered in AIDS treatment facilities; characterized by very high early mortality rates. Therapy for resistant TB is best guided by sensitivity test results. (3: Prevention) Strict adherence to isolation protocols is vital to avoid nosocomial infection. INH prophylaxis (for 9 rather than 6 months) is effective in PPD+/HIV+ pts.

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