Etiology
A large number of viruses are found in the human intestine. These include viruses that infect bacteria (bacteriophages), viruses that use the intestines as a portal of entry (e.g., poliovirus, enterovirus, hepatovirus, and some adenoviruses), and viruses that cause gastrointestinal infection. The latter group of viruses causes damage to the gastrointestinal tract, resulting in vomiting, diarrhea, or both. Rotaviruses, caliciviruses, astroviruses, coronaviruses, noroviruses, and enteric adenoviruses are the primary pathogens causing gastroenteritis in humans. HIV, cytomegalovirus, and herpes simplex virus mainly cause opportunistic infections in immunocompromised hosts.[11][12]
Pathophysiology
Rotavirus spreads from person to person, primarily by the fecal-oral route.[13] After ingestion (an infectious dose may be as low as 100 virus particles), rotavirus infects the mature enterocytes. The rate of death of rotavirus-infected mature villous enterocytes exceeds the rate of production of new enterocytes in crypts. This imbalance causes structural damage to the small bowel mucosa, including villous shortening, crypt hyperplasia, and a mononuclear inflammatory infiltrate in the lamina propria. Immature enterocytes are secretory in nature, and there is loss of absorptive surface. Rotavirus damages the villous brush border, causing osmotic diarrhea, and also produces an enterotoxin (NSP4) that causes calcium-mediated secretory diarrhea.[14] The loss of brush-border enzymes can cause osmotic diarrhea, and the permeable mucosa can cause food intolerance, which may exacerbate and prolong the diarrhea.
In general, an infection with rotavirus confers high protection against symptomatic reinfection, but in developing countries where exposure is high, the protection is less. The major correlate of protective immunity seems to be antirotavirus immunoglobulin A coproantibodies.
Norovirus transmission occurs by a number of routes: person-to-person (fecal-oral route, by ingestion of aerosolized vomitus, or by indirect exposure via fomites or contaminated environmental surfaces), foodborne, and waterborne.[15] Norovirus incubation period is typically 12-48 hours.[15]
Noroviruses particularly cause severe disease in older people and people with chronic diseases. Noroviruses also cause persistent infection in immunosuppressed patients. Host susceptibility factors for noroviruses have been identified and include histo-blood group antigens that likely function as initial receptors for some noroviruses. Noroviruses are resistant to commonly used disinfectants, and infected people can shed the virus for prolonged periods even after the illness stops. Genetic variability of noroviruses also counteracts immunity.
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