By no means do all viruses found in feces cause gastroenteritis. Some do, but others cause "silent" infections of the gastrointestinal tract, which is their portal of entry to the body (e.g., reovrruses and many enteroviruses and adenoviruses); some may then move on, usually via the bloodstream, to their target organ(s) in other parts of the body. For this reason, it has not been easy to pin down which viruses actually cause gastroenteritis, especially as enteritis is so very common and not always easy to distinguish from minor changes in bowel habits arising from time to time for dietary, psychologic, or other reasons.
Assiduous searches have revealed a fascinating range of viruses in feces, none of which are human parasites, including bacteriophages parasitizing enteric bacteria and plant or animal viruses from ingested food. There is much current interest in two small viruses with genomes consisting of two or three segments of double-stranded RNA, which because of similarities to the family Birnavindae have been tentatively named picobirnaviruses and picotrirna-viruses, respectively Miscellaneous other "small round viruses" and "parvovirus-like" agents have also been carefully described, as well as parvoviruses causing enteritis in cats and dogs, but a clear etiologic association has yet to be nailed down in humans. The same applies to the enteric coro-naviruses, and perhaps enteric toroviruses, which have been frequently visualized by electron microscopy in feces from patients or even outbreaks of human gastroenteritis, especially in psychogeriatric patients, AIDS patients, and immunocompromised children, but not proved beyond a shadow of a doubt to cause the disease from which they were recovered. It is quite probable that some human enteric viruses are harmless passengers in most people most of the time but are capable rarely of causing diarrhea under certain circumstances in certain (especially immunocompromised) individuals. In Table 36-2 we list only those enteric viruses that unequivocally cause gastroenteritis in humans.
Gastroenteritis vies with upper respiratory infection for the mantle of commonest of all infectious diseases and is the greatest cause of death. It has been estimated that 5-10 million children die each year in the Third World from diarrheal diseases, rotavirus infections in malnourished infants being a major contributor. Rotaviruses infect young children, causing severe diarrhea which may last up to a week and lead to dehydration requiring fluid and electrolyte replacement. Most infections are sporadic, but nosocomial outbreaks occur frequently in hospital nurseries. Nearly all rotavirus infections are caused by group A serotypes and occur mainly in infants under the age of 2 years; group B rotaviruses have been asssociated with some very large waterborne outbreaks in China, affecting adults as well as children; group C rotaviruses cause only occasional zoonotic infections.
Enteric adenoviruses were first visualized in feces by electron microscopy and assigned to the correct family on the basis of their characteristic morphology; because of the copious numbers of virions excreted, they could be demonstrated by direct immunoassay to be distinct from other known members of that family, even though they themselves had never been cultured. Only later
were suitable technicjuesjieyeloged^ the "laMidious" or "enteric"
adenoviruses, types 40 and 41, and later higher-numbered serotypes so far found only in AIDS patients. The sequence of events leading to the discovery of the other groups of viruses causing human enteritis has been essentially the same. These particular serotypes of human adenoviruses (but not the numerous other types that replicate in the respiratory and/or gastrointestinal tract but cause no disease in the gastrointestinal tract) have turned out to be common causes of gastroenteritis, especially in young children, for example, in outbreaks in day-care centers.
The "typical" enteric caliciviruses are also common, especially in young children. In contrast, the Norwalk group of viruses, now classified as members of the family CaHciviridae, tend to infect older children and adults, often in common-source outbreaks. The illness consists of an explosive episode of nausea, vomiting, diarrhea, and abdominal cramps, sometimes accompanied by headache, myalgia, and/or low-grade fever.
Astroviruses display many of the epidemiologic and clinical features of rotaviruses but are not as common and not as virulent. They appear to be endemic worldwide, with occasional epidemics, causing a relatively mild form of enteritis with watery diarrhea, mainly in young children; outbreaks have also occurred among immunosuppressed and institutionalized geriatric patients.
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