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Viral Gastroenteritis

Symptomatic human infection is caused by several distinct groups of viruses ( Table 17-7 ). Rotavirus accounts for an estimated 140 million cases and 1 million deaths worldwide per year.

The target population is children age 6 to 24 months, but young infants and debilitated adults are susceptible to symptomatic infection. This virus accounts for 25% to 65% of severe

diarrhea in infants and young children.[50] Rotavirus is an encapsulated virus with a segmented double-stranded RNA genome. Rotavirus selectively infects and destroys mature enterocytes

in the small intestine, without infecting crypt cells. The surface epithelium of the villus is repopulated by immature secretory cells. With the loss of absorptive function and excess of

secretory cells, there is net secretion of water and electrolytes, compounded by an osmotic diarrhea from incompletely absorbed nutrients. The minimal infective inoculum is approximately

10 viral particles, whereas an individual with rotavirus gastroenteritis typically sheds up to 1012 particles/mL stool. Thus, outbreaks among pediatric populations in hospitals and day-care

centers are very common. The clinical syndrome has an incubation period of approximately 2 days, which is followed by vomiting and watery diarrhea for several days. Viral infection can

induce protective immunity, but the protection for reinfection is often short-lived. Antirotavirus

TABLE 17-7-- Common Gastrointestinal Viruses

Virus Genome Size (nm)

% of U.S. Childhood

Viral Enterocolitis Host Age Mode of Transmission

Prodrome/Duration of

Illness

Rotavirus (Group A) dsRNA 70 60 6–24 months Person-to-person, food, water 2 days/3–5 days

Caliciviruses ssRNA 35–40 20 Child or adult Person-to-person, water, cold

foods, raw shellfish

1–3 days/4 days

••Norwalk-like viruses

••Sapporo-like viruses

Enteric adenoviruses dsDNA 80 8 Child <2 years Person-to-person 3–10 days/7+ days

Astroviruses ssRNA 28 4 Child Person-to-person, water, raw

shellfish

24–36 hours/1–4 days

Data from Goodgame RW: Viral causes of diarrhea. Gastroenterol Clin North Am 30:779,2001.

ds, double-stranded; ss, single stranded.

antibodies are present in mother's milk, so rotavirus infection is most frequent at the time of weaning.

Among the numerous types of adenovirus, the subtypes (enteric serotypes) Ad40, Ad41, and Ad31 appear to be responsible for enteric infections and are a common cause of diarrhea

among infants. They can be distinguished from adenoviruses that cause respiratory disease by their failure to grow easily in culture. Adenoviruses cause a moderate gastroenteritis with

diarrhea and vomiting, lasting for a week to 10 days after an incubation period of approximately 1 week. In the small intestine, adenoviral infection causes atrophy of the villi and

compensatory hyperplasia of the crypts similar to rotavirus, resulting in malabsorption and fluid loss. The virus can also cause colitis. Immunohistochemical stain of nuclear inclusions



facilitates the diagnosis.

Caliciviruses include two major groups: the classic Caliciviruses (Sapporo-like viruses) and the Norwalk-like viruses (small round structured viruses). Sapporo-like viral infection is rare,

while Norwalk virus, the prototype of Norwalk-like viruses, is responsible for the majority of cases of nonbacterial food-borne epidemic gastroenteritis in all age groups. Norwalk-like

viruses are small icosahedral viruses containing a single-stranded RNA genome. They cause epidemic gastroenteritis with diarrhea, nausea, and vomiting among children. Outbreaks occur

following exposure of multiple individuals to a common source. The clinical syndrome has an incubation period of 1 to 2 days, which is followed by 12 to 60 hours of nausea, vomiting,

watery diarrhea, and abdominal pain.

Astrovirus is named after its starlike appearance. It primarily affects children, (it accounts for 4% of acute gastroenteritis in young children), and has a worldwide distribution. Those

infected develop anorexia, headache, and fever. Other viruses such as enterotrophic coronaviruses and toroviruses are occasionally implicated in human diarrheal disease.

Despite the high incidence of viral gastroenteritis, insights into disease pathogenesis have been slow in coming.

Morphology.

Although the enteric viruses are genetically and morphologically different from each other, the lesions they cause in the intestinal tract are similar. The small intestinal mucosa usually

exhibits modestly shortened villi and infiltration of the lamina propria by lymphocytes. Vacuolization and loss of the microvillus brush border in surface epithelial cells may be evident, and

the crypts become hypertrophied. Viral particles may be visualized by electron microscopy within surface epithelial cells. In infants, rotavirus can produce a flat mucosa resembling celiac

sprue (discussed later).


Date: 2016-04-22; view: 196


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