The prototype “Norwalk agent” was
discovered in an outbreak of diarrhea and vomiting among the
elementary school students in Norwalk, Ohio. The virus has icosahedral
symmetry, contain single-stranded RNA, and thought to be a Calicivirus.
Norwalk-like viruses have been
associated with gastroenteritis, primarily in adults and older
children.
Diarrhea and vomiting are of short
duration, and dehydration is rare (as compared with
rotavirus diarrhea
).
The outbreaks have occurred at all
times of the year and have been associated with contaminated water and
foods (eg. oysters) infected food-handlers, and person-to-person
spread.
Outbreaks have occurred in schools and
college campuses, among families, in nursing homes, in communities
with contaminated drinking or swimming water, and on cruise ships.
Rotavirus and Norwalk-like viruses
multiply in epithelial cells lining villi of the small intestine,
causing nonspecific inflammatory changes in the mucosa, including
blunting of the villi, cytoplasmic vacuolization of epithelial cells,
and a patchy, mononuclear, infiltrate of the lamina propria.
The lysis of epithelial cells leaves
the villi denuded and allows leakage of fluid and electrolytes into
the lumen of the small intestine.
Later there is contraction of denuded
villi and transient replacement of mature, tall columnar epithelial
cells by immature, cuboidal cells that migrate up from the crypts.
Severe watery diarrhea and dehydration
result from the decreased absorptive surface of the villi, decrease in
the enzymatic activity of the brush border (including disaccharidases),
and secretions by the immature crypt cells.
Since the diet of young children is
often based on milk, lactose malabsorption is of special concern.
The diarrhea lasts only 1 to 3 days in
most patients, and the intestinal mucosa is histologically normal
within 3 to 4 weeks after onset of diarrhea.
Histologic changes, however, persist in
some children, especially among those less than 6 months old.
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