Clinical Manifestations And Pathogenesis

Clinical features of human calicivirus infections are variable, ranging from severe gastroenteritis to subclinical infections. Characteristic symptoms of NL illness include vomiting (69%), nonbloody diarrhea (66%), nausea (79%), abdominal cramps (30%), fever (37%), chills (32%), myalgia (26%), and sore throat (18%) (11). Norwalk virus has an incubation period of approximately 48 hours, with illness typically lasting 24-48 hours after onset (31). Experimentally-induced Norwalk illness causes blunting (shortening) and broadening of the villi, crypt hypertrophy, increased cellularity of the lamina propria, and increased mitosis in the crypts as early as 12 hours after virus ingestion and persisting for at least 5-6 days, usually 2-4 days after clearance of clinical symptoms (32,33). Changes in the absorptive cells lining the villi include decreased height, extensive vacuolization within the cytoplasm, and disordered nuclear polarity of the epithelial cells. Mononuclear cells increase in number within intracellular spaces of the epithelial cells. The lamina propria undergoes increased cellularity and infiltration by polymorphonuclear leukocytes.

Inflammation of the mucosa of the small bowel reduces the absorptive capacity of the villi contributing to diarrhea. Both crypt hypertrophy and epithelial cell proliferation may represent the body's response for replacing virus-damaged absorptive cells (33). The colonic mucosa and stomach remain relatively unaffected during Norwalk virus infection; however, the involvement of the distal portion of the small intestine is uncertain. Although the propagation of Norwalk virus reportedly occurs in the jejunum, the presence of virions within these cells was not observed (33,34).

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