Pathogen. Caliciviruses are 30-35 nm, possess only one capsid protein and a polyadenylated, 7500-nucleotide RNA with a VPg at the 5' end. The surface of the viruses has a characteristic structure with small, regular, calyxlike concavities that give the capsid the form of a Star of David.
Caliciviruses are classified based on genomic similarities as either human caliciviruses (HuCV) or "small, round-structured viruses," SRSV. This designation stems from their initial identification under the electron microscope as "small, round, virus particles." The SRSV are grouped in two subtypes, I and II. Type I includes the Norwalk virus and a number of similar viruses named for their geographic venues, some with antigenicity differing from the Norwalk type.
Clinical picture. Caliciviruses cause enteritis. Together with rotaviruses (p. 456) and adenoviruses (p. 416), they are the most frequent viral enteritis pathogens in children, often causing minor epidemics during the winter months ("winter vomiting disease").
Diagnosis. Detection by means of electron microscopy or antigen assay in stool.
Epidemiology. Two-thirds of the adult population in the temperate zone carry antibodies to the Norwalk virus. SRSVare regularly implicated in minor epidemics and family outbreaks. The transmission route of the Norwalk virus has been described: in addition to the fecal-oral route, water and uncooked foods are involved.
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