These viruses have a diameter of 28-30 nm and possess a positive-strand RNA genome coding for four structural polypeptides. A surface structure with a five- or six-pointed star-like appearance (hence
astrovirus) can be seen by electron microscopy on 5— 10% of particles (Fig. 5). These viruses were first described in association with an outbreak of mild diarrhea in newborn infants. Strains have been identified in many animals including calf, lamb, pig, cat, dog, duck and turkey. Experimental infections are usually asymptomatic or produce only mild symptoms, although infected ducklings can develop fatal hepatitis. Infections appear to be host-specific. Seven human serotypes that are serologically distinct from animal strains have been described. Serotype 1 has been responsible for >50% of all human astrovirus infections described to date.
Human astrovirus infections occur worldwide, with peak incidence in winter/spring in temperate climates. Illness is usually mild, with an incubation period of 3-4 days, and is accompanied by seroconversion. Symptomatic astrovirus infection is mainly restricted to young children and can account for severe diarrhea in <5% of hospitalized children. Outbreaks of astrovirus gastroenteritis have been observed in nursing homes for the elderly (Marin county agent), among military recruits, and in immunocompromised patients. An enzyme immunoassay incorporating monoclonal antibodies and an RNA probe hybridization assay have been developed recently to facilitate diagnosis. Antibody prevalence surveys show that 65% of 3-4-year-old children and 87% of 5—10-year-old children possess serum antibody. The virus can be propagated in human embryo kidney cells in the presence of trypsin, and in human colonic carcinoma cell lines (CaC02, HT-29).
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