Human, small round structured viruses, caliciviruses and astroviruses.

Epidemiological studies on SRSVs, human calicivirus and astroviruses have been limited by the problems of establishing them in cell culture and the inability to transmit them to animals or to use strains from animals as a source of antigen for diagnostic tests. The use of EM and the subsequent development of RIAs and EIAs in a few research centres has shown that they are a cause of outbreaks and sporadic cases of diarrhoea and vomiting. SRSVs have increasingly been recognized as a major cause of outbreaks of gastroenteritis in the community and in hospital wards. The symptoms of illness are generally mild and of short duration and patients seldom require medical attention. However, because of the high attack rates and large numbers of persons of all age groups involved, there is often considerable economic loss and disruption of services. Evidence is accumulating that polluted water, molluscan shellfish, and contaminated cold foods are major sources of infection. Recently a SRSV has been shown to be the cause of epidemics and sporadic cases of waterborne enterically transmitted non-A, non-B hepatitis (hepatitis E virus) which have occurred in the USSR, India, Mexico and Africa. Astroviruses and human caliciviruses are occasional causes of outbreaks of vomiting and diarrhoea in infants and the elderly which can necessitate the closure of hospital wards and cause considerable disruption. Symptoms are generally mild and of short duration and therefore the majority of cases are unlikely to be investigated by laboratories. Diagnosis of infections is at present limited to the few laboratories that have developed their own assays or have access to electronmicroscopy facilities.