AN OUTBREAK OF NORWALK VIRUS GASTROENTERITIS ASSOCIATED WITH EATING RAW OYSTERS - IMPLICATIONS FOR MAINTAINING SAFE OYSTER BEDS

Objective.-To determine the characteristics and the cause of an outbre ak of gastroenteritis associated with eating raw oysters. Design.-Surv ey of groups of persons reporting illness to the health department aft er eating oysters; survey of convenience sample of oyster harvesters; and tracing of implicated oysters. Setting.-General community. Main Ou tcome Measures.-Relative risk for illness after oyster consumption, so urce bed of contaminated oysters, presence of antibodies to Norwalk vi rus in serum, presence of a Norwalk virus in stool by direct electron microscopy and reverse transcription-polymerase chain reaction (RT-PCR ), and DNA sequences of RT-PCR products. Results.-Seventy (83%) of 84 persons who ate raw oysters became ill vs three (7%) of 43 people who did not eat raw oysters (relative risk, 11.9; 95% confidence interval, 4.0 to 34.2). Eleven (79%) of 14 serum pairs had at least a fourfold increase in antibody to Norwalk virus. All 12 stool samples tested wer e positive by electron microscopy and/or RT-PCR for Norwalk virus. The RT-PCR products from all seven stool samples tested had identical DNA sequences. Implicated oysters were harvested November 9 through 13, 1 993, from a remote oyster bed. Crews from 22 (85%) of 26 oyster harves ting boats working in this area reported routine overboard disposal of sewage. One harvester with a high level of antibodies to Norwalk viru s reported having gastroenteritis November 7 through 10 and overboard disposal of feces into the oyster bed. Conclusions.-This outbreak was caused by contamination of oysters in the oyster bed, probably by stoo l from one or more ill harvesters. Education of oyster harvesters and enforcement of regulations governing waste disposal by oyster harvesti ng boats might prevent similar outbreaks.