Raw oyster consumption and outbreaks of viral gastroenteritis in New Zealand: evidence for risk to the public's health.

Abstract

To investigate the relationship between consumption of raw Pacific half shell oysters and outbreaks of Norwalk-like virus (NLV) gastroenteritis in Auckland in the last third of 1999. Ten outbreaks were investigated as retrospective cohorts using standardised questionnaires relating to food and drink exposures. Trace back of oysters and site inspections of implicated commercial growing areas were performed. Virological analyses compared oysters linked to outbreaks and faecal samples from cases. Eighty-six cases were identified, of whom 32 (37.2%) were confirmed NLV positive on faecal analysis. The summary risk estimate for illness among oyster consumers for all outbreaks was RR 8.23 (95% CI 4.55-14.90; p< 0.001) and in five of seven outbreaks permitting statistical analysis, the risk for those consuming raw oysters was greater than five-fold that of non-consumers. NLVs were identified in two batches of oysters from different growing areas, implicated in four outbreaks. In both the strain (Genogroup II/3 'Mexico-like virus') from cases matched that in the oysters from the same harvest batch. The epidemiological and virological evidence implicates oysters as the source of a number of outbreaks of NLV gastroenteritis. This is the first time NLVs have been identified in commercially farmed Pacific oysters in New Zealand. Sewage effluent from recreational boats was the likely source of faecal contamination of growing waters in one site. Combined use of virological and epidemiological methods have proved invaluable in investigating NLV outbreaks. Further research is needed into enteric viral contamination of commercial oyster farms.