Objective: To investigate the relationship between consumption of raw Pacif
ic half shell oysters and outbreaks of Norwalk-like virus (NLV) gastroenter
itis in Auckland in the last third of 1999.
Method: Ten outbreaks were investigated as retrospective cohorts using stan
dardised questionnaires relating to food and drink exposures. Trace back of
oysters and site inspections of implicated commercial growing areas were p
erformed. Virological analyses compared oysters linked to outbreaks and fae
cal samples from cases.
Results: Eighty-six cases were identified, of whom 32 (37.2%) were confirme
d NLV positive on faecal analysis. The summary risk estimate for illness am
ong 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 g
rowing areas, implicated in four outbreaks. In both the strain (Genogroup I
I/3 'Mexico-like virus') from cases matched that in the oysters from the sa
me harvest batch.
Conclusion: 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 oyster
s in New Zealand. Sewage effluent from recreational boats was the likely so
urce of faecal contamination of growing waters in one site.
Implications: Combined use of virological and epidemiological methods have
proved invaluable in investigating NLV outbreaks. Further research is neede
d into enteric viral contamination of commercial oyster farms.