| Resumo: |
In 1990 there were 57 separate outbreaks of viral gastro-enteritis over 18 days implicating raw oysters from a large oyster producing area. Of 1,750 people involved in 11 outbreaks, 550 ate oysters and of these 446 (81 %) were ill (mean incubation period 36 hours) with symptoms persisting for several days. Norwalk virus was detected in patients' stools and there was serological evidence of Norwalk viral infection in blood specimens. Remaining opened oysters (11 samples) from implicated batches were examined for standard plate count, faecal coliforms and Escherichia coli with only 1 sample complying with micro biological standards. Vibrio vulnificus and Vibrio parahaemolyticus were detected at low levels while Salmonella, Campylobacter jejuni, and Staphylococcus aureus were not able to be isolated using electron microscopy. Some oysters could not be traced back to their source due to inadequate records and mixing of oysters from different estuaries at processing level. Large volumes of untreated sewage and stormwater run-off overflowed and polluted harvesting areas following heavy rainfall. E. coli (counts up to 60/g) was detected in oysters and harvesting and purification were voluntarily suspended, preventing any likelihood of food poisoning outbreaks. Despite abatement of the sewage overflow, extending the suspension period by 3 days to allow a period of natural cleansing, testing harvesting areas, purifying oysters and attempting to monitor purified oysters for E. coli prior to sale, Norwalk virus persisted in oysters causing the outbreaks. Besides improvements to the sewerage system, changes have been made to suspension periods, testing protocols and supervision by regulatory agencies. Using purification on its own as a pollution curative for oysters infected by Norwalk virus presents problems in New South Wales at least.
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