Purpose: To undertake a national prevalence survey for norovirus (NoV) and hepatitis A virus (HAV) in market-ready Australian grown oysters.
Methods: Sampling of oysters was done during 2014-15 in two rounds to capture “peak” and “off-peak” times for NoV circulating within the community. All commercial Australian production areas were included in the survey. A sample size of 150 per round was calculated based on the probability of 0.95 of detecting at least one positive if ≥2% of the samples were contaminated. Sample number allocation to production area was determined by its proportional contribution to national oyster production. Samples were only collected from growing areas in the Open Status for harvest, as defined by the Australian Shellfish Quality Assurance Program’s Operations Manual. NoV GI, NoV GII and HAV were determined by quantitative RT-PCR (ISO/TS 15216-1).
Results: A total of 33 oyster production regions in New South Wales, South Australia, Tasmania and Queensland were involved in the survey. One hundred forty-nine and 148 samples were collected during round one and two of sampling, respectively. No NoV or HAV was detected in oysters collected in either sampling round. For each of the two sampling rounds this translated into an estimated prevalence for NoV and HAV in Australian oysters of <2% with a 95% confidence interval.
Significance: Results of this survey show that the Australian oyster industry is producing a safe product with respect to viral contamination. The low estimated prevalence of foodborne viruses in oysters was supported by epidemiological evidence, with no oyster-related foodborne illness reported during the survey period. There is no evidence that end product viral limits are necessary in the Australian context. Any end product viral limits should be applied on a risk assessment basis.