Minimising the risk of norovirus contamination in Australian commercial oysters
Creators: name: Brake, F
Publisher: name: University Of Tasmania
Year: 2023

description: Outbreaks of human gastroenteritis caused by norovirus (NoV)-contaminated oysters occur worldwide and have periodically been linked to the consumption of Australian oysters. Contamination with NoV can occur when human excrement (faeces and vomit) containing the virus flows into oyster growing areas. Minimal data about the occurrence of NoV in Australian oysters was available at the commencement of this thesis due to the expense involved in testing oysters for viruses because no Australian commercial laboratories had the capacity to perform the test. To that time, all Australian samples were tested in New Zealand. The aim of this Thesis was to establish an informed strategy for minimising the risk of NoV in commercial Australian oysters. To collect the NoV data, a sensitive and accurate method to detect NoV was adopted in this study and validated for Australian oysters and conditions. Epidemiological evidence suggests that the occurrence of NoV in Australian oysters is low; therefore a sampling regime for the detection of NoV at low occurrence and concentration in oysters was designed. Using this sampling programme, NoV was detected in oysters (8.3%, n = 163) from a harvest area that had been associated with 2 NoV illness outbreaks. The results showed the need for a comprehensive sampling regime to ensure the detection of NoV. To address the lack of systematically collected data on the occurrence of NoV in Australian growing areas and due to the impending imposition of international standards for NoV in Australia, a survey was conducted. This involved the collection of oysters from two geographically distinct oyster growing areas on four occasions from each of the three main oyster producing States in Australia and testing samples for NoV and E. coli (n = 120). The growing areas selected were considered by Australian shellfish authorities to be the most compromised in those States with respect to the potential for human faecal contamination, as identified by shoreline surveys. NoV GII was detected in two samples (1.7%) but NoV GI was not detected. Some of the samples were found to have more than the guidance concentration of 230 E. coli per 100 g of shellfish flesh but these samples did not contain detectable concentrations of NoV. These results reinforce epidemiological data suggesting that NoV contamination of commercially produced Australian oysters is rare. There is minimal data about NoV dispersal in waterways following sewage overflows. The persistence of NoV in Sydney Rock oysters (SRO) following sewage overflows was also unknown. These knowledge gaps were addressed by strategically placing SRO in an estuary downstream from a sewage pump station known to overflow periodically and initiating sampling after an overflow event. After the event NoV GII was detected up to 5.29 km downstream and persisted in SRO closest to the pump station outfall for 42 days. NoV GII concentrations decreased significantly over time; a reduction rate of 8.5% per day was observed in oysters located at two sites near the outfall (p&lt;0.001). Five days after the overflow, NoV GII concentrations were found to decrease significantly as a function of distance at a rate of 5.8% per km (p&lt;0.001). The decline in E. coli concentration with distance was 20.1% per km (p&lt;0.001). NoV GI and Hepatitis A virus were not detected. A comparison of NoV GII reduction rates from oysters over time, derived from this study and other, published, research collectively suggest that GII reduction rates from oysters may be broadly similar, regardless of environmental conditions, oyster species and genotype. In the final phase of the project, an investigation of risk management options for Australian commercial oyster harvest areas to protect oyster consumers from NoV was conducted. The result, a culmination of data and research, is a combination of strategies recommended for minimising the risk of NoV prevalence in Australian oysters. It was found that the risk is real, as demonstrated by the occurrence of 1-2 outbreaks of NoV illness associated with oysters annually. This was supported by the findings of a NoV contamination rate of 1.7% of oysters from Australian growing areas. Recommendations were made for risk management strategies to prevent contamination of oyster growing areas and to enable rapid detection and notification of contamination events when they occur. It was found that communication between local councils, water utility operators and shellfish authorities regarding reporting sewage spills and the condition of potential sources of sewage spills (i.e. on-site sewage management systems (OSMS)) was lacking. A theoretical NoV contamination event in oyster growing areas was considered, showing that it was possible for a small human faecal mishap to cause illness in oyster consumers. Recommendations for a riskbased virus monitoring programme are proposed, including: regulation of OSMS by local councils; workshops on water quality to stimulate increased communication between Environmental Health Officers and shellfish authorities; upgrading sewage treatment plants where the effluent flows into oyster growing areas; mandatory dye studies of STP effluent flows and potential sources of sewage overflows for each growing area to enable more effective closure times and delineation of the impacted area closed for harvesting; ASQAP to include guidance to put local management committees in place for each growing area, ASQAP to include guidance for portable toilets to be on oyster harvesting boats, and the development of an alternative indicator for NoV other than faecal coliforms i.e. bacteriophage testing. The results of this Thesis will improve NoV risk management strategies used by shellfish authorities to protect the Australian oyster consumer and help to ensure that future control measures are commensurate with the risk associated with the growing area rather than requiring mandatory end product testing regimes for all areas.