Vibrios are naturally occurring bacteria that are ubiquitous in fresh, estuarine and marine environments. Many Vibrio species are non-pathogenic, but some can cause disease in animals, and others are pathogenic to humans. People can contract vibriosis by consuming raw, undercooked or cross-contaminated seafood (predominantly oysters, crabs and shrimp) or exposing a wound to seawater. Bivalve molluscs, such as oysters, are a known vector for pathogenic bacteria as they are often consumed raw, and their filter feeding action concentrates bacteria within their tissues. Historically Vibrio parahaemolyticus has been rarely implicated in illnesses attributed to the consumption of Australian seafood. However, several recent outbreaks of gastroenteritis caused by V. parahaemolyticus in oysters have occurred in Australia.
An improved understanding of the environmental determinants was warranted to assist in future risk management considerations and food safety requirements. This report describes the results of the study undertaken which recorded the available and relevant environmental information and considered known risk factors relevant to V. parahaemolyticus that could be used for future investigations and to help underpin risk management considerations. The identification of data gaps and tools that could be used to identify and assess potential Vibrio risk factors may help guide where additional effort is required to assist future understanding of this complex food safety issue.
Vibrios are part of the normal microbiota of many oysters and are ubiquitous in many other aquatic products. Vibrios multiply in oyster tissues at temperature-dependent rates before, during and after harvest. Across the two outbreaks, three sequence types (ST36, ST50 and ST417) were identified from clinical isolates and only one sequence type (ST417) was isolated from oysters as part of investigations following the second outbreak. The environmental conditions, notably sea surface temperature, oyster basket temperature and salinity, during the onset periods of the two Vibrio outbreaks (February 2021 and September 2021) were conducive to the growth of V. parahaemolyticus. However, there were no evident climatological anomalies in the collated data sets that help to substantiate why these Vibrio outbreaks occurred in South Australia at these times given that there had not been any significant changes in oyster production, harvest and post-harvest practices.
This project has also highlighted several data gaps. Poor traceability through supply chain hampered traceback investigations and the identification of the unique harvest date, harvest location, and subsequent production, harvest and post-harvest conditions was limited. There is no information publicly available on the levels of detection of V. parahaemolyticus in the implicated oysters. The occurrence of these two and similar recent Vibrio outbreaks in Australia demonstrates that vibrios are a risk that requires effective control mechanisms. A range of tools and approaches are available that could be used to identify and assess potential risk factors and improved surveillance. These tools include in-situ data collection, remote sensing of the environment, microbiological sampling and molecular diagnostics.
Recommendations
1. In-situ environmental monitoring is improved through use of loggers in more growing and harvest areas.
2. Further work needs to be undertaken within the supply chain to ensure that legislated responsibilities on labelling, traceability and control of co-mingling are adhered to.
3. Vibrio parahaemolyticus isolates should be collected during vibriosis events (clinical and oyster) and an Australian isolate collection curated and maintained.
4. A review and refresh of growers recall plans is necessary and growers should participate in simulation training of recall events to improve the practices supporting speedy recalls.
5. Open lines of communication between regulators and industry should be maintained to determine what type of data can be shared and when.
6. Authorities should implement timely closure of growing areas following multiple illnesses in line with ASQAP guidelines.
7. Food Safety Management plans should be reviewed and closely adhered to, especially if there are any future outbreaks.
8. Regulators should hold a post event review that includes industry and research representatives to strengthen working relationships and improve joint outcomes.