Survey of foodborne pathogenic Vibrio species in commercial Tasmanian bivalve shellfish and predictors of risk at harvest

There has been an increase in foodborne vibriosis outbreaks globally, with Vibrio parahaemolyticus emerging as a foodborne issue in temperate commercial shellfish growing regions, including southern Australia. The food safety concerns associated with these microorganisms have led to the need for specific guidance on potential risk management strategies for their control. This is the first Australian multi-seasonal survey of V. parahaemolyticus and Vibrio vulnificus in commercial shellfish growing areas aimed at understanding their occurrence and regional environmental drivers of risk at harvest. Eleven commercial Tasmanian shellfish (oyster and mussel) growing areas were surveyed for the prevalence and levels of V. parahaemolyticus, including presence of pathogenicity associated tdh and trh genes, and V. vulnificus at harvest between 2020 and 2022. Vibrio parahaemolyticus was detected in all surveyed growing areas during the three summer/autumn sampling periods, with a prevalence of 8–100 %. Prevalence was generally higher in the north (north-west, Moulting Bay and upper east coast) as compared with the mid-east coast and south-east and Bruny regions. Vibrio parahaemolyticus was only detected in five of the eleven growing areas during the two surveyed winter/spring seasons: Duck Bay, Moulting Bay, Great Swanport, Little Swanport and Boomer Bay East. Where it was detected, the prevalence was much lower in the winter/spring seasons (17–33 %). Levels of V. parahaemolyticus detected during the survey were generally low (<10 MPN/g) for most growing areas. Some higher levels (100–1100 MPN/g) were observed in Duck Bay, Moulting Bay, Great Swanport and Little Swanport. Only one sample from Great Swanport had a level of over 1000 MPN/g (1,100 MPN/g). The higher levels were only observed in the summer/autumn sampling periods. Vibrio parahaemolyticus tdh, trh or tdh/trh gene detections only occurred in the summer/autumn months with a prevalence of 0–21 %, 0–18 % or 0–7 %, respectively, depending on the growing area surveyed. Despite low levels of V. parahaemolyticus being detected in southern commercial growing areas, five sporadic cases of vibriosis associated with oysters from southern Tasmania were reported during the survey period, predominantly from oysters harvested recreationally. Levels of V. vulnificus detected were generally very low in most Tasmanian growing areas (<1 MPN/g). However, levels of 35–460 MPN/g were detected in shellfish at harvest in one area (Great Swanport). Predictive models for V. parahaemolyticus at harvest were developed from survey data which were area specific. Water temperature was the sole or primary driver in most areas. Predictive models for V. vulnificus at harvest were developed for Great Swanport and were driven by river flow and rainfall.