Modelling the impact of rainfall events on increasing Vibrio vulnificus concentrations in Pacific oysters (Crassostrea gigas)

Vibrio vulnificus occurs naturally in seawater and causes debilitating, often fatal illnesses, particularly in people with underlying health issues such as liver disease. The illness can occur when raw molluscan shellfish that have bio-accumulated the organism are consumed. New Zealand seafood is not known to have caused any illnesses, although there have been wound infections. This study sought to better understand the effect of environmental conditions on concentrations of V. vulnificus in Pacific oysters in three harbours over the summer months of 2016-19. Fortnightly sampling at two harbours only once gave a most probable number (MPN) of >10 per g of oyster meat, while the third harbour regularly produced much higher counts (up to 220,000 per g). From 2017-19, weekly samples from three sites in this harbour (four in 2018) were tested. Eleven peaks in concentration were observed, all when seawater temperatures exceeded 20{degrees}C and after heavy rainfall had reduced the seawater salinity, usually to <25{per thousand} from the average of 32{per thousand}. A fitted structural equation model with temperature and salinity terms accounted for 60% of the variance in concentrations and rates of decline after concentration peaks averaged 1.05 log10 MPN per week. The rate of decline was highly variable so microbiological testing would be required to confirm this rate for use in food safety management. However, the results of the study will enable better risk management. ImportanceVibrio vulnificus is a deadly bacterium that naturally occurs in some seawater and can be concentrated in shellfish such as oysters. Consumers with underlying health issues who eat contaminated shellfish may suffer serious illnesses. New Zealand shellfish have never been known to cause such illnesses although the species is present in our seawater. The strains present may not be able to cause foodborne illness. We found that V. vulnificus could be present in high concentrations in farmed Pacific oysters in one of three studied harbours. This happened when seawater temperatures were warm and heavy rainfall reduced salinity. We developed a model based on seawater temperature and salinity that would be able to predict concentrations in shellfish, at least in this harbour. This will help industry and regulators manage the food safety risk should this organism become a public health issue. TweetWe modelled concentrations of Vibrio vulnificus in Pacific oysters after flood events and showed that seawater salinity and temperatures in previous 168 h affected them.