Vibrio parahaemolyticus: Predicting effects of storage temperature on growth in Crassostrea gigas harvested in New Zealand

The frequency of gastroenteritis outbreaks caused by Vibrio parahaemolyticus has been increasing in many parts of the world including New Zealand. Some outbreaks in New Zealand have occurred at times when V. parahaemolyticus concentrations in shellfish in growing waters are low. V. parahaemolyticus is mesophilic, so is not expected to grow under refrigeration. Therefore, one possible explanation for the outbreaks is that the organism was able to grow to hazardous concentrations (e. g. >10,000/g) because of inadequate post-harvest refrigeration. To model the response of growth rates to temperature, we collected Pacific oysters (Crassostrea gigas) containing natural populations of V. parahaemolyticus over three consecutive summers, stored them for various times at 5, 10, 15, 20, 25 and 30 °C and determined V. parahaemolyticus concentrations. A simple log-linear fit was best to determine the growth rates at each temperature for each season. Applying a square root model (√growth rate (MPN/h) = b x (Temperature (°C) - T0) to all the growth rates gave a good fit (R2 = 0.84, RMSE = 0.0178) with b = 0.0096 and T0 = 8.44 °C. Fitted growth rates were considerably lower than previously reported, but the hypothetical minimal growth temperature was also lower. Thus, concentrations of V. parahaemolyticus are likely to increase more slowly in New Zealand oysters than was previously thought, but the organism may also be able to grow at lower temperatures. Both models suggest that the recommended minimum storage temperature of 7 °C in current New Zealand and US regulatory schemes is adequate. This information will be useful for evaluating food safety risks from storing Pacific oysters at different temperatures for different times. It will also assist in formulating time-temperature recommendations for safe handling of Pacific oysters. •We modelled temperature effects on Vibrio parahaemolyticus growth in Pacific oysters.•Square root modelling gave lower growth rates than previously reported.•Oyster farmers can now select safe post-harvest storage temperatures and times.•Public health risk management will be improved, reducing vibriosis illness rates.