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 degree celsius 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 (root growth rate (MPN/h) = b x (Temperature (degree celsius) -T0) to all the growth rates gave a good fit (R-2 = 0.84, RMSE = 0.0178) with b = 0.0096 and T0 = 8.44 degree celsius. 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 degree celsius 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.

Automated summary

The frequency of gastroenteritis outbreaks caused by Vibrio parahaemolyticus has been increasing worldwide, including in New Zealand. Inadequate post-harvest refrigeration may allow the organism to grow to hazardous concentrations. Experimental and modeling studies show that the growth rates of V. parahaemolyticus are best described by log-linear and square root models, suggesting a slower growth rate in New Zealand oysters but the ability to grow at lower temperatures. The recommended minimum storage temperature of 7 degrees Celsius is adequate according to both models.