Long-term study of Vibrio parahaemolyticus prevalence and distribution in New Zealand shellfish

The food-borne pathogen Vibrio parahaemolyticus has been reported as being present in New Zealand (NZ) seawaters, but there have been no reported outbreaks of food-borne infection from commercially grown NZ seafood. Our study determined the current incidence of V. parahaemolyticus in NZ oysters and Greenshell mussels and the prevalence of V. parahaemolyticus tdh and trh strains. Pacific (235) and dredge (21) oyster samples and mussel samples (55) were obtained from commercial shellfish-growing areas between December 2009 and June 2012. Total V. parahaemolyticus numbers and the presence of pathogenic genes tdh and trh were determined using the FDA most-probable-number (MPN) method and confirmed using PCR analysis. In samples from the North Island of NZ, V. parahaemolyticus was detected in 81% of Pacific oysters and 34% of mussel samples, while the numbers of V. parahaemolyticus tdh and trh strains were low, with just 3/215 Pacific oyster samples carrying the tdh gene. V. parahaemolyticus organisms carrying tdh and trh were not detected in South Island samples, and V. parahaemolyticus was detected in just 1/21 dredge oyster and 2/16 mussel samples. Numbers of V. parahaemolyticus organisms increased when seawater temperatures were high, the season when most commercial shellfish-growing areas are not harvested. The numbers of V. parahaemolyticus organisms in samples exceeded 1,000 MPN/g only when the seawater temperatures exceeded 19°C, so this environmental parameter could be used as a trigger warning of potential hazard. There is some evidence that the total V. parahaemolyticus numbers increased compared with those reported from a previous 1981 to 1984 study, but the analytical methods differed significantly.