bThe 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.
Because of the halophilic nature and marine habitat of Vibrio parahaemolyticus, raw seafood can naturally harbor this microorganism and is the main food source responsible for the gastroenteritis the microorganism causes (1). A recent report released by the U.S. Centers for Disease Control and Prevention estimated that the average annual incidence of Vibrio species infections in the United States increased by 43% from the 2006 to 2008 period to 2012 (2). The incidence rate of V. parahaemolyticus infection in New Zealand (NZ) is calculated to be 1.6/100,000, increasing to 15.3/100,000 in the Pacific Islander population, with most cases linked to imported seafood (3).An international risk assessment of V. parahaemolyticus in raw seafood highlighted the importance of exposure to raw oysters based on the incidence of V. parahaemolyticus harboring the thermostable direct hemolysin (tdh) and tdh-related hemolysin (trh) genes at harvest (4).NZ has two main islands (Fig. 1) that extend from latitudes 34°t o 47°south. This means that there are diverse habitats and significant differences in water temperature along the length of the country, with cooler waters in the south. The first reported NZ isolation of V. parahaemolyticus was from Bay of Islands' shellfish (5). Subsequently, Fletcher (6) conducted a 3-year surve...