HIGH FREQUENCIES OF VIRULENT Vibrio parahaemolyticus IN SOUTH CAROLINA Crassostrea virginica OYSTERS  

Seafood-associated gastroenteritis caused by the emergent, pandemic pathogen Vibrio parahaemolyticus is the number one public health issue for seafood safety in the US.  V. parahaemolyticus infections occur after ingestion of raw or undercooked shellfish, particularly oysters.  Wild Crassostrea virginica (Eastern) oysters were harvested during summer 2014 at two sampling sites, one a site of commercial oyster harvest (Beaufort, SC) and the other a pristine comparison site (North Inlet, SC).  These oysters were screened for virulent V. parahaemolyticus strains.  Oysters from both sites contained high frequencies of virulent V. parahaemolyticus.

Virulent strains of V. parahaemolyticus are differentiated from avirulent strains by the content of two hemolysin genes, tdh (the thermostable direct hemolysin) and trh (the tdh-related hemolysin).  These hemolysin genes cause damage to host cells by acting as porins.  V. parahaemolyticus strains containing tdh and/or trh are frequently isolated from patients suffering from gastroenteritis; while environmental strains of V. parahaemolyticus rarely contain tdh and/or trh (i.e.: 1-2% of environmental strains).  However, in this study, we were able to detect tdh and trh at high frequencies from 379 environmental, oyster-derived V. parahaemolyticus strains.  Using improved PCR detection methods, we detected tdh and trh in 47% and 55%, respectively, of V. parahaemolyticus strains isolated from oysters.  These results contradict the long accepted idea that tdh and trh are a rarity in environmental V. parahaemolyticus strains.

In 2013, shellfish harvest areas in Virginia, Massachusetts, Connecticut and New York were subjected to closures due to association with V. parahaemolyticus outbreaks.  Improved detection of virulent V. parahaemolyticus can lead to enhanced monitoring of this pathogen and better prediction of potential outbreaks before shellfish closures have to occur.