Environmental investigations of Vibrio parahaemolyticus in oysters after outbreaks in Washington, Texas, and New York (1997 and 1998)

Abstract

Total Vibrio parahaemolyticus densities and the occurrence of pathogenic strains in shellfish were determined following outbreaks in Washington, Texas, and New York. Recently developed nonradioactive DNA probes were utilized for the first time for direct enumeration of V. parahaemolyticus in environmental shellfish samples. V. parahaemolyticus was prevalent in oysters from Puget Sound, Wash.; Galveston Bay, Tex.; and Long Island Sound, N.Y., in the weeks following shellfish-associated outbreaks linked to these areas. However, only two samples (one each from Washington and Texas) were found to harbor total V. parahaemolyticus densities exceeding the level of concern of 10,000 g(-1). Pathogenic strains, defined as those hybridizing with tdh and/or trh probes, were detected in a few samples, mostly Puget Sound oysters, and at low densities (usually <10 g(-1)). Intensive sampling in Galveston Bay demonstrated relatively constant water temperature (27.8 to 31.7 degrees C) and V. parahaemolyticus levels (100 to 1,000 g(-1)) during the summer. Salinity varied from 14.9 to 29.3 ppt. A slight but significant (P < 0.05) negative correlation (-0.25) was observed between V. parahaemolyticus density and salinity. Based on our data, findings of more than 10,000 g(-1) total V. parahaemolyticus or >10 g(-1) tdh- and/or trh-positive V. parahaemolyticus in environmental oysters should be considered extraordinary.

FIG. 1

Mean V. parahaemolyticus densities in Galveston Bay oysters obtained from 29 June through 21 September 1998. Symbols: ●, direct plating of three lease sites sampled throughout the study; ○, direct plating during August for the expanded sampling set of 30 lease sites; ■, MPN of three to five lease sites sampled throughout the study. Values are mean CFU per gram ± standard deviation. Dates are month/day.