High Salinity Relaying to Reduce V. parahaemolyicus and V. vulnificus in Chesapeake Bay Oysters

Introduction: Cases of Vibrio infections have tripled from 1996 to 2010 and these infections are most often associated with the consumption of V. parahaemolyticus (Vp) and V. vulnificus (Vv) contaminated seafood, particularly oysters. Information is inadequate on evaluation and validation of high salinity relaying method to reduce these bacteria in oysters.

Purpose: The purpose of this study was to evaluate and validate a low cost method to reduce the abundance of Vp and Vv in oysters.

Methods: High salinity relaying was carried out in on-shore closed recirculating tanks (RAS) and in the Chesapeake Bay (field) to reduce Vp and Vv numbers in oysters.  Moreover, moderate salinity relaying was conducted in the field. Oysters were collected from approved harvest water, temperature abused for 4 hours, then transferred to high (28-34 ppt.) and moderate (13-18 ppt.) salinities. After 7, 14 and 21 days of relaying, oysters were analyzed for Vp and Vv using Multiplex Real Time PCR, and for total coliforms and Escherichia coli using petrifilm. Water samples were analyzed for total coliforms, E .coli, and total Vibrio using membrane filtration procedure.

Results: The counts of Vp and Vv ranged from 3 to 438,399 MPN/g with 2-5 log reductions. The total mortalities of oysters were not higher than 4%. Relaying of oysters to high salinity field sites was found to be more effective in reducing Vv than Vp. In RAS tanks, additional days were required to achieve comparable reductions in Vv and Vp.  E. coli and total coliforms counts in oyster and water samples ranged from <10 to 18,000 CFU/g and <10 to 340 CFU/100ml, respectively. No concomitant reduction of total coliforms or E. coli was observed during relaying trials.

Significance: These data suggest that high salinity relaying of oysters is an effective postharvest mitigation strategy to reduce Vibrio in oysters.