THE EFFECTS OF HARVESTING AND HANDLING PRACTICES OF OYSTER ON Vibrio parahaemolyticus  

The leading bacterial cause of morbidity in United States' shellfish consumers is Vibrio parahaemolyticus.  Infections are mostly associated with the consumption of raw or undercooked oysters.  In recent years, in an effort to identify the best management practices, the Food and Drug Administration (FDA), as well as the Interstate Shellfish Sanitation Conference (ISSC) and member states, along with academia have examined harvesting and handling practices of oysters with respect to the levels of V. parahaemolyticus.  Multiple practices were investigated:  air storage/refrigeration, air storage/icing, air storage/re-submersion, and intertidal harvest/re-submersion.  In addition, V. parahaemolyticus levels were observed upon initial harvest of different molluscan shellfish typically consumed in the United States: oysters, cage cultured and wild; clams; and mussels. For air storage/refrigeration, an average log increase of V. parahaemolyticus of 0.56 log, 1.10 log, and 1.63 log was observed in oysters following a 1-, 5-, and 9-hr, respectively, ambient storage (20-28^oC) and overnight refrigeration.  For air storage/icing, a slightly lower increase (between 0.2-0.3 log) in V. parahaemolyticus levels was observed in oysters following a 1-, 5-, and 9- hr ambient storage and 15 min icing; 0.24 log, 0.76 log, and 1.32 log, respectively.  For air storage/re-submersion and intertidal harvest/re-submersion, previous studies demonstrated similar results with an initial increase of up to 0.9 log of V. parahaemolyticus observed after 5 hr ambient air storage with no reduction observed after overnight refrigeration.  Oysters that were harvested and left exposed to ambient air for 24 hours demonstrated an average log increase of 0.88 of V. parahaemolyticus.  After one day of re-submersion V. parahaemolyticus levels increased an average 1.12 log, returning to within 0.3 log of background oysters (not previously handled) by 3 days with no discernable difference noticed by day 14.  Interestingly, comparing levels across shellfish types, wild oysters had an average of 1.41 log higher levels of V. parahaemolyticus as compared to caged oysters harvested from the same waters.  Additionally, wild oysters from the same harvest area also contained an average of 1.06 log higher levels of V. parahaemolyticus as compared to wild mussels.  Similarly, oysters harvested from the same waters as clams also contained higher levels of V. parahaemolyticus.  Vibrio parahaemolyticus levels ranged from 1.24 logs to 0.28 logs higher in oysters than clams taken from the same areas.  These data are being used by the ISSC and member states to assist in refining state vibrio plans in effort to reduce risk of vibrio illness resulting from shellfish consumption.