Effects of tumbling, refrigeration, and resubmersion on Vibrio parahaemolyticus and V. vulnificus levels in North Carolina cultured oysters (C. virginica)

In oyster aquaculture, routine handling practices (tumbling, desiccation) are pre-harvest growing techniques that produce high-quality products. These practices involve the removal of oysters from the water for extended periods, which can inadvertently increase the levels of human pathogenic vibrio bacteria within the oyster. Vibrio vulnificus and V. parahaemolyticus are naturally found in estuarine waters and can be concentrated in oysters during filter feeding, creating a potential public health risk to raw oyster consumers. To mitigate the public health risk before harvest, oysters can be resubmersed in the water to allow elevated vibrio levels to decline or "recover" to naturally occurring levels. Previous research is almost exclusively from Alabama waters, leaving open the question of applicability to other oyster growing regions; therefore, this study investigated recovery times at a farm off the coast of North Carolina. Four handling treatments (tumbled/refrigerated [TR], tumbled/desiccated [TNR], refrigerated only [NTR], and desiccated only [NTNR]) were applied to oysters, followed by resubmersion. Levels of V. vulnificus and total and pathogenic (tdh+/trh+) V. parahaemolyticus were measured before and after handling, and 1, 3, 7, 10, and 14 days after resubmersion. After handling, the refrigerated oysters did not have significant increases in vibrio levels (-0.08-0.46 log MPN/g), while the non-refrigerated oysters did (1.06-2.06 log MPN/g). The refrigerated oysters had a significant spike (0.60-1.03 log MPN/g) in V. parahaemolyticus levels after one to three days of resubmersion, but this effect was not seen for V. vulnificus. Elevated V. vulnificus levels recovered to ambient levels after 1 day of resubmersion in all treatments. Total and pathogenic V. parahaemolyticus recovered in 1 to 7 days in all but the TR treatment (14 days), which supports the previous finding that the combination of tumbling and refrigeration would not be a recommended handling practice, as the recovery time was the longest. Excluding the TR treatment, the vibrio recovery times observed in this study were similar to those found previously in Alabama studies.