Effects of desiccation practices and ploidy in cultured oysters, Crassostrea virginica, on Vibrio spp. abundances in Portersville Bay (Alabama, USA)

Off-bottom cultivation of oysters, Crassostrea virginica, is increasing in the Gulf of Mexico. The warm ambient air and water temperatures found in the Gulf of Mexico, coupled with the target market for off-bottom cultivated oysters for live raw consumption, raise concerns about the potential infections by human health pathogens, Vibrio parahaemolyticus and V. vulnificus. Regular practices associated with off-bottom cultivation, such as desiccation, expose oysters to ambient air to eliminate bio-fouling and are also known to increase these Vibrio spp. levels in oysters. Along with cultivation methods being introduced in the Gulf of Mexico, the use of triploid oysters is becoming increasingly popular. Triploid oysters are used a majority of the time in off-bottom cultivation due to their sterility, which results in rapid growth and high summer meat quality. Research also suggests that the lack of gonad tissue may correlate with lower Vibrio spp. levels in oysters. In this study, triploid and diploid oysters were cultured in Australian long line systems and subjected to two typical desiccation practices, air dried and freshwater dipped/air dried, and then evaluated for V. parahaemolyticus and V. vulnificus abundances over time. Three two-week long studies determined that Vibrio spp. levels in oysters that underwent either desiccation treatment returned to levels similar to those of submersed oysters by day three, referred to as returning to background levels. However, the Vibrio spp. levels in the treated oysters remained not significantly different from the elevated levels seen immediately following the desiccation treatment until seven days after re-submersion. There was no significant difference in Vibrio spp. levels between triploid and diploid oysters, nor a difference in the time of re-submersion needed to return levels to background. These results suggest that oysters that have been desiccated should be re-submersed for at least seven days prior to harvest to mitigate any human health risk contributed by desiccation practices, regardless of oyster ploidy.