Pathogenic Vibrio parahaemolyticus Increase in Intertidal-Farmed Oysters in the Mid-Atlantic Region, but Only at Low Tide

Human-pathogenic marine bacteria in the genus Vibrio proliferate rapidly in shellfish tissues when exposed to warm ambient air conditions, such as during intertidal exposure. However, it remains unclear whether intertidal-cultured shellfish are inherently associated with elevated Vibrio concentrations. Here we spatially matched eastern oysters (Crassostrea virginica) held in subtidal and intertidal aquaculture systems at six experimental oyster farms along the mid-Atlantic coast and tested whether intertidal aquaculture elevates concentrations of V. parahaemolyticus, the predominant species associated with seafood-related gastroenteritis, as well as concentrations of V. parahaemolyticus carrying the tdh and/or trh genes associated with pathogenicity in humans. Across the sites and over time, we found only slight evidence for increasing V. parahaemolyticus concentrations with exposure to ambient air at low tide; however, tdh/trh+ V. parahaemolyticus increased about 1.5-fold. We also found that total V. parahaemolyticus concentrations and concentrations of tdh/trh+ V. parahaemolyticus did not differ between intertidal- and subtidal-cultured oysters when submerged at high tides. Any changes to V. parahaemolyticus concentrations within oyster tissues, including concentrations of tdh/trh+ V. parahaemolyticus, therefore diminish with the incoming tide. Our results collectively support the existing time-to-cooling restrictions that are in place in many state Vibrio control plans, which involve minimizing the amount of time harvested shellfish can be held prior to cooling or refrigeration, but oppose more austere measures such as restricting or prohibiting intertidal harvest entirely.