Vibrios in farmed oysters in relation to season and different farms across the Chesapeake and Maryland Coastal Bays, USA

Oyster farming is one of the most important marine aquaculture practices contributing to the Chesapeake and the Maryland Coastal Bays habitat and ecosystem. Illness caused by Vibrio parahaemolyticus and V. vulnificus can be contracted from consuming raw or undercooked oysters. Oyster- Vibrio -associated outbreaks, specifically in the summer, emphasize the importance of aquaculture risk assessment. This study investigated the seasonal prevalence and abundance of total and pathogenic V. parahaemolyticus and V. vulnificus in farmed oysters and surrounding seawater across three Chesapeake Bay (CB) farms and one in the Maryland Coastal Bays (MD), USA from June 2019 to October 2021. A total of 108 oyster and 112 water samples were analyzed using most probable number–quantitative PCR (MPN-qPCR), targeting tlh , tdh , trh ( V. parahaemolyticus ), and vvhA , vcgC ( V. vulnificus ) genes. Detection rates were high for tlh and vvhA in both oysters and water (>80%). Pathogenic genes were frequently detected, with vcg C exceeding 50% in both sample types. Vibrio levels were significantly lower at the MD farm than at the CB farms. During the warm season, V. vulnificus abundance ( vvh A+) exceeded V. parahaemolyticus abundance ( tlh +) in CB oysters, while the reverse was true in MD oysters. Correlation analysis revealed strong positive associations between Vibrio abundance and water temperature, while dissolved oxygen showed significant negative correlations. Generalized additive modeling revealed that water temperature, salinity, turbidity, and chlorophyll a were predictive of Vibrio levels. These findings highlight the influence of seasonal and environmental factors on Vibrio dynamics and support targeted monitoring strategies to enhance seafood safety. • Seasonal study of Vibrio spp. in farmed oysters across four MD aquaculture sites • Water temperature was a strong and consistent predictor of Vibrio gene abundance • Salinity, turbidity, and chlorophyll a were also predictive of Vibrio levels • Flexible modeling can uncover relationships that correlation analysis might miss • One oyster sample exceeded the 10,000 MPN/g limit for V. parahaemolyticus