Seasonality of Vibrio parahaemolyticus in the Eastern oyster, Crassostrea virginica, along the coast of Maine, USA

As the Gulf of Maine warms, characterizing seasonal baseline levels of the human pathogen Vibrio parahaemolyticus (Vp) in Eastern oysters (Crassostrea virginica) is crucial, along with synoptic environmental data, to develop predictive capacity and risk management strategies. In Maine, USA, from May to November 2023, oyster samples were collected from Scarborough River, Maquoit Bay, and the Damariscotta River. Vp was quantified using the U.S. Food and Drug Administration-approved three-tube most-probable-number (MPN) real-time PCR method, detecting the presence of total Vp (i.e., tlh gene) and potentially pathogenic Vp (i.e., tdh and/or trh genes). tlh was detected at all sites once water temperatures reached 16 °C, with densities ranging from <3 to >1,100 MPN g⁻¹. Generalized additive models revealed significant positive relationships between both potential pathogenic markers (tdh and trh) and sea surface temperature. tdh in the Damariscotta River, the largest oyster-growing area in northern New England, was the most responsive to water temperature, and trh in Maquoit Bay was the most responsive to salinity. Statewide Vp regulations for harvesting and handling oysters were implemented in Maine in 2024, and these baseline data already demonstrate the importance of maintaining cold chain storage, as potentially pathogenic strains were detected at all three sites. Given the growing number of Vp-associated illnesses and the clear response of Vp to warming conditions in the region, these baseline data provide timely insights for harvesters and inform policies to safeguard public health.