Seasonality of Vibrio parahaemolyticus in the Eastern Oyster, Crassostrea virginica, along the Coast of Maine, USA

Rising numbers of vibriosis illnesses associated with consumption of raw oysters, including a 2023 outbreak in Maine, U.S., have highlighted the need to characterize the current concentrations and seasonality of the human pathogen, Vibrio parahaemolyticus (Vp). As the Gulf of Maine (GoM) warms and the oyster industry grows, characterizing seasonal baseline levels of Vp in Eastern oysters (Crassostrea virginica) in Maine is crucial along with synoptic environmental data to enable future predictive capacities. From May - November 2023, oyster samples were collected from three representative oyster growing areas: Scarborough River, Maquoit Bay, and the Damariscotta River. Vp was quantified using the U.S. Food and Drug Administration (FDA) 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 when the water temperatures reached 16°C and densities ranged from < 3 MPN g-1 to >1100 MPN g-1. Generalized additive models (GAMs) revealed significant positive relationships between both potential pathogenic markers (tdh and trh) and sea surface temperature, and a nonsignificant negative relationship between trh and salinity. 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 went into effect in Maine in 2024. These data show the importance of statewide control on oysters due to potentially pathogenic strains detected at all three sites even though Maquoit Bay was not under Vibrio control at the time of this study. Due to the growing number of documented illnesses caused by Vp, and Vp’s clear response to warming temperatures in the region, these established baseline Vp levels are already providing valuable insights for harvesters and informing policies to safeguard public health.