Oyster farming is one of the most important marine aquaculture practices that contributes to the Chesapeake and the Maryland Coastal Bays habitat and ecosystem. Illness caused by Vibrio parahaemolyticus (Vp) and V. vulnificus (Vv) can be contracted from consuming raw or undercooked oysters. Oyster-Vibrio associated outbreaks, specifically in the summer, emphasize the importance of aquaculture risk assessment. Also, it causes economic burden to the oyster farming industry. The aim of this study was to assess the effects of season on the prevalence of total and pathogenic Vp and Vv in oyster and seawater samples using MPN-qPCR. Oyster (n=108) and seawater (n=112) samples were collected from three farming sites in the Chesapeake Bay (CB) and one site in the Maryland Coastal Bays (MCB), USA from June 2019-October 2021. Weather conditions and physico-chemical parameters of the water were recorded per sampling event. MPN-qPCR was performed to quantify total and pathogenic Vp (tlh, tdh, trh genes) and Vv (vvhA, vcgC genes). Positivity rates for oyster and water samples were 88 and 92% for tlh, 81 and 80% for tdh, 37 and 48% for trh, 45 and 54% for vvhA, and 54 and 56% for vcgC, respectively.
The detection frequency of total Vp (tlh+) and pathogenic Vv (vcgC+) from oyster and water was significantly lower in MCB than in CB farms in the colder and warmer months, respectively. Moreover, Vp was more abundant than Vv in the MCB. During the warmer months, the average log MPN of Vp (tlh+) in the oysters from the CB farms was significantly lower than Vv (vvhA+). However, the average log MPN of Vp (1.9) was significantly higher than Vv (0.5) in the oysters from MCB. The average log MPN of Vp (1.5) and Vv (2.1) in oyster were higher than in water. These results indicate that Vp and Vv were more prevalent in the CB than the MCB.