Levels of Vibrio parahaemolyticus in Pacific Oysters (Crassostrea gigas) From Washington State Following Ambient Exposure and Chilling

Vibrio parahaemolyticus illnesses, often associated with the consumption of raw or undercooked oysters, are most common in summer months when higher temperatures increase V. parahaemolyticus levels in the environment. In Washington, post-harvest controls focus on the time from harvest to temperature-controlled storage to minimize V. parahaemolyticus illness risk. This study examined the effect of post-harvest ambient storage on V. parahaemolyticus levels in Pacific oysters. Additionally, the effects of cooling method, icing and/or refrigeration, on V. parahaemolyticus levels in oysters were evaluated. Five independent trials were conducted during July and August of 2015. For each trial, oysters were harvested from Totten Inlet and exposed to ambient conditions for 0 h (immediately cooled), 1 h, 5 h, or 9 h, and then either iced or refrigerated. Total and pathogenic (tdh+/trh+) V. parahaemolyticus levels were determined via MPN real-time PCR. Data from each trial were analyzed independently due to differences in initial V. parahaemolyticus levels. Total V. parahaemolyticus levels in oysters increased relative to control (0 h I) levels after the 1 h ambient exposure in only one trial, but pathogenic V. parahaemolyticus levels did not significantly increase after the 1 h exposure. Total and pathogenic V. parahaemolyticus levels increased by 0.8–1.9 log MPN/g in 5 h exposed oysters and by 1.0–2.9 log MPN/g in 9 h exposed oysters compared to levels in 0 h I samples. Mean maximum temperature of 5 h and 9 h exposed samples increased to ≈29°C compared to ≈21°C in 0 h and 1 h exposures, which likely contributed to observed increases in V. parahaemolyticus levels. Total and pathogenic V. parahaemolyticus levels increased more often in oysters cooled by refrigeration than by ice; this was most notable for the longer ambient exposure samples. Overall, these data support shorter post-harvest ambient exposure as well as rapid cooling of oysters to minimize risk of V. parahaemolyticus illness