A Case-Control Analysis of Traceback Investigations for Vibrio parahaemolyticus Infections (Vibriosis) and Pre-harvest Environmental Conditions in Washington State, 2013-2018

Abstract

Background/Aim: Vibrio parahaemolyticus is a major cause of seafood-borne illness. It is naturally prevalent in brackish waters and accumulates in shellfish. Vibriosis cases are rising globally, likely due to warming temperatures. We aimed to identify pre-harvest risk factors of vibriosis in Washington State using environmental and genetic variables sampled from shellfish. Methods: Successful traceback investigations of vibriosis were spatiotemporally matched to routine intertidal oyster (Crassostrea gigas) sampling events, which included measurements of temperature, salinity, and V. parahaemolyticus genetic targets. Remaining sampling events were treated as controls. Ecological variable importance was assessed using logistic regression models. Results: Systematic differences were observed across Washington harvesting zones. These included positive associations between the odds of vibriosis and genetic targets in South Puget Sound, with a particularly high odds ratio (OR) = 13.0 (95% CI: 1.5, 115.0) for a 1-log10 increase in the tdh pathogenic marker when total bacterium abundance was low (<1 log10). A positive association also occurred for a 1°C increase in tissue temperature OR=1.20 (95% CI: 1.10, 1.30) while a negative association occurred for a similar increase in water temperature OR= 0.70 (95% CI: 0.59, 0.81). The coastal bays displayed positive associations for water temperature OR= 2.16 (95% CI: 1.15, 4.05), and for a 1-log10 increase in the tdh:trh pathogenic marker ratio OR= 5.85 (95% CI: 1.06, 32.26). Conclusions: The zonal variation in associations indicates unique pathogenic strain prominence, suggesting tdh+/trh+ strains in South Puget Sound, such as the O4:K12 serotype, and tdh+/trh- strains in the coastal bays. The temperature discrepancy between water and oyster tissue suggests that South Puget Sound pathogenic strains flourish with exposure to relatively warm air during low tide. These findings identify new ecological risk factors for vibriosis in Washington State that can be used in future prevention efforts.