Date of Award

Fall 2020

Project Type


Program or Major

Natural Resources

Degree Name

Master of Science

First Advisor

Stephen H Jones

Second Advisor

Cheryl Whistler

Third Advisor

Kenneth La Valley


Vibrio parahaemolyticus (Vp.) outbreaks and sporadic cases associated with the consumption of oysters harvested from the New England Region of the U.S. have increased since 2011. Increasing Vp. infections in Massachusetts have resulted in harvest areas closures, product recalls, and the implementation of costly control measures on the harvesting and handling of oysters in the state during summer months. A combination of factors such as increased summertime production and consumption of raw shellfish, and climate related changes leading to warmer sea surface temperatures and more variable salinities favorable for Vp. bacteria, have been attributed to the recent increase in infections. However, the 2012 introduction and ecosystem establishment of a highly virulent Pacific-endemic strain of Vp. called sequence type (ST) 36 has been implicated in most illnesses and appears to be driving risk. The growing public health and economic burden associated with managing Vp. risk in Massachusetts have made the collection of information to support the development of effective Vp. mitigation strategies and risk assessment models a major priority in the region.

Northeast oyster production is targeted almost exclusively toward the raw half shell market and Massachusetts production has increased nearly fivefold between 2008 and 2018, from 11 million to over 50 million oysters. Thus, the observed reported increase in Vp. illnesses in the state may not reflect a significant change in the risk per serving to consumers. Both production and illnesses vary across harvest areas and should be evaluated to accurately assess effects of environmental conditions on exposure and risk. Massachusetts collected limited information on the distribution and abundance of total and potentially pathogenic (tdh+ and trh+) Vp. in shellfish harvest areas prior to the introduction of ST36 to inform Vp risk assessment models and localized risk management strategies. Our aim for this study was to characterize trends in Vp. infection risk in Massachusetts harvest areas in relation to environmental conditions, total and potentially pathogenic Vp. abundance, and clinical strains implicated in regional infections. Characterizing Vp risk in MA is critical to managing risk and sustaining the oyster industry. We conclude: 1. There is significant variability in Vp. risk between Massachusetts harvest areas. 2. Current Vp. surveillance methodology and temperature driven risk assessment models cannot accurately capture differential risk. 3. Spatial variability in Vp. population composition is likely the largest driver of differential Vp. infection risk between Massachusetts harvest areas.