Quantitative Risk Assessment on the Public Health Impact of Pathogenic Vibrio parahaemolyticus in Raw Oysters

July 19, 2005

The Food and Drug Administration (FDA) conducted a quantitative risk assessment to characterize the factors influencing the public health impact associated with the consumption of raw oysters containing pathogenic Vibrio parahaemolyticus. This effort was initiated in January 1999 and a draft risk assessment was made available for public comment in 2001. The risk assessment was conducted in response to four outbreaks in 1997 and 1998 in the United States involving over 700 cases of illness. These outbreaks renewed concern for this pathogen as a serious foodborne threat to public health and raised new concerns about the effectiveness of risk management guidance available at that time. These outbreaks also raised questions about the criteria used to close and reopen shellfish waters to harvesting and the FDA guidance for the maximum number of V. parahaemolyticus per gram in shellfish.  FDA decided to conduct a quantitative risk assessment to provide new insights into how to better manage the presence of this pathogenic microorganism in shellfish.

This risk assessment focused on raw oysters, because that is the food in the United States predominately linked to illness from this pathogen. The risk assessment gathers available knowledge of V. parahaemolyticus in a systematic manner, and includes sophisticated, mathematical models. The levels of the pathogen in oysters were estimated beginning with harvest of the oysters through post-harvest handling, processing, and storage to predict human exposure from consumption of raw oysters and subsequent illnesses. The number of illnesses (on a per serving and a per year basis) were predicted for six regions in the United States and each season for a total of 24 region/season combinations. Total cases of illness include both gastroenteritis and septicemia.  In addition, the probability of gastroenteritis progressing to septicemia in individuals with underlying medical conditions (such as diabetes, alcoholic liver disease, hepatitis, and those receiving immunosuppressive treatments for cancer or AIDS) was compared to that of healthy individuals. Once developed, the baseline model was used to develop "what-if" scenarios to evaluate the likely impact of potential intervention strategies on the exposure to pathogenic V. parahaemolyticus from consumption of raw oysters.

Vibrio parahaemolyticus is a gram-negative, salt tolerant bacterium that occurs naturally in estuaries. It has been long recognized as an important bacterial seafood-borne pathogen throughout the world. It was first isolated and implicated in an outbreak of food poisoning in Japan in 1950. Vibrio parahaemolyticus has been associated with outbreaks and individual cases of illness in the United States since 1969. These bacteria are normally present in many types of raw seafood, including fish, crustaceans, and molluscan shellfish. The microorganism concentrates, colonizes, and multiplies in the gut of filter-feeding molluscan shellfish such as oysters, clams, and mussels. Not all strains of V. parahaemolyticus cause illness; on the contrary, pathogenic strains represent a small percentage of the total V. parahaemolyticus present in the environment or seafood.

The full assessment is available in PDF (4.22 MB).

Interpretive Summary available in PDF (449 KB).

Quantitative Risk Assessment on the Public Health Impact of Pathogenic Vibrio parahaemolyticus in Raw Oysters; Risk Assessment, Federal Register Notice of Availability; July 20, 2005

Quantitative Risk Assessment on the Public Health Impact of Pathogenic Vibrio parahaemolyticus in Raw Oysters; Risk Assessment Federal Register Notice of Public Meeting; July 20, 2005

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