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Vibrio parahaemolyticus Risk Assessment - I. Introduction

July 19, 2005

Table of Contents

The Food and Drug Administration (FDA) conducted this risk assessment on the public health impact of Vibrio parahaemolyticus transmitted by raw oysters. This is a "product pathway" risk assessment and provides a systematic evaluation of the factors affecting V. parahaemolyticus in oysters and the sequence of events leading to consumer illnesses.


Vibrio parahaemolyticus is a marine bacterium that occurs naturally in the estuarine environment and can accumulate in filter-feeding molluscan shellfish. This microorganism was first identified as a foodborne pathogen in Japan in the 1950s. It has been associated with outbreaks and individual cases of illness in the United States since 1969. In 1997 and 1998, over 700 cases of illness from four outbreaks were associated with consumption of raw oysters in three regions of the country, the Gulf Coast, Pacific Northwest, and Northeast. These outbreaks renewed concern for this pathogen as a serious foodborne threat to public health and raised new concerns about the effectiveness of current risk management guidance

The Centers for Disease Control and Prevention (CDC) estimates that each year there are approximately 2,800 cases of V. parahaemolyticus illness associated with the consumption of raw oysters. The most common clinical manifestation of V. parahaemolyticus infection is gastroenteritis. In at-risk populations (individuals with underlying chronic medical conditions), infection can lead to more serious outcomes (septicemia and death).

FDA announced the initiation of this risk assessment in 1999 in the Federal Register (FDA, 1999). The public was invited to comment on the planned assessment and submit scientific data and information for use in the assessment. The advice and recommendations of the National Advisory Committee on Microbiological Criteria for Foods (NACMCF) were sought on the assumptions and the model structure to be used. During the conduct of this risk assessment, FDA solicited the technical advice and opinions of scientific experts both within and outside of the Federal government. The availability of the draft risk assessment was announced in the Federal Register (Federal Register Docket No. 99N 1075) in January 2001 (FDA, 2001). A comment period was established during which FDA actively sought comments, suggestions, and additional data sources. The draft risk assessment was presented to stakeholders and other interested parties during a public meeting on March 20, 2001. The risk assessment report and model were modified based on the public comments received and availability of new data. The revised document and model were subjected to extensive review. A chronology of the technical and scientific review involved in the development of this risk assessment is provided in Appendix 1. A summary of the modifications made to the 2001 model is provided in Appendix 2.


This risk assessment is a quantitative product pathway analysis in which the key steps from harvest through post-harvest handling and processing to consumption were modeled. The likelihood of illness following exposure to pathogenic V. parahaemolyticus from consumption of raw oysters was calculated. The levels of V. parahaemolyticus in oysters at the time of consumption can be influenced by the harvest methods and handling of oysters after harvest and these practices may vary considerably in different geographic areas and at different times of year. The impact of regional and seasonal conditions on the predicted risk was evaluated.

The risk assessment had two main objectives: (1) to determine the factors that contribute to the risk of becoming ill from the consumption of pathogenic V. parahaemolyticus in raw oysters and (2) to evaluate the likely public health impact of different control measures, including the effectiveness of current and alternative microbiological standards

The risk assessment addresses the following questions:

  • What is known about the dose-response relationship between consumption of V. parahaemolyticus and illnesses?
  • What is the frequency and extent of pathogenic strains of V. parahaemolyticus in shellfish waters and in oysters?
  • What environmental parameters (e.g., water temperature, salinity) can be used to predict the presence of V. parahaemolyticus in oysters?
  • How do levels of V. parahaemolyticus in oysters at-harvest compare to levels at consumption?
  • What is the role of post-harvest handling on the level of V. parahaemolyticus in oysters?
  • What reductions in risk can be anticipated with different potential intervention strategies?

Risk Assessment Overview

The Vibrio parahaemolyticus Risk Assessment follows the risk assessment structure of the Joint Food and Agriculture Organization/World Health Organization Expert Consultation on the Application of Risk Analysis to Food Standards Issues (FAO/WHO, 1998). The structure consists of four components: (1) hazard identification, (2) hazard characterization, (3) exposure assessment, and (4) risk characterization. Figure I-1 shows the organization and components of the risk assessment including the types of data and modeling techniques used.

Hazard Identification

The Hazard Identification component of a microbial risk assessment is the identification of the pathogenic organism that may be present in a particular food or group of foods that are capable of causing adverse health effects. The hazard on which this risk assessment is focused is pathogenic V. parahaemolyticus in raw oysters. The adverse health effect considered is the number of illnesses characterized by gastroenteritis and septicemia. See Chapter II: Hazard Identification for details.

Hazard Characterization/Dose Response/Severity Assessment

The Hazard Characterization component of a microbial risk assessment is often referred to as Dose-Response because it characterizes the relationship between the level of exposure to a pathogen (the dose) and the likelihood of an adverse health effect for individuals and populations (the response). For this risk assessment, a quantitative relationship was developed to predict the number and severity of illnesses resulting from ingesting different amounts of pathogenic V. parahaemolyticus. The Dose-Response model was developed using human clinical volunteer feeding studies and epidemiological surveillance data. See Chapter III: Hazard Characterization for details.

Exposure Assessment

The Exposure Assessment component of a microbial risk assessment defines the frequency and likely level of exposure to a pathogenic microorganism. In this risk assessment, the likelihood of exposure to pathogenic V. parahaemolyticus from consumption of raw oysters was evaluated. The Exposure Assessment was divided into three modules: Harvest, Post-Harvest, and Consumption. The levels of V. parahaemolyticus in oysters at the time of consumption can be influenced by the harvest methods and handling of oysters after harvest and these practices may vary considerably in different geographic areas and at different times of year.

Oysters are harvested in the United States from the Gulf Coast, Mid-Atlantic, Northeast Atlantic, and Pacific Northwest. In the Gulf Coast, the harvest duration for Louisiana is typically much longer than for other states in that region (Florida, Mississippi, Texas, and Alabama), therefore it was divided into two distinct regions: Gulf Coast (Louisiana) and Gulf Coast (Non-Louisiana). Likewise, the Pacific Northwest was divided into two distinct regions: Pacific Northwest (Intertidal) and Pacific Northwest (Dredged). In the Pacific Northwest, oysters are harvested by two methods: dredging and intertidal. For the intertidal harvest method, oysters are hand-picked when oyster reefs are exposed during the tide cycle and left in baskets until the tide rises to a sufficient depth to allow a boat to retrieve the basket. The risk assessment considered six oyster harvest regions and four seasons, for a total of 24 region/season combinations. See Chapter IV: Exposure Assessment for details.

Risk Characterization

Risk Characterization is the integration of the Dose-Response relationship with the Exposure Assessment to predict the probability of potential adverse outcomes for individuals or populations. For this risk assessment, the likelihood and severity of illness (gastroenteritis alone or gastroenteritis followed by septicemia) from the consumption of raw oysters containing pathogenic V. parahaemolyticus was predicted on both a per serving and a per annum basis. The uncertainties associated with the predicted risk estimates were also determined. See Chapter V: Risk Characterization for details.

Using the Model as a Tool: "What-If" Scenarios

The baseline risk assessment model can be used to estimate the likely impact of intervention strategies on the predicted number of illnesses. "What-if" scenarios were conducted by changing one or more model inputs and measuring the resulting change to the model outputs. Various control measures and mitigation strategies were evaluated. See Chapter VI: What-If Scenarios for details.


Chapter II: Hazard Identification

  • Characteristics of Vibrio parahaemolyticus
  • Endpoints of concern: Gastroenteritis, Septicemia
  • Susceptible populations
  • Food considered: Raw Oysters
  • Incidence : Outbreaks; Sporadic Cases; Seasonality

Chapter III: Hazard Characterization


Chapter IV: Exposure Assessment
(Harvest, Post-Harvest, Consumption)


  • Human clinical studies
  • Surveillance
  • Host susceptibility


  • Dose-response curves
  • Adjustment factor(s)


  • Water temperature
  • Total vs. pathogenic Vibrio parahaemolyticus in oysters
  • Time-to-refrigeration
  • Air temperature
  • Growth rates
  • Oysters consumed/serving


  • Pathogenic Vibrio parahaemolyticus levels in oysters at harvest
  • Growth between harvest and refrigeration
  • Pathogenic Vibrio parahaemolyticus in raw oysters at consumption

Chapter V: Risk Characterization

  • Number of illnesses: per serving and per annum
  • Severity of illness (gastroenteritis vs. septicemia)
  • Uncertainty and variability analysis
  • Model validation

Chapter VI: 'What-If' Scenario

  • 4.5-log10 reduction (heat; ultra high pressure)
  • 2-log10 reduction (freezing)
  • approximately 1-log10 reduction (immediate cooling)
  • Impact of time-to-refrigeration after harvest
  • Sample-based control plans

Figure I-1. Overview of Vibrio parahaemolyticus Risk Assessment Document


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