NOAA/Saltonstall-Kennedy Grant Program: Harvest Management Tools to Control the Levels of Vibrio parahaemolyticus in Oysters and Other Bivalve Shellfish

Purpose or Scope:

The emphasis of this research will be on manipulative experiments and experimental analyses of factors influencing V. parahaemolyticus levels and post-harvest methods to minimize the risk of V. parahaemolyticus-induced food poisoning. This study will provide much needed information for enhanced control of pathogenic V. parahaemolyticus in shellfish. This project combines information gained from previous short-term studies with ongoing sampling and research programs in a two-year study. All research will be directed to studies of V. parahaemolyticus in oysters, the most common species implicated in shellfish-related food poisoning. However, the research findings will likely be applicable to other bivalve shellfish species consumed raw or uncooked.

Approach:

The primary hypotheses to be tested are:

  1. When oysters exhibit elevated V. parahaemolyticus levels following intertidal exposure or unrefrigerated holding following harvest, a reduction of V. parahaemolyticus to acceptable levels can be achieved by placing the animals in ambient seawater at the harvest site.
  2. Reductions in V. parahaemolyticus levels will be significantly greater than have been achieved by placing oysters in controlled-temperature sterilized seawater depuration systems.
  3. Ambient seawater V. parahaemolyticus reduction rates and levels are independent of the culture method, culture location, post-harvest holding practice, and oyster size or age.
  4. Other than water and air temperature, other environmental factors are not correlated with the ambient seawater V. parahaemolyticus reduction rates and levels.
  5. Observations can be replicated to comply with public health guidance across a range of sampled conditions.

The primary objectives of the proposed research are:

  1. Examine the effects of different holding practices on V. parahaemolyticus clearance under natural conditions during tidal inundation.
  2. Evaluate the rate and variability of the V. parahaemolyticus growth and clearance responses at multiple sites under natural conditions before and during tidal inundation.
  3. Assess key water quality and environmental parameters for possible correlations and interactions with V. parahaemolyticus growth and clearance.
  4. Produce a post-harvest management report complementing the existing USFDA national V. parahaemolyticus risk assessment report, and the National Shellfish Sanitation Program (NSSP) V. parahaemolyticus control plan.

Background:

It is estimated that 20 million Americans eat raw shellfish, and consumption of raw and undercooked shellfish has been reported to account for more than 90 percent of 113,000 seafood-poisoning cases each year in the United States. The Centers for Disease Control and Prevention (CDC) estimates that each year there are approximately 2,800 cases of V. parahaemolyticus illness, 40 hospitalizations, and seven deaths associated with the consumption of raw oysters (CDC 2006). Outbreaks of V. parahaemolyticus infections appear to be increasing, despite increased public awareness of the risk factors and improved surveillance and monitoring. Outbreaks involving more than 1,000 confirmed cases of illness associated with consumption of raw shellfish occurred between 1997 and 2007 in the Gulf Coast, Pacific Northwest, Alaska, and Atlantic Northeast regions of the United States (CDC 1998; CDC 1999; CDC 2006; Guichard 2008; McLaughlin et al. 2005; U.S. Food and Drug Administration 2001). Recent outbreaks on the North American west coast include the following:

Washington

Between 1997 and 2007, several major outbreaks of V. parahaemolyticus gastroenteritis occurred in Washington caused largely by the consumption of raw oysters. In the summer of 2006, a total of 169 confirmed cases (three hospitalized and no fatalities) of V. parahaemolyticus infection were reported in New York, Oregon, and Washington between May 20 and July 31, 2006 (CDC 2006). The number of confirmed cases, from the outbreak was greater than the average number reported from 2000 to 2004 in the entire U.S (CDC 2006). Between the larger outbreaks of 1997-1998 and 2007, sporadic cases of V. parahaemolyticus attributed to Washington State oysters continued to be reported on an annual seasonal basis (Figure 1).Figure 1

British Columbia

The first recorded outbreak of V. parahaemolyticus in British Columbia occurred in 1997, coinciding with a large outbreak in Washington State. A total of 101 raw oyster consumption illnesses were reported and then confirmed in British Columbia from July to August of that year. Since then, the pattern of confirmed V. parahaemolyticus cases has paralleled those reported in Washington State.

Alaska

Alaskan waters had been thought to be too cold to support levels of V. parahaemolyticus high enough to cause disease. However, in the summer of 2004 a large outbreak of V. parahaemolyticus occurred stemming from the consumption of oysters produced in Alaska. This correlated with record-high ambient air, and water temperatures that met or exceeded 15°C. This outbreak expanded the range of epidemiologically confirmed V. parahaemolyticus illness to a latitude higher than 60° - more than 1,000 km north of British Columbia, previously the northernmost area reported to have locally acquired illness (McLaughlin et al. 2005).

Economic impacts to the shellfish industry

Bivalve shellfish account for about 13% of the total amount, by weight, of seafood consumed in the United States. Production in the United States is growing. For example, the Pacific Coast Shellfish Growers Association (PSCGA) estimated that in 2005, approximately 106 million pounds of shellfish were produced by shellfish farms in West Coast waters. Table 1The value of these harvests to farmers was $111 million. When V. parahaemolyticus-caused human illness occurs due to raw shellfish consumption, and is reported, the shellfish industry experiences a reduction in oyster sales, as well as a consequently significant financial hardship and loss of revenue to largely rural agricultural communities. Food and Drug Administration (FDA) consumer advisories recommending against the purchase and consumption of raw shellfish during the outbreaks tend to worsen the losses for all producers, including those outside V. parahaemolyticus affected waters. For example, during the summer of 2006 outbreak in Puget Sound, the largest shellfish grower reported lost revenues for all shellfish sales of over $1.4 million (Table 1). At least 90 farms in Washington State were implicated and affected by harvest restrictions and reduced sales. Revenue losses for all shellfish farms on the West Coast were estimated to be at least $4 million (Pacific Coast Shellfish Growers Association 2008).

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