• Decrease font size
  • Return font size to normal
  • Increase font size
U.S. Department of Health and Human Services

Food

  • Print
  • Share
  • E-mail

Vibrio parahaemolyticus Risk Assessment: VII. Interpretation and Conclusions

July 19, 2005

Table of Contents


This risk assessment included an analysis of the available scientific information and data in the development of a model to predict the public health impact of pathogenic V. parahaemolyticus in raw oysters. The assessment focuses on comparing the relative risk of consuming raw oysters acquired from different geographic regions, seasons, and harvest practices. The scientific evaluations and the mathematical models developed during the risk assessment also facilitate a systematic evaluation of strategies to minimize the public health impact of pathogenic V. parahaemolyticus.

Regional and seasonal differences in climates and oyster harvesting practices occur within the United States. Therefore, the risk assessment was structured to assess regional, seasonal and harvesting practices influences on illness rates. Six separate geographic regions and harvesting practices combinations were considered: Northeast Atlantic, Mid-Atlantic, Pacific Northwest (Dredging), Pacific Northwest (Intertidal), Gulf Coast (Louisiana), Gulf Coast (non-Louisiana states). The predicted risk estimates must of course be evaluated in relation to the uncertainties as a result of limited scientific data and knowledge.

Although the risk assessment modeled sporadic V. parahaemolyticus illnesses, steps taken to reduce sporadic cases would be expected to reduce the size and frequency of outbreaks. The proportional reduction would depend on the virulence of the outbreak strain and on the survivability and growth of the strain following post-harvest treatments. Mitigation or control measures aimed at decreasing levels of V. parahaemolyticus in oysters will also likely decrease levels of other species in the Vibrio genus (or family), such as Vibrio vulnificus.

Below are the responses to the questions that the risk assessment team was charged with answering.

What is known about the dose-response relationship between consumption of V. parahaemolyticus and illnesses?

  • Although an individual may become ill from consumption of low levels of V. parahaemolyticus, it is much more likely that he or she will become ill if the level is high. The probability of illness is relatively low (<0.001%) for consumption of 10,000 V. parahaemolyticus cells/serving (equivalent to about 50 cells/gram oysters). Consumption of about 100 million V. parahaemolyticus cells/serving (500 thousand cells/gram oysters) increases the probability of illness to about 50%.
  • Anyone exposed to V. parahaemolyticus can become infected and develop gastroenteritis. However there is a greater probability of gastroenteritis developing into septicemia (and possibly death) among the subpopulation with concurrent underlying chronic medical conditions.
  • The model predicts about 2,800 V. parahaemolyticus illnesses from oyster consumption each year. Of infected individuals, approximately 7 cases of gastroenteritis will progress to septicemia each year for the total population, of which 2 individuals would be from the healthy subpopulation and 5 would be from the immunocompromised subpopulation.
  • This risk assessment assumed that pathogenic strains of V. parahaemolyticus are TDH+ and that all strains possessing this characteristic are equally virulent. Modifications can be made to the risk assessment if data become available for new virulence determinants. For example, data from outbreaks suggest that fewer microorganisms of V. parahaemolyticus O3:K6 are required to cause illness compared to other strains.

What is the frequency and extent of pathogenic strains of V. parahaemolyticus in shellfish waters and in shellfish?

  • Pathogenic V. parahaemolyticus (i.e., TDH+ strains) usually occur at low levels in shellfish waters and oysters. This makes it difficult to monitor shellfish waters for pathogenic V. parahaemolyticus to prevent illnesses from this microorganism. As shown in Table VII-1, the predicted levels of pathogenic V parahaemolyticus in oysters at the time of harvest are only a small fraction of the total V. parahaemolyticus levels. There are differences among regions. For example, the ratio of pathogenic to total V. parahaemolyticus is lower in the Gulf Coast (approximately 0.2%) compared to the Pacific Northwest (approximately 2.0%).
Table VII-1. Predicted Mean Levels of Total and Pathogenic Vibrio parahaemolyticus in Raw Oysters At-Harvest
RegionVibrio parahaemolyticusMean Predicted Levels of V. parahaemolyticus per grama
SummerFallWinterSpring
Gulf Coast bTotal2,10022052940
Pathogenic3.6<1.0<1.01.6
Mid-AtlanticTotal780513.5200
Pathogenic1.3<1.0<1.0<1.0
Northeast AtlanticTotal230333.742
Pathogenic<1.0<1.0<1.0<1.0
Pacific Northwest (Dredged)Total5.0<1.0<1.0<1.0
Pathogenic<1.0<1.0<1.0<1.0
Pacific Northwest (Intertidal)cTotal6502.3<1.061
Pathogenic15<1.0<1.01.4
a Values rounded to 2 significant digits. See Appendix 7 for actual values of levels.
b The at-harvest levels are similar for the Gulf Coast (Louisiana) and Gulf Coast (non-Louisiana) regions; this is a function of the model construction. Differences between these regions occur in the post-harvest module because time from harvest to refrigeration is typically shorter for Louisiana compared to non-Louisiana states (Florida, Mississippi, Texas, and Alabama).
c Oysters harvested using intertidal methods are typically exposed to ambient air temperatures for longer times before refrigeration compared with dredged methods.

What environmental parameters (e.g., water temperature, salinity) can be used to predict the presence of V. parahaemolyticus in shellfish?

  • The primary driving factor to predict the presence of Vibrio parahaemolyticus in oysters is water temperature. Salinity was a factor evaluated but not incorporated into the model. Salinity is not a strong determinant of Vibrio parahaemolyticus levels in the regions that account for essentially all the commercial harvest. Other factors such as oyster physiology and disease status may also be important but no quantifiable data were available to include these factors in the model.
  • There are large differences in the predicted levels of V. parahaemolyticus in oysters at harvest among regions and seasons (see Table VII-1 above). For all regions, the highest levels of V. parahaemolyticus were predicted in the summer and spring and the lowest levels in the fall and winter. Overall, the highest levels of total and pathogenic V. parahaemolyticus were predicted for the Gulf Coast and the lowest levels in the Pacific Northwest (dredged).
  • After harvest, air temperature is also an important determinant of the levels of V. parahaemolyticus in oysters. Vibrio parahaemolyticus can continue to grow and multiply in oysters until they are adequately chilled.
  • Levels of Vibrio parahaemolyticus are lower in oysters after harvest in the cooler vs. warmer months (see Table VII-2 below). This means that reducing the time between harvest and cooling will be more important in the summer and spring than in the fall and winter.
Table VII-2. Predicted Mean Levels of Pathogenic Vibrio parahaemolyticus per Serving in Raw Oysters At-Harvest and At-Consumption
RegionPathway StepMean Predicted Levels of V. parahaemolyticus per Servinga
SummerFallWinterSpring
Gulf Coast (Louisiana)At-harvest7208018320
At-consumption21,0002,000987,900
Gulf Coast (Non-Louisiana)bAt-harvest At-consumption7208018320
At-consumption15,000880475,600
Mid-AtlanticAt-harvest At-consumption260181.266
At-consumption4,300110<1.01,500
Northeast AtlanticAt harvest At-consumption78121.214
At-consumption86017<1.0180
Pacific Northwest (Dredged)At-harvest At consumption24<1.0<1.04
At-consumption4601.2<1.042
Pacific Northwest (Intertidal)cAt-harvest At-consumption3,00010<1.0280
At-consumption7,50017<1.0740
a Values rounded to 2 significant digits. See Appendix 7 for actual values of levels.
b Includes oysters harvested from Florida, Mississippi, Texas, and Alabama. The time from harvest to refrigeration in these states is typically shorter than for Louisiana.
c Oysters harvested using intertidal methods are typically exposed to higher ambient air temperature for longer times before refrigeration compared with dredge methods.

How do levels of V. parahaemolyticus in shellfish at harvest compare to levels at consumption?

  • Absent mitigation treatments, levels of V. parahaemolyticus are higher in oysters at consumption then at harvest (see Table VII-2 above). The difference between V. parahaemolyticus densities at-harvest versus at-consumption is largely attributable to the extent of growth that occurs before the oysters are cooled to no-growth temperatures.
  • Levels of V. parahaemolyticus in oysters vary by region and season and are highest during the summer.
  • During intertidal harvest, oysters are exposed to ambient air temperatures for longer times, allowing additional growth of Vibrio parahaemolyticus in oysters and leading to higher predicted risk of illness.
  • Preventing growth of V. parahaemolyticus in oysters after harvest (particularly in the summer) will lower the levels of V. parahaemolyticus in oysters and as a consequence, lower the number of illnesses associated with the consumption of raw oysters.

What is the role of post-harvest handling on the level of V. parahaemolyticus in shellfish?

  • Post-harvest measures aimed at reducing the V. parahaemolyticus levels in oysters reduced the model-predicted risk of illness associated with this pathogen.
  • Reducing the time between harvest and chilling has a large impact on reducing levels of Vibrio parahaemolyticus in oysters and the number of illnesses. Predicted reductions were greater for shorter times to refrigeration using ice (oysters reach no-growth temperature in 1 hour) compared to cooling under conventional refrigeration (which may take up to 10 hours until oysters reach a no-growth temperature).

What reductions in risk can be anticipated with different potential intervention strategies?

  • Overall. The most influential factor predicted to affect risk of illness was the levels of total V. parahaemolyticus in oysters at harvest. Intervention strategies should be aimed at reducing levels of V. parahaemolyticus and/or preventing its growth in oysters after harvest. These strategies, either at-harvest or post-harvest, must consider regional/seasonal differences. For example, the use of ice on harvest boats to cool oysters to the no-growth temperature of V. parahaemolyticus will have a larger impact on reducing illnesses in the summer than in the winter when air temperatures are cooler and V. parahaemolyticus levels are lower.
  • Regional/seasonal Differences. Table VII-3 shows the relationship between the predicted number of illnesses and region/season combinations. The risk of V. parahaemolyticus illness is increased during the warmer months of the year, with the magnitude of this increase a function of the extent to which the growing waters (and ambient air temperatures) are at temperatures that support the growth of the pathogen (e.g., temperatures above 10°C). For each region, the predicted numbers of illnesses are much higher for the summer compared to the winter months. Intervention measures that depend on cooling oysters to no-growth temperatures for V. parahaemolyticus may be more important in warmer seasons and regions.

    The risk of V. parahaemolyticus illness is substantial in the Gulf Coast region where water temperatures are warm over a large part of the year as compared to the Northeast Atlantic region where water temperatures support the growth of V. parahaemolyticus only during a relatively small portion of the year. A difference is seen among the regions due to different harvesting methods. Within the Gulf Coast, the predicted number of illnesses is much higher in Louisiana compared to other states in this region because the harvest boats in Louisiana are typically on the water longer, i.e., leading to a longer time from harvest to refrigeration. Harvest volume is also a determining factor; in the summer, Louisiana accounts for approximately 77% of the Gulf Coast harvest. This is also seen in the Pacific Northwest by comparing intertidal versus dredged harvesting. Intertidal harvesting accounts for 75% of the Pacific Northwest harvest and exposes oysters to higher temperatures longer, allowing greater growth of V. parahaemolyticus. Overnight submersion for a single tidal cycle, reduces levels of V. parahaemolyticus in oysters and the risk of illness.

Table VII-3. Predicted Mean Annual Number of Illnesses Associated with the Consumption of Vibrio parahaemolyticus in Raw Oysters
RegionSummer
(July to September)
Fall
(October to December)
Winter
(January to March)
Spring
(April to June)
Total
Gulf Coast (Louisiana)1,40613275052,050
Gulf Coast (Non-Louisiana)a299513193546
Mid-Atlantic74<1415
Northeast Atlantic142<1319
Pacific Northwest (Dredged)b4<1<1<14
Pacific Northwest (Intertidal)b1731<118192
TOTAL1,903190107232,826
a Includes oysters harvested from Florida, Mississippi, Texas, and Alabama. The time from harvest to refrigeration in these states is typically shorter than for Louisiana.
b Oysters harvested using intertidal methods are typically exposed to higher ambient air temperature for longer times before refrigeration compared with dredged methods.
  • Post-Harvest Treatments. Measures aimed at reducing the levels of V. parahaemolyticus in oysters reduce the predicted risk of illness associated with this pathogen (Table VII-4). Post-harvest treatments that reduce levels of V. parahaemolyticus by 2 to 4.5-logs were found to be effective for all seasons and regions, with the most pronounced effects seen for regions and seasons with higher baseline risk. The model shows that any treatment that causes at least a 4.5-log decrease in the number of V. parahaemolyticus bacteria reduces the probability of illness to such an extent that few illnesses would be identified by epidemiological surveillance. However, some outbreak strains (e.g., O3:K6) are more resistant to mitigations than endemic pathogenic V .parahaemolyticus strains, and the duration or extent of treatment may need to be more stringent to achieve an equivalent degree of reduction. Studies have shown that both V. parahaemolyticus and V. vulnificus respond similarly to control measures such as ultra high pressure, mild heat treatment, and freezing. Therefore, mitigations aimed at decreasing levels of V. parahaemolyticus will also likely decrease levels of V. vulnificus.

    Table VII-4. Predicted Mean Number of Illnesses per Annum from Reduction of Levels of Pathogenic Vibrio parahaemolyticus in Oysters
    RegionPredicted Mean Number of Illnesses per Annum
    BaselineImmediate Refrigerationa2-log10 Reductionb4.5-log10 Reductionc
    Gulf Coast (Louisiana)2,05020222<1
    Gulf Coast (Non-Louisiana)546806<1
    Mid-Atlantic152<1<1
    Northeast Atlantic193<1<1
    Pacific Northwest (Dredged)4<1<1<1
    Pacific Northwest (Intertidal)1731002<1
    TOTAL2,82639130<1
    aRepresents refrigeration immediately after harvest; the effectiveness of which varies both regionally and seasonally and is typically approximately 1-log10 reduction.
    bRepresents any process which reduces levels of V. parahaemolyticus in oysters 2-log10 reduction, e.g. such as may be expected for freezing (-30°C).
    cRepresents any process which reduces levels of V. parahaemolyticus in oysters achieving a 4.5-log10 reduction, e.g. such as mild heat treatment (5 min at 50°C), irradiation, or ultra high hydrostatic pressure.

    The model also demonstrated that if oysters are not refrigerated soon after harvest, V. parahaemolyticus rapidly multiply resulting in higher levels. For example, the model indicates that for the Gulf Coast there is a significant reduction (~10-fold) in the probability of illness when the oysters are placed in a refrigerator immediately after harvest. Less pronounced reductions are predicted for the other regions. Predicted reduction in illness is less in colder seasons because oysters harvested in cooler weather are already at or below the temperature threshold for V. parahaemolyticus growth and as such refrigeration has little additional impact on levels of V. parahaemolyticus.
  • At-Harvest and At-Retail Controls. Controlling the levels of V. parahaemolyticus in oysters at-harvest or at-retail (after refrigeration and storage) drastically reduces the number of predicted illnesses but would require diversion of oysters from the raw market or modification of handling practices to reduce post-harvest Vibrio parahaemolyticus growth. For the Gulf Coast (Louisiana) region in the summer, excluding all oysters with at least 10,000 V. parahaemolyticus/g at-harvest would reduce illness by approximately 16% with an impact of approximately 3% of the total harvest; and this same control level at-retail would reduce illness by about 99% with a 43% loss from the raw consumption market. The effectiveness of the control level either at-harvest or at-retail to reduce illnesses depends on the extent of compliance with that control level (see Table VII-5).
Table VII-5. Effect of Compliance with Guidance Levels for Vibrio parahaemolyticus In Raw Oysters At-Harvest and At-Retail for the Gulf Coast (Louisiana)/ Summer Harvest
Guidance LevelaCompliance Level
(%)
At-HarvestAt-Retail
Harvest Diverted
(%)b
Illnesses Averted
(%)c
Harvest Diverted
(%)b
Illnesses Averted
(%)c
1005033654774
100669894100
1,0005011373769
100216875100
5,000503142663
10062853100
10,00050182260
1003164399
a Guidance level is the level of total V. parahaemolyticus per gram of oyster. Assumes that the level of V. parahaemolyticus is known either at the time of harvest or at retail.
b Refers to the amount of the total oyster harvest that would need to be diverted from the raw oyster market or subjected to preventive controls.
c Refers to the number of illnesses that would be prevented in comparison to the baseline model predictions.
  • In a sample-based control strategy, a reasonable surrogate for pathogenic V. parahaemolyticus may be total levels of this microorganism. Criteria for rejection of oysters based on the levels of this surrogate might have to vary by region. For example, an at-harvest control criterion based on total V. parahaemolyticus levels in the Pacific Northwest might need to be more stringent than in the Gulf Coast because the incidence of pathogenic strains appears to be higher in the Pacific Northwest. However, in an outbreak, the ratio of pathogenic to total V. parahaemolyticus may not be the same or consistent, and the model does not evaluate how well total Vibrio parahaemolyticus would serve as a surrogate for pathogenic V. parahaemolyticus in an outbreak situation.

In conclusion, the risk assessment illustrates that the levels of V. parahaemolyticus at-harvest play an important role in causing human illness. However, other factors that either reduce or allow growth of V. parahaemolyticus in oysters are also important in determining the number of illnesses. For example, shortening the time-to-refrigeration of oysters in the summer controls growth of V. parahaemolyticus in oysters and subsequently reduces illnesses associated with this microorganism.

The results of this risk assessment are influenced by the data and assumptions that were used to develop the Exposure Assessment and Dose-Response models. The predicted risk of illness among consumers of raw oysters and the most significant factors which influence the incidence of illness could change as a result of future data obtained from continuing surveillance studies. It is anticipated that periodic updates to the model when new data and knowledge become available will reduce the degree of uncertainty associated with the factors that influence the risk. This risk assessment provides an understanding of the relative importance and interactions among the factors influencing risk. It will hopefully provide a useful tool to facilitate the formulation of effective guidance and requirements and the evaluation of risk mitigation strategies.


Return to Table of Contents