##### ![](media/image1.wmf)Interpretive Summary: 

##### 

##### Quantitative Risk Assessment on the

**Public Health Impact of Pathogenic**

***Vibrio parahaemolyticus***

**In Raw Oysters**

Center for Food Safety and Applied Nutrition

Food and Drug Administration

U.S. Department of Health and Human Services

July 2005

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

**PREFACE**

This Interpretive Summary provides an overview of the 2004 Food and Drug
Administration (FDA) *Vibrio parahaemolyticus* risk assessment. Its
purpose is to briefly describe, in non-technical language, the material
covered in the complete risk assessment. This includes background
information on *Vibrio parahaemolyticus*, the techniques and data used
to develop the risk assessment, the results of the risk assessment, and
the interpretation, implications and limitations of those findings. A
full understanding of the risk assessment requires the reader to
consider the complete risk assessment. The complete risk assessment may
be obtained on the Internet at
[TUwww.cfsan.fda.govUT](http://www.cfsan.fda.gov/). A printed copy will
be provided upon request. Requests may be faxed to the CFSAN Outreach
and Information Center at 1-877-366-3322.

**INTRODUCTION**

*Vibrio parahaemolyticus* is a bacterium that occurs naturally in
coastal marine waters and estuaries (where rivers flow into the sea). It
is recognized world-wide as a significant cause of bacterial
seafood-borne illness. The United States Centers for Disease Control and
Prevention (CDC) estimates that of the approximately 7,880 *Vibrio*
illnesses each year in the United States, approximately 2,800 are
estimated to be associated with *Vibrio parahaemolyticus* and raw oyster
consumption. *Vibrio parahaemolyticus* is normally present in many types
of raw seafood, including fish, crustaceans, and molluscan shellfish. It
multiplies and colonizes in the gut of filter-feeding shellfish such as
oysters, clams, and mussels. Not all strains of *Vibrio
parahaemolyticus* cause illness; on the contrary, pathogenic strains
represent a small percentage of the total *Vibrio parahaemolyticus*
present in the environment or seafood.

FDA conducted this "product pathway" risk assessment to characterize the
factors influencing the public health impact associated with the
consumption of raw oysters containing pathogenic *Vibrio
parahaemolyticus.* This is referred to as a "product pathway" risk
assessment because the factors that influence the risk associated with
*Vibrio parahaemolyticus* in oysters are examined from harvest through
post-harvest handling to consumption. The risk assessment was conducted
in response to outbreaks in 1997 and 1998 in the United States involving
more than 700 cases of *Vibrio parahaemolyticus* illness. These
outbreaks renewed concern for this pathogen as a serious foodborne
threat to public health and raised concerns about the effectiveness of
the risk management guidance available at that time.

**SCOPE AND GENERAL APPROACH**

This risk assessment was initiated in January 1999 and a draft risk
assessment was made available for public comment in 2001. The draft risk
assessment has been modified to take into account public comments, to
incorporate additional scientific data and knowledge that has become
available since 2001, and to take advantage of improvements in modeling
techniques. Modifications made to the draft risk assessment are provided
in Summary Table 1.

**Summary Table 1. Modifications Made to the 2001 Draft *Vibrio
parahaemolyticus* Risk Assessment**

+------------------+---------------------------------------------------+
| **Topic**        | **Modifications**                                 |
+==================+===================================================+
| Assumptions      | Additional information was obtained that further  |
|                  | support the following assumptions:                |
|                  |                                                   |
|                  | -   Growth rates for pathogenic and               |
|                  |     non-pathogenic *Vibrio parahaemolyticus* are  |
|                  |     similar;                                      |
|                  |                                                   |
|                  | -   Time required for refrigerated oysters to     |
|                  |     cool down to temperatures that do not support |
|                  |     the growth of *Vibrio parahaemolyticus* is    |
|                  |     variable and may range from 1 to 10 hours.    |
+------------------+---------------------------------------------------+
| Additional Data/ | -   Prevalence of total and pathogenic *Vibrio    |
|                  |     parahaemolyticus* at harvest for Pacific      |
| Information      |     Northwest (PNW) and Gulf Coast regions;       |
|                  |                                                   |
|                  | -   Relationship between water temperature and    |
|                  |     *Vibrio parahaemolyticus* levels in oysters;  |
|                  |                                                   |
|                  | -   Time-to-refrigeration after harvest for the   |
|                  |     PNW region.                                   |
+------------------+---------------------------------------------------+
| Model/           | -   Included intertidal harvesting in the PNW as  |
|                  |     an additional harvest region;                 |
| Modeling         |                                                   |
| Techniques       | -   Evaluated mitigation effect of specific       |
|                  |     reduction levels of *Vibrio parahaemolyticus* |
|                  |     in addition to types of interventions;        |
|                  |                                                   |
|                  | -   Included regression-based sensitivity         |
|                  |     analysis;                                     |
|                  |                                                   |
|                  | -   Added two additional uncertainty parameters   |
|                  |     (total *Vibrio parahaemolyticus* in oysters   |
|                  |     based on water temperature and dose-response  |
|                  |     relationship) to the examination of factors   |
|                  |     that influence risk predictions;              |
|                  |                                                   |
|                  | -   Oyster meat weights at retail were used       |
|                  |     rather than those at harvest;                 |
|                  |                                                   |
|                  | -   Comparison of the model-predicted number of   |
|                  |     illnesses using both retail survey and        |
|                  |     epidemiological data.                         |
+------------------+---------------------------------------------------+

This risk assessment is based on a quantitative simulation model. The
focus is on raw oysters, because that is the food in the United States
predominately linked to outbreaks of illness associated with this
pathogen since 1997. The risk assessment examines events occurring from
oyster harvest to consumption that influence the levels of *Vibrio
parahaemolyticus* likely to be present in raw oysters at the time of
consumption. The levels of *Vibrio parahaemolyticus* in oysters at the
time of consumption are influenced by the harvest methods and
environmental conditions, as well as the handling of oysters after
harvest. These practices and conditions vary considerably among
different geographic areas and at different times of the year.
Therefore, the model was constructed to predict illnesses for each
harvest region and season in the United States. The likelihood and
severity of illness following exposure to pathogenic *Vibrio
parahaemolyticus* from consumption of raw oysters was estimated. 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 *Vibrio parahaemolyticus* from consumption of raw
oysters.

The risk assessment had two main objectives:

-   determine the factors that contribute to the risk of becoming ill
    from the consumption of pathogenic *Vibrio parahaemolyticus* in raw
    oysters

-   evaluate the likely public health impact of different control
    measures, including the effectiveness of current and alternative
    microbiological standards

**Risk Assessment FRAMEWORK**

Microbial risk assessments generally include four components: Hazard
Identification, Hazard Characterization, Exposure Assessment, and Risk
Characterization. These components are defined and discussed in detail
below.

-   U[Hazard Identification]{.underline}: Identifies the pathogenic
    microorganism (i.e., the hazard) that may be present in a particular
    food or group of foods that is capable of causing adverse health
    effects. The hazard on which this risk assessment is focused is
    pathogenic *Vibrio parahaemolyticus* in raw oysters. The adverse
    health effects are the primary illnesses caused by oral ingestion of
    *Vibrio parahaemolyticus*: gastroenteritis alone or gastroenteritis
    followed by septicemia.

-   U[Hazard Characterization/Dose-Response/Severity
    Assessment]{.underline}: Characterizes the relationship between the
    level of exposure to a pathogen (dose) and the likelihood of an
    adverse health effect for individuals within a population
    (response). For this risk assessment, a quantitative relationship
    was developed to predict the number and severity of illnesses
    resulting from ingestion of different amounts of pathogenic *Vibrio
    parahaemolyticus*. The risk assessment considered two
    subpopulations, "healthy" individuals, and individuals with impaired
    immune systems, in evaluating the likelihood of septicemia.

-   U[Exposure Assessment]{.underline}: Defines the frequency and likely
    level of exposure to *Vibrio parahaemolyticus* from consumption of
    raw oysters containing these microorganisms.

-   U[Risk Characterization:]{.underline} Integrates Dose-Response and
    Exposure Assessment to predict the probability of potential adverse
    outcomes for individuals within a population or a specified
    subpopulation. For this risk assessment the likelihood and severity
    of illness from the consumption of raw oysters containing pathogenic
    *Vibrio parahaemolyticus* were predicted. An important part of this
    step is determining the uncertainties associated with these
    predicted risk estimates distinguishing, to the extent possible,
    uncertainty from the inherent variation that occurs in any
    biological and environmental system.

Summary Figure 1 depicts a schematic representation of the components of
the *Vibrio parahaemolyticus* risk assessment model. The Exposure
Assessment model was separated into three modules: harvest,
post-harvest, and consumption. The model outputs from the Exposure
Assessment were then combined with the Dose-Response model to relate
these exposures to public health outcomes. The model inputs are
expressed as distributions instead of single point estimates (such as a
mean). Using a distribution allows a range of values, each with a
specific frequency of occurrence, to be included in the model.
Distributions are commonly used in simulation modeling to account for
the inherent biological variability in nature and our uncertainty of the
"true" values, resulting in a more accurate prediction of the risk.

Data for this risk assessment were obtained from many sources including
published and unpublished scientific literature and reports produced by
various organizations such as State shellfish control authorities, the
CDC, the shellfish industry, the Interstate Shellfish Sanitation
Conference (ISSC), and state health departments. In some instances, the
conduct of the risk assessment required that assumptions be made when
data were incomplete for the purposes of modeling. To the extent
possible, research undertaken to address the data gaps identified in the
2001 draft risk assessment have been incorporated into the model. The
criteria used to select data for the risk assessment modeling are
described in detail in the complete risk assessment.

For the risk assessment, 6 harvest regions and 4 seasons (winter,
spring, summer, and fall) were considered separately in the model for a
total of 24 region/season combinations (i.e., there were predictions of
illnesses for 24 regions/seasons). The oyster harvest regions included:
Gulf Coast (Louisiana), Gulf Coast (non-Louisiana), Mid-Atlantic,
Northeast Atlantic, Pacific Northwest (Dredged), and Pacific Northwest
(Intertidal). In the Gulf Coast, the harvest duration (i.e., time
between removal of the oysters from the water to unloading them at the
dock) for Louisiana is longer than for other states in that region
(Florida, Mississippi, Texas, and Alabama). Since harvest duration can
affect the levels of *Vibrio parahaemolyticus* in raw oysters, the Gulf
Coast was divided into these two distinct regions. The Pacific Northwest
was also divided into two regions, but in this case it was based on
harvest methods, intertidal versus dredged. Oysters harvested in
intertidal areas are typically exposed to higher temperatures before
refrigeration than those harvested using dredging, leading to the need
to define two harvest practice-based regions within the Pacific
Northwest.

**Summary Figure 1. Schematic Representation of the V*ibrio
parahaemolyticus* Risk Assessment Model** \[The light grey boxes with
black lettering show the Harvest Module, the gray boxes with black
lettering show the Post-Harvest Module, the dark grey boxes with white
lettering show the Consumption Module, the white boxes with black
lettering show the Dose-Response model, and the white boxes with dark
black outline show the Risk Characterization. Vp= *Vibrio
parahaemolyticus*\]

**HAZARD IDENTIFICATION**

*Vibrio parahaemolyticus* is a salt tolerant bacterium and a normal
inhabitant of the marine environment. This bacterium is found in many
types of seafood, including fish, crustaceans, and molluscan shellfish.
It was first isolated in 1950 and implicated in an outbreak of food
poisoning in Japan. In the United States, the first confirmed outbreak
of *Vibrio parahaemolyticus* illness occurred in Maryland in 1971. Since
1997, several large outbreaks, associated with the consumption of raw
oysters, have been reported in the United States. These outbreaks are
shown in Summary Table 2.

**Summary Table 2. Outbreaks of Illnesses from *Vibrio
parahaemolyticus***

**Associated with Consumption of Raw Oysters in the United States**

+------------------+-------------------+------------------------------+
| **Year**         | **Location**      | **Number of Cases**          |
+------------------+-------------------+------------------------------+
| 1997             | Pacific           | 209                          |
|                  | Northwest^a^      |                              |
+------------------+-------------------+------------------------------+
| 1998             | Pacific           | 48                           |
|                  | Northwest^a^      |                              |
| 1998             |                   | 416^b^                       |
|                  | Texas             |                              |
| 1998             |                   | 10                           |
|                  | Northeast         |                              |
|                  | Atlantic          |                              |
+------------------+-------------------+------------------------------+
| 2002             | New York          | 7                            |
+------------------+-------------------+------------------------------+
| 2002             | New Jersey        | 11                           |
+------------------+-------------------+------------------------------+
| 2004             | Alaska            | 46                           |
+------------------+-------------------+------------------------------+

^a^ The Pacific Northwest includes California, Oregon, Washington State,
and British Columbia.

^b^ 296 cases in Texas and 120 cases in other states that were traced
back to oysters harvested from Texas.

Human illnesses from ingestion of *Vibrio parahaemolyticus* have been
well documented. Any exposed individual can become infected with *Vibrio
parahaemolyticus* and develop illness. The most common clinical
manifestation of *Vibrio parahaemolyticus* infection is gastroenteritis,
an inflammation of the gastrointestinal tract. Gastroenteritis is
usually an illness of short duration and moderate severity that is
characterized by diarrhea, vomiting, and abdominal cramps. *Vibrio
parahaemolyticus* infections can also lead to septicemia, a severe,
life-threatening disease caused by the multiplication of pathogenic
microorganisms and/or the presence and persistence of their toxins in
circulating blood. Individuals with underlying chronic medical
conditions (such as diabetes, alcoholic liver disease, hepatitis, and
those receiving immunosuppressive treatments for cancer or AIDS) do not
appear to be at a higher risk of acquiring the initial infection than
otherwise healthy people. However, individuals with underlying chronic
conditions do appear to have a higher risk of the initial infection
developing into septicemia.

The CDC estimates that of the total *Vibrio* illnesses in the United
States (average 7,880 per year), there are approximately 4,500 *Vibrio
parahaemolyticus* illnesses and of those approximately 2,800 are
estimated to be associated with raw oyster consumption. There have been
reports of *Vibrio parahaemolyticus* illness associated with various
types of cooked and raw seafood including crayfish, lobster, shrimp,
crab, oysters, and clams. *Vibrio* illnesses associated with cooked
seafood are likely due to inadequate heating or recontamination after
cooking. Although thorough cooking destroys *Vibrio*, oysters are often
eaten raw, which may explain why it is the most common seafood
associated with *Vibrio* infection in the United States. Epidemiological
data indicate that consumers of raw oysters are 2.8 times more likely to
experience *Vibrio parahaemolyticus* illness compared to non-raw oyster
eaters. Food intake surveys indicate that raw shellfish is not a
commonly consumed food in the United States: only 10 to 20% of the
population consumes raw shellfish at least once a year. Among oyster
consumers, raw oysters are typically eaten approximately once every 6
weeks and the typical serving size ranges from 6 to 24 oysters, with 12
being the most frequent.

Not all strains of *Vibrio parahaemolyticus* cause illness; on the
contrary, pathogenic strains generally represent a small percentage of
the total *Vibrio parahaemolyticus* present in the environment or
seafood. Pathogenic *Vibrio parahaemolyticus* strains are more likely to
produce symptomatic infections and have one or more distinctive traits
that are generally absent in non-pathogenic strains. Two important
virulence indicators are the ability to produce thermostable direct
hemolysin (TDH) and the ability to produce a related toxin, thermostable
related hemolysin (TRH). Hemolysin is an enzyme that breaks down red
blood cells on a blood agar plate, which is referred to as the Kanagawa
phenomenon. The vast majority of *Vibrio parahaemolyticus* strains
isolated from the stools of patients with *Vibrio parahaemolyticus*
gastroenteritis are TDH-positive (TDH^+^). The role of traits other than
TDH has not yet been determined. Therefore, for the purposes of this
risk assessment, pathogenic *Vibrio parahaemolyticus* is defined as
strains that are TDH^+^.

*Vibrio parahaemolyticus* infections occur throughout the year, peaking
in spring and summer. Cases are most often associated with the regions
of the country within close proximity to marine environments. The
geographical distribution of cases attributed to oysters from specific
harvest areas likely reflects the propensity for individuals in close
proximity to coastal areas to consume raw shellfish. Likewise, the
volume of oysters harvested in the U.S. each year varies by season.
Approximately 66% of the annual oyster harvest occurs in the winter and
fall with the remainder in spring and summer. There are also regional
differences in the oyster harvest volume; the Gulf Coast accounts for
approximately one-half of the oyster harvest, the Pacific Northwest
about a fourth, and less than a tenth from the Mid-Altantic region. In
addition, regional climatic differences (e.g., water temperatures) and
post-harvest handling practices influence the levels of *Vibrio
parahaemolyticus* in shellfish and consequently the potential for
illness.

# Hazard Characterization

In a quantitative risk assessment, the Hazard Characterization typically
entails the determination of a dose-response relationship for a
specified population, relating the incidence of an identified adverse
effect with the level of exposure to a particular microorganism (or
substance). This dose-response relationship is often expressed as a
relation between different levels of exposure and the likelihood (or
probabilities) that such exposures will result in illness. For this risk
assessment, a quantitative relationship was developed to predict the
number and severity of illnesses resulting from ingestion of pathogenic
*Vibrio parahaemolyticus*.

A quantitative dose-response model for *Vibrio parahaemolyticus* was
developed based on human clinical feeding studies. The model
extrapolates the observed illness rates from the studies to doses and
illness rates that are more likely to be encountered with contaminated
oysters. Next, the dose-response curve was adjusted to account for the
estimates of the annual illness burden (2,800 cases per year) as
determined by CDC. This approach is typically referred to as "anchoring"
to epidemiological data. There is uncertainty in the dose-response
relationship because of the limited data from the clinical studies. This
uncertainty was accounted for in the model by multiple curve-fitting of
the data.

Summary Figure 2 shows the dose-response model. Using the most likely
estimate of the dose-response curve (i.e., the dashed line), the
probability of illness is approximately 0.5 (50%) for a dose of
approximately 100 million (i.e., 1x10^8^) *Vibrio parahaemolyticus*
cells/serving. This means for every 100 individuals eating a serving of
oysters that contains 1x10^8^ cells of pathogenic *Vibrio
parahaemolyticus*, approximately 50 individuals will become ill. At
lower exposure levels (1x10^3^ or 1x10^4^), the probability of illness
is much lower (\<0.001). Using the risk assessment results and available
epidemiological data, the likelihood that a *Vibrio parahaemolyticus*
illness (gastroenteritis) will lead to septicemia was determined for
healthy and immunocompromised individuals. (See section entitled, Risk
Characterization, for the results of the assessment.)

**Summary Figure 2**. **The Dose-Response Model for *Vibrio
parahaemolyticus (Vp)*** \[The solid line is the best estimate of the
model fit to pooled human feeding studies. The dashed line shows the
shift adjustment so that the model predictions agree with
epidemiological surveillance data. MLE denotes the maximum likelihood
estimate. ID~50~ is the dose corresponding to a 50% probability of
gastroenteritis.\]

**EXPOSURE ASSESSMENT**

The purpose of the Exposure Assessment is to determine the likelihood of
ingesting pathogenic *Vibrio parahaemolyticus* from consumption of raw
oysters, and the likely level of exposure. Insufficient data are
available on the levels of pathogenic *Vibrio parahaemolyticus* in raw
oysters at the moment of consumption. Therefore, the model predicts
these levels using available data on the factors that influence the
levels of the pathogen present in oysters at harvest. These factors
include the environmental conditions that contribute to the likely
presence of *Vibrio parahaemolyticus* in oysters at harvest and the
impact of post-harvest handling and processing practices on the growth
or decline of *Vibrio parahaemolyticus* in oysters prior to consumption.
In addition, the frequency of oyster meals and the amount of oysters
consumed per serving were considered.

The Exposure Assessment was divided into three modules that reflect the
chain of events from oyster harvest to consumption: Harvest,
Post-Harvest, and Consumption. The levels of total and pathogenic
*Vibrio parahaemolyticus* in oysters were estimated for each handling or
processing event. The predicted levels of *Vibrio parahaemolyticus* from
each module were used as inputs for the subsequent module (e.g., results
from the Harvest module served as the input to the Post-Harvest module).
Because *Vibrio parahaemolyticus* levels may be affected by climate and
region-specific oyster harvesting practices, modeling was conducted
separately for each of the 24 harvest region/season combinations
described in the "Risk Assessment Framework" section above.

**[Harvest Module]{.underline}.** In the Harvest Module of the Exposure
Assessment model, factors identified as potentially influencing the
variation of levels of *Vibrio parahaemolyticus* at the time of harvest
were evaluated and the effects of those factors that could be suitably
quantified were incorporated into the quantitative simulation model.

The available data suggest that a number of factors can affect the
presence and growth of *Vibrio parahaemolyticus* in oysters at the time
of harvest. Once present in the environment, *Vibrio parahaemolyticus*
levels are affected primarily by water temperatures and to a lesser
extent by salinity levels. Such factors as the amount of zooplankton in
the shellfish growing area, the rate of tidal flushing, levels of
dissolved oxygen in the water, and the presence of pollutants have less
certain effects on *Vibrio parahaemolyticus* levels. Oyster-specific
factors, such as the physiology and health of the oyster also contribute
to the ability of *Vibrio parahaemolyticus* to colonize and grow in the
oysters. Bacteriophages, toxins, or other proteins produced by bacterial
strains that infect or colonize oysters at the same time as *Vibrio
parahaemolyticus* may affect the survival of the *Vibrio
parahaemolyticus*. In addition, the percentage of the total *Vibrio
parahaemolyticus* that is pathogenic may vary. Several studies suggest
that the average percentage of pathogenic *Vibrio parahaemolyticus* is
higher on the West Coast than in other areas of the country.

Although a number of potential factors affecting *Vibrio
parahaemolyticus* levels at the time of harvest were identified, there
were little data available to quantify the effects of most of these
factors. Furthermore, accompanying analyses indicated that in most
instances water temperature is overwhelmingly the primary determinant
that controls *Vibrio parahaemolyticus* levels in oysters. Water
salinity was not included as a variable in the model because preliminary
modeling indicated that the small variability in water salinity in the
major commercial harvest regions was not a strong determinant of *Vibrio
parahaemolyticus* prevalence and growth in oysters. Additionally, the
impact on the model of varying water salinity was overshadowed by the
impact of varying water temperatures. Levels of pathogenic *Vibrio
parahaemolyticus* in oysters at-harvest were predicted using data on: 1)
the relationship between total *Vibrio parahaemolyticus* in oysters and
water temperature, 2) water temperature distributions, and 3) the ratio
of pathogenic to total *Vibrio parahaemolyticus* in oysters.

The relationship between total *Vibrio parahaemolyticus* levels in
oysters and water temperature was modeled based on the assumption that
*Vibrio parahaemolyticus* may be present at levels below the sensitivity
of the analytical method (e.g., less than the limit of detection) but
not actually zero, even at low temperatures. The distribution of
pathogenic *Vibrio parahaemolyticus* in oysters at harvest was
determined using the distribution of total *Vibrio parahaemolyticus* in
oysters at harvest and the appropriate pathogenic percentage for each
region (i.e., 2.3% for the Pacific Northwest and 0.18 % for the Gulf
Coast, Mid-Atlantic, and Northeast Atlantic regions).

Summary Table 3 provides the predicted mean levels of *Vibrio
parahaemolyticus* at harvest for each of the 24 region/season
combinations. Across all regions, the predicted levels are much higher
in the warmer months compared to the cooler months. The predicted levels
for the Gulf Coast region are considerably higher than the other regions
due to the warmer water temperatures. During the summer, the levels of
*Vibrio parahaemolyticus* in the mid-Atlantic and Northeast Atlantic are
higher than those of the Pacific Northwest (when harvest occurs by
dredging). Even during the summer, air temperatures in the Pacific
Northwest are cooler, on average, than in the Gulf and Atlantic regions.
However, exposure to higher temperatures for longer time periods, such
as occurs during intertidal harvest in some Pacific Northwest areas,
allows for additional growth, resulting in an increase of total and
pathogenic *Vibrio parahaemolyticus* to levels higher than that of the
Northeast Atlantic region.

**Summary Table 3. Predicted Mean Levels of *Vibrio parahaemolyticus
***per gram in Oysters At-Harvest

+-----------------+-----------+---------+---------+---------+--------+
| **Region**      | **Level** | **Summ  | **Fa    | **Wint  | *      |
|                 |           | er^a^** | ll^a^** | er^a^** | *Sprin |
|                 |           |         |         |         | g^a^** |
+-----------------+-----------+---------+---------+---------+--------+
| Gulf Coast      | Total     | 2,100   | 220     | 52      | 940    |
+-----------------+-----------+---------+---------+---------+--------+
| (Louisiana)^b^  | P         | 4       | \<1     | \<1     | 2      |
|                 | athogenic |         |         |         |        |
+-----------------+-----------+---------+---------+---------+--------+
| Gulf Coast      | Total     | 2,100   | 220     | 52      | 940    |
|                 |           |         |         |         |        |
| (No             |           |         |         |         |        |
| n-Louisiana)^b^ |           |         |         |         |        |
+-----------------+-----------+---------+---------+---------+--------+
|                 | P         | 4       | \<1     | \<1     | 2      |
|                 | athogenic |         |         |         |        |
+-----------------+-----------+---------+---------+---------+--------+
| Mid-Atlantic    | Total     | 780     | 51      | 3       | 200    |
+-----------------+-----------+---------+---------+---------+--------+
|                 | P         | 1       | \<1     | \<1     | \<1    |
|                 | athogenic |         |         |         |        |
+-----------------+-----------+---------+---------+---------+--------+
| Northeast       | Total     | 230     | 33      | 4       | 42     |
| Atlantic        |           |         |         |         |        |
+-----------------+-----------+---------+---------+---------+--------+
|                 | P         | \<1     | \<1     | \<1     | \<1    |
|                 | athogenic |         |         |         |        |
+-----------------+-----------+---------+---------+---------+--------+
| Pacific         | Total     | 5       | \<1     | \<1     | \<1    |
| Northwest       |           |         |         |         |        |
| (Dredged) ^c^   |           |         |         |         |        |
+-----------------+-----------+---------+---------+---------+--------+
|                 | P         | \<1     | \<1     | \<1     | \<1    |
|                 | athogenic |         |         |         |        |
+-----------------+-----------+---------+---------+---------+--------+
| Pacific         | Total     | 650     | 2       | \<1     | 61     |
| Northwest       |           |         |         |         |        |
| (Intertidal)    |           |         |         |         |        |
| ^d^             |           |         |         |         |        |
+-----------------+-----------+---------+---------+---------+--------+
|                 | P         | 15      | \<1     | \<1     | 1      |
|                 | athogenic |         |         |         |        |
+-----------------+-----------+---------+---------+---------+--------+

^a^ Predicted mean levels of total and pathogenic *Vibrio
parahaemolyticus* per gram of raw oysters. Values rounded to 2
significant digits. ^b^ Note: the values for Louisiana and non-Louisiana
areas are the same because the water temperature is similar for these
regions. Differences in the Gulf Coast states occur in the post-harvest
portion of the model (See Summary Table 4). ^c^ Predicted mean levels
when oyster reefs are submerged. ^d^ Predicted mean levels after
intertidal exposure.

**[Post-Harvest Module]{.underline}.** The Post-Harvest Module of the
Exposure Assessment model predicts the effects of typical industry
practices on *Vibrio parahaemolyticus* densities in oysters during
transportation, distribution, and storage from harvest through retail.
After oysters are harvested, levels of *Vibrio parahaemolyticus* can
increase or decline in oysters during handling and storage before
consumption. After harvesting, oysters are typically stored
unrefrigerated on the oyster boat for a period of time ranging from a
few hours to more than half a day. The potential growth of *Vibrio
parahaemolyticus* in the oysters during this period of unrefrigerated
holding is a function of the air temperature at the time of harvest and
the length of time oysters are unrefrigerated. Once the oysters are
placed under refrigeration (e.g., during transport or after arrival at
wholesalers), the rate of growth slows until oysters reach a "no-growth"
temperature (i.e., below 10˚C) for *Vibrio parahaemolyticus*. The length
of time during which *Vibrio parahaemolyticus* growth occurs after the
start of refrigeration and the (reduced) rate of growth during this
period of time were estimated. When held at a refrigeration temperature
of 45°F (7.2°C), levels of *Vibrio parahaemolyticus* decrease slowly as
cells die under this storage condition; this effect was included in the
Post-Harvest model. The post-harvest levels are carried forward to the
Consumption Module where the dose levels of *Vibrio parahaemolyticus*
consumed are modeled.

Summary Table 4 provides the predicted mean levels for total and
pathogenic *Vibrio parahaemolyticus* in oysters post-harvest. These
results, in comparison to those shown in Summary Table 3, are indicative
of the effects of current post-harvest handling and processing practices
on *Vibrio parahaemolyticus* levels. The predicted total and pathogenic
*Vibrio parahaemolyticus* levels in oysters post-harvest are highest in
both the Louisiana and non-Louisiana Gulf Coast regions because the
levels at-harvest were the highest and ambient temperature is much
higher in this region than in the other regions, allowing for more
growth. Predicted levels in the Gulf Coast (Louisiana) are higher than
those in the Gulf Coast (non-Louisiana), reflecting a longer time from
harvest to refrigeration. The type of harvesting also has an impact on
the levels of *Vibrio parahaemolyticus*. In the Pacific Northwest, the
typically longer exposure to warmer air temperatures during intertidal
harvesting can elevate oyster temperatures, allowing for additional
growth of *Vibrio parahaemolyticus* during intertidal harvesting.

**Summary Table 4. Predicted Mean Levels of Total and Pathogenic *Vibrio
parahaemolyticus ***per gram in Oysters Post-Harvest

  ------------------ ------------ --------------- ------------- --------------- ---------------
  **Region**         **Level**    **Summer^a^**   **Fall^a^**   **Winter^a^**   **Spring^a^**

  Gulf Coast         Total        60,000          5,700         290             23,000

  (Louisiana)        Pathogenic   100             10            \<1             39

  Gulf Coast         Total        42,000          2,500         135             16,000

  (Non-Louisiana)    Pathogenic   73              4             \<1             28

  Mid-Atlantic       Total        12,000          310           1               4,200

                     Pathogenic   21              \<1           \<1             7

  Northeast Atlantic Total        2,500           52            1               510

                     Pathogenic   4               \<1           \<1             \<1

  Pacific Northwest  Total        100             \<1           \<1             9
  (Dredged) ^b^                                                                 

                     Pathogenic   2               \<1           \<1             \<1

  Pacific Northwest  Total        1,700           4             \<1             150
  (Intertidal) ^c^                                                              

                     Pathogenic   38              \<1           \<1             4
  ------------------ ------------ --------------- ------------- --------------- ---------------

^a^ Predicted mean levels of total and pathogenic *Vibrio
parahaemolyticus* per gram of raw oysters. Values rounded to 2
significant digits. ^b^ Predicted mean levels when oyster reefs are
submerged. ^c^ Predicted mean levels after intertidal exposure.

**[Consumption Module]{.underline}.** The Consumption Module of the
Exposure Assessment model estimates the levels of total and pathogenic
*Vibrio parahaemolyticus* in a single serving of an oyster meal. The
number of oyster meals or servings eaten, the quantity of oysters
consumed per serving, and the pathogenic *Vibrio parahaemolyticus*/g
oyster at consumption are included in this module. The number of
servings eaten refers to the oysters harvested from a specific region.
As such, the risk assessment model predicts illness associated with
oysters harvested from specific regions but does not predict illness
associated with the location (region) where the oysters were consumed or
illness reported. Summary Table 5 provides the mean predicted levels of
total and pathogenic *Vibrio parahaemolyticus* at consumption.

**Summary Table 5. Predicted Mean Levels of Total and Pathogenic *Vibrio
parahaemolyticus*** per Serving of Oysters at Consumption

+---------------------+--------+---------+--------+-------+---------+
| **Region**          | **L    | **Summ  | **Fal  | **W   | **Spri  |
|                     | evel** | er^a^** | l^a^** | inter | ng^a^** |
|                     |        |         |        | ^a^** |         |
+---------------------+--------+---------+--------+-------+---------+
| Gulf Coast          | Total  | 12,     | 1,2    | 5     | 4,      |
| (Louisiana)         |        | 000,000 | 00,000 | 8,000 | 600,000 |
+---------------------+--------+---------+--------+-------+---------+
|                     | Path   | 21,000  | 2,000  | 98    | 7,900   |
|                     | ogenic |         |        |       |         |
+---------------------+--------+---------+--------+-------+---------+
| Gulf Coast          | Total  | 8,      | 5      | 2     | 3,      |
| (Non-Louisiana)     |        | 500,000 | 00,000 | 7,000 | 200,000 |
+---------------------+--------+---------+--------+-------+---------+
|                     | Path   | 15,000  | 880    | 47    | 5,600   |
|                     | ogenic |         |        |       |         |
+---------------------+--------+---------+--------+-------+---------+
| Mid-Atlantic        | Total  | 2,      | 62,000 | 280   | 850,000 |
|                     |        | 500,000 |        |       |         |
+---------------------+--------+---------+--------+-------+---------+
|                     | Path   | 4,300   | 110    | \<1   | 1,500   |
|                     | ogenic |         |        |       |         |
+---------------------+--------+---------+--------+-------+---------+
| Northeast Atlantic  | Total  | 500,000 | 11,000 | 300   | 100,000 |
+---------------------+--------+---------+--------+-------+---------+
|                     | Path   | 860     | 17     | \<1   | 180     |
|                     | ogenic |         |        |       |         |
+---------------------+--------+---------+--------+-------+---------+
| Pacific Northwest   | Total  | 21,000  | 46     | 2     | 1,900   |
|                     |        |         |        |       |         |
| (Dredged) ^b^       |        |         |        |       |         |
+---------------------+--------+---------+--------+-------+---------+
|                     | Path   | 460     | 1      | \<1   | 43      |
|                     | ogenic |         |        |       |         |
+---------------------+--------+---------+--------+-------+---------+
| Pacific Northwest   | Total  | 330,000 | 800    | 3     | 30,000  |
| (Intertidal) ^c^    |        |         |        |       |         |
+---------------------+--------+---------+--------+-------+---------+
|                     | Path   | 7,500   | 17     | \<1   | 740     |
|                     | ogenic |         |        |       |         |
+---------------------+--------+---------+--------+-------+---------+

^a^ Predicted mean levels of total and pathogenic *Vibrio
parahaemolyticus* per serving of raw oysters. Values rounded to 2
significant digits. ^b^ Predicted mean levels when oyster reefs are
submerged. ^c^ Predicted mean levels after intertidal exposure.

**Risk characterization**

The Risk Characterization combines the results of the Exposure
Assessment model with the Dose-Response model to predict the number of
illnesses and the severity of illness associated with different regions
and seasons. Estimates of the uncertainty associated with these
predictions of risk and illness burden (i.e., upper and lower bounds)
are also determined. For simplicity, the results of these regional and
seasonal predictions of illness are presented below as the mean of the
distribution (i.e., the mean number of predicted illnesses). A detailed
description of the uncertainty distributions can be found in the
complete risk assessment. Sensitivity analyses were conducted to
evaluate the importance of the various input factors on the model
results. The model was validated by comparing the results to a retail
study and epidemiological data.

**Predicted Illness Burden**

[Risk per Serving.]{.underline} The "risk per serving" is the risk of an
individual becoming ill (gastroenteritis alone or gastroenteritis
followed by septicemia) when he or she consumes a single serving of
oysters. As shown in Summary Table 6, the predicted risk per serving is
highest for the Gulf Coast (Louisiana) region and lowest for Pacific
Northwest (Dredged) region. Within a region, the risk per serving is
highest for the warmer months and lowest for the cooler months. For
example, for the Northeast Atlantic, the risk per serving in the winter
is on the order of 1x10^-8^, meaning only one illness in every 100
million servings. For this same region, the risk per serving in the
summer is approximately 3 orders of magnitude higher (one illness in
every 100,000 servings). For the Pacific Northwest region during the
summer and spring, the risk per serving is higher for oysters harvested
by intertidal compared with dredged methods.

^Summary\ Table\ 6.\ Predicted\ Mean\ Risk\ per\ Serving\ Associated\ with\ the\ Consumption\ of\ Pathogenic\ *Vibrio\ parahaemolyticus*\ in\ Raw\ Oysters^

  ---------------------- -------------- ---------- ------------ ------------ -----------
  **Region**             **Mean Risk                                         
                         Per                                                 
                         Serving^a^**                                        

                         **Summer**     **Fall**   **Winter**   **Spring**   **Total**

  Gulf Coast (Louisiana) 4.4 x 10^-4^   4.3 x      2.1 x        1.7 x        6.6 x
                                        10^-5^     10^-6 ^      10^-4 ^      10^-4^ 

  Gulf Coast             3.1 x 10^-4^   1.9 x      1.1 x        1.2 x 10^-4^ 4.5 x
  (Non-Louisiana)^b^                    10^-5^     10^-6^                    10^-4^ 

  Mid-Atlantic           9.2 x 10^-5^   2.2 x      1.1 x        3.1 x        1.3 x
                                        10^-6^     10^-8^       10^-5^       10^-4^ 

  Northeast Atlantic     1.8 x 10^-5^   4.0 x      1.1 x        3.6 x        2.2 x
                                        10^-7^     10^-8^       10^-6^       10^-5^

  Pacific Northwest      1.0 x 10^-5^   2.6 x      8.1 x        8.7 x        1.1 x
  (Dredged)                             10^-8^     10^-10^      10^-7^       10^-5^ 

  Pacific Northwest      1.4 x 10^-4^   3.9 x      1.7 x        1.3 x 10^-5^ 1.5 x
  (Intertidal)^c^                       10^-7^     10^-9^                    10^-4^ 
  ---------------------- -------------- ---------- ------------ ------------ -----------

^a^ Risk per serving refers to the predicted risk of an individual
becoming ill (gastroenteritis alone or gastroenteritis followed by
septicemia) when he or she consumes a single serving of raw oysters.

^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 temperature for longer times before refrigeration compared with
dredged methods.

[Risk per Annum]{.underline}**.** The "risk per annum" is the predicted
number of illnesses (gastroenteritis alone or gastroenteritis followed
by septicemia) in the United States each year. As shown in Summary Table
7, for each region, the highest number of predicted cases of illnesses
is associated with oysters harvested in the summer and spring and the
lowest in the winter and fall. Of the total annual predicted *Vibrio
parahaemolyticus* illnesses, approximately 92% are attributed to oysters
harvested from the Gulf Coast (Louisiana and non-Louisiana states)
region in the spring, summer and fall and from the Pacific Northwest
(intertidal) region in the summer. The lower numbers of illnesses
predicted for the Northeast Atlantic and Mid-Atlantic oyster harvests
are attributable both to the colder water temperatures and the smaller
harvest from these regions. The harvesting practice also has an impact
on the illness rate. Intertidal harvesting in the Pacific Northwest
poses a much greater risk than dredging in this region (192 vs. 4
illnesses per year). This is likely attributable to elevation of oyster
temperatures during intertidal exposure leading to *Vibrio
parahaemolyticus* growth.

**Summary Table 7. Predicted Mean Annual Number of Illnesses Associated
with the Consumption of *Vibrio parahaemolyticus*** in Raw Oysters

  ------------------------- ---------------- ---------- ------------ ------------ -----------
  **Region**                **Mean Annual                                         
                            Illnesses^a^**                                        

                            **Summer**       **Fall**   **Winter**   **Spring**   **Total**

  Gulf Coast (Louisiana)    1,406            132        7            505          2,050

  Gulf Coast                299              51         3            193          546
  (Non-Louisiana)^b^                                                              

  Mid-Atlantic              7                4          \<1          4            15

  Northeast Atlantic        14               2          \<1          3            19

  Pacific Northwest         4                \<1        \<1          \<1          4
  (Dredged)                                                                       

  Pacific Northwest         173              1          \<1          18           192
  (Intertidal)^c^                                                                 

  **TOTAL**                 1,903            190        10           723          2,826
  ------------------------- ---------------- ---------- ------------ ------------ -----------

^a^ Mean annual illnesses refers to the predicted number of illnesses
(gastroenteritis alone or gastroenteritis followed by septicemia) in the
United States each year.

^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 temperature for
longer times before refrigeration compared with dredged methods.

[Severity of Illness]{.underline}. The predicted number of cases of
septicemia was determined for the total United States population as
shown in Summary Table 8. The number of predicted cases of septicemia
was calculated by multiplying the mean number of predicted illnesses
(Summary Table 7) by the probability of gastroenteritis progressing to
septicemia (0.0023). The calculation of the probability of
gastroenteritis progressing to septicemia is described in the complete
risk assessment. Since most of the cases of illness are predicted to be
associated with the Gulf Coast (Louisiana) harvest, this is also the
harvest that would be expected to be associated with the highest number
of cases of septicemia.

Anyone exposed to *Vibrio parahaemolyticus* can become infected and
develop gastroenteritis. However, compared to the healthy population,
there is about a 40-fold higher probability of an infected individual
with a concurrent underlying chronic medical condition developing
septicemia. The model predicts about 7 cases of septicemia each year for
the total population, of which 2 would be expected to occur in healthy
individuals and 5 would be expected to occur among the immunocompromised
population.

**Summary Table 8. Predicted Mean Number of Cases of *Vibrio
parahaemolyticus*** Septicemia Associated with the Consumption of Raw
Oysters

  ------------------------- ----------------- ---------- ------------ ------------ -----------
  **Region**                **Mean Annual                                          
                            Cases of                                               
                            Septicemia^a^**                                        

                            **Summer**        **Fall**   **Winter**   **Spring**   **Total**

  Gulf Coast (Louisiana)    3                 \<1        \<1          1            4

  Gulf Coast                \<1               \<1        \<1          \<1          1
  (Non-Louisiana)^b^                                                               

  Mid-Atlantic              \<1               \<1        \<1          \<1          \<1

  Northeast Atlantic        \<1               \<1        \<1          \<1          \<1

  Pacific Northwest         \<1               \<1        \<1          \<1          \<1
  (Dredged)                                                                        

  Pacific Northwest         \<1               \<1        \<1          \<1          \<1
  (Intertidal)                                                                     

  **TOTAL**                 4                 \<1        \<1          2            7
  ------------------------- ----------------- ---------- ------------ ------------ -----------

^a^ Calculated by multiplying the probability of septicemia (0.0023) by
the mean predicted number of illnesses (see Summary Table 7). ^b^
Includes oysters harvested from Florida, Mississippi, Texas, and
Alabama. The typical time from harvest to refrigeration of oysters for
these states is shorter than for Louisiana.

**Sensitivity Analysis**

A sensitivity analysis was conducted to determine which model input
factors have the strongest influence on the predicted probability of
illness. A representative example of this type of evaluation is shown in
Summary Figure 3. The graph (referred to as a Tornado Plot) shows the
rank and magnitude of influence of factors (from highest to lowest) on
the probability of illness. For example, in the Gulf Coast (Louisiana)
Summer harvest, the model prediction of risk is influenced the most by
the level of *Vibrio parahaemolyticus* in the environment and secondly
by the percent of pathogenic *Vibrio parahaemolyticus* in oysters at the
time of harvest. The length of time oysters are unrefrigerated after
harvest and air temperature are also important factors. The ranking is
similar for all regions, except for intertidal-harvested oysters in the
Pacific Northwest. For the Pacific Northwest intertidal harvest, the
second and third most influential factors are air and oyster
temperatures. Thus, for this region, higher levels of risk are
associated with oysters that have been collected on warm sunny days.
Since the levels of *Vibrio parahaemolyticus* decrease during cold
storage, the length of time the oysters are refrigerated is negatively
correlated with the risk for all regions and seasons and the factor
points to the left rather than to the right on the Tornado Plot.

![](media/image2.wmf){width="5.827083333333333in"
height="3.9618055555555554in"}**Summary Figure 3. Tornado Plot of
Influential Variability Factors of *Vibrio parahaemolyticus* (Vp)
Illness per Serving of Raw Oysters in the Gulf Coast (Louisiana) Summer
Harvest**

**Model Validation**

Exposure predictions were validated by comparing predicted *Vibrio
parahaemolyticus* levels in oysters at the time of consumption to data
from a 1998-1999 survey of *Vibrio parahaemolyticus* levels in oysters
at retail conducted collaboratively by the Interstate Shellfish
Sanitation Conference (ISSC) and the FDA (Summary Figure 4). These data
were not used in the development of the risk assessment model. In
general, the mean *Vibrio parahaemolyticus* levels predicted by the
model compared well with the mean levels from the ISSC/FDA survey,
particularly for the Gulf and Mid-Atlantic summer when the risk of
illness is highest. For the Pacific Northwest, the model predictions are
higher than the ISSC/FDA estimates, but there is substantial uncertainty
associated with the ISSC/FDA data for this region due to the relatively
small number of samples. Based on the generally good agreement between
model-predicted *V. parahaemolyticus* densities and observed densities
at retail, the exposure assessment portion of the model is considered to
be validated.

**Summary Figure 4. Observed log~10~ Density of Total *Vibrio
parahaemolyticus* at Retail Compared to Model Predictions for the Gulf
Coast (Louisiana and non-Louisiana states)** \[The error bars indicate
one standard deviation above and below either the model predictions
(square box) or observed values (filled circle).\]

The corresponding validation of the risk estimates based on a comparison
of the risk assessment predictions and available epidemiological data
showed a higher degree of uncertainty. The surveillance data reported to
CDC are the only data available to validate the model predictions of
illness for each region and season. Temporally, the model predictions
and CDC data both indicate that the risk of illness is higher in the
spring and summer than in the winter and fall. However, agreement
between the surveillance data and the regional predictions of risk were
less clear cut, though both showing similar trends (e.g., the highest
number of illnesses are associated with Gulf Coast oysters followed by
Pacific Northwest oysters). In part, this uncertainty reflects the fact
that the surveillance data indicate where (location) the illness
occurred and the model predicts illnesses attributed to where (region)
oysters were harvested. It is difficult to trace the oysters that caused
an illness back to the harvest region. Because of the intrinsic
difference in what the two systems measure (location of illness
occurrence vs. harvest region of oysters that cause illness), full
validation of the regional model predictions of illness based on
regional surveillance data will require additional research and targeted
surveillance initiatives with more thorough traceback data.

**What-If Scenarios**

The risk assessment model can be used to estimate the likely impact of
intervention strategies on the predicted number of illnesses. The impact
of different harvesting methods, seasons (i.e., water and air
temperatures), time until refrigeration, and length of storage before
consumption were parameters considered in the baseline model. By
changing one or more of the input parameters and measuring the resulting
change in the model outputs, the likely impact of new or different
processing procedures or regulatory actions can be evaluated. These
changes to the baseline model are commonly referred to as "what-if"
scenarios. The what-if scenarios evaluated include the following:
reducing levels of *Vibrio parahaemolyticus* in oysters (representing
various post-harvest mitigation controls); reducing
time-to-refrigeration; re-submersion of intertidally harvested oysters;
and sample-based control plans.

[**Reducing Levels of *Vibrio parahaemolyticus* in
Oysters**.]{.underline}

Post-harvest mitigation control scenarios included an evaluation of
treatments that reduce levels of *Vibrio parahaemolyticus* in oysters.
The reduction levels represent a range of potential mitigation controls:
immediate refrigeration (i.e., cooling immediately after harvest); 2-log
reduction (e.g., freezing and cold storage); and 4.5-log reduction
(e.g., mild heat treatment, irradiation, or ultra high hydrostatic
pressure). The effectiveness of immediate refrigeration may be expected
to vary both regionally and seasonally and is typically approximately
1-log reduction.

Measures that control or reduce the levels of *Vibrio parahaemolyticus*
in oysters reduced the predicted risk of illness associated with this
pathogen (Summary Table 9). Treatment such as immediate refrigeration
decreased the number of predicted illnesses by approximately 10-fold.
The effect of immediate refrigeration is less pronounced in the cooler
regions than in the warmer Gulf Coast. Treatment causing a 2-log
decrease in the levels of *Vibrio parahaemolyticus* in oysters reduces
the probability of illness by approximately 100-fold. Treatment causing
a 4.5-log decrease in the number of *Vibrio parahaemolyticus* bacteria
reduces predicted illness to an extent that makes it unlikely that
illnesses would be observed.

**Summary Table 9. Predicted Mean Number of Illnesses per Annum from
Reduction of Levels of Pathogenic *Vibrio parahaemolyticus*** in Oysters

  ----------------------- -------------- -------------------- ------------------ ----------------
  **Region**              **Predicted                                            
                          Mean Number of                                         
                          Illnesses per                                          
                          Annum**                                                

                          **Baseline**   **Immediate          **2-log**          **4.5-log
                                         Refrigeration^a^**   **Reduction^b^**   Reduction^c^**

  Gulf Coast (Louisiana)  2,050          202                  22                 \<1

  Gulf Coast              546            80                   6                  \<1
  (Non-Louisiana)                                                                

  Mid-Atlantic            15             2                    \<1                \<1

  Northeast Atlantic      19             3                    \<1                \<1

  Pacific Northwest       4              \<1                  \<1                \<1
  (Dredged)                                                                      

  Pacific Northwest       192            106                  2                  \<1
  (Intertidal)                                                                   

  **TOTAL**               2,826          391                  30                 \<1
  ----------------------- -------------- -------------------- ------------------ ----------------

^a^ Represents refrigeration immediately after harvest; the
effectiveness of which varies both regionally and seasonally and is
typically approximately 1-log reduction.

^b^ Represents any process which reduces levels of *Vibrio
parahaemolyticus* in oysters 2-log, e.g., freezing.

^c^ Represents any process which reduces levels of *Vibrio
parahaemolyticus* in oysters 4.5-log, e.g., mild heat treatment,
irradiation, or ultra high hydrostatic pressure.

**[Reducing the Time-to-Refrigeration]{.underline}**

For this scenario, the impact of "rapid" cooling (i.e., using ice or an
ice slurry after harvest) such that oysters would be chilled to a
"no-growth" temperature (\<10˚C) within 1 hour of harvest were compared
to "conventional" cooling (i.e., refrigeration after harvest) such that
up to 10 hours were presumed for oysters to reach the no-growth
temperature. For the Gulf Coast Louisiana/ Summer harvest, the greatest
reductions were predicted for shorter times to refrigeration and using
cooling with ice compared to cooling under conventional refrigeration
(Summary Figure 5). Predicted reduction in *Vibrio parahaemolyticus*
illnesses from oysters cooled within 1 hour after harvest ranged from
86% (conventional refrigeration) to 97% (cooling with ice). The lower
temperatures associated with the other regions result in predicted
reductions that are less dramatic.

**Summary Figure 5. Predicted Effectiveness of Two Different Methods of
Cooling on *Vibrio parahaemolyticus* Risk for the Gulf Coast Region
(Louisiana and non-Louisiana) Summer Harvest** \[Errors bars denote
central 95% of uncertainty distribution about the mean % reduction.\]

**[Re-submersion of Intertidally Harvested Oysters]{.underline}**

As an example of a harvest practice scenario, the impact of overnight
submersion of oysters was evaluated. The model predicts the levels of
*Vibrio parahaemolyticus* in intertidally-harvested oysters, e.g.,
oysters are placed into baskets and removed after the tide rises, a
typical practice in the Pacific Northwest. *Vibrio* *parahaemolyticus*
levels can increase in oysters during intertidal exposure but overnight
submersion of the oysters in water has been shown to reduce these
levels. Delaying harvest until near the end of the tidal cycle, just
before oysters are exposed again, was predicted to reduce the risk of
illness by approximately 90%. Research is needed to determine whether
the predicted level of reduction can be achieved when oysters are
stacked in baskets.

**[Sample-Based Control Plans]{.underline}**

The FDA/ISSC recommends that the levels of *Vibrio parahaemolyticus* in
oysters not exceed 10,000 cells/gram and the ISSC interim control plan
(ICP) recommends monitoring of oyster meats for the presence of *Vibrio
parahaemolyticus*. Prior to 2001, ISSC recommended that shellfish
harvest waters be re-sampled for pathogenic *Vibrio parahaemolyticus* if
the levels of total *Vibrio parahaemolyticus* in oyster meats at harvest
exceed 10,000 cells/gram. In 2001, the ICP was revised to recommend that
shellfish harvest waters be re-sampled for pathogenic *Vibrio
parahaemolyticus* if the levels of total *Vibrio parahaemolyticus* in
oyster meats at harvest are above 5,000 cells/gram.

The incidence of illness was evaluated assuming that it was possible to
identify and exclude oysters from the raw market which contained various
specified levels of *Vibrio parahaemolyticus* either at harvest or at
retail. The Gulf Coast region (Louisiana)/ Summer harvest is presented
here as an example. As shown in Summary Figures 6 and 7, restricting the
levels of *Vibrio parahaemolyticus* in oysters either at-harvest or
at-retail reduces the number of predicted illnesses, but requires
diversion of oysters from the raw market (or modification of handling
practices to reduce post-harvest *Vibrio parahaemolyticus* growth). For
the Gulf Coast region (Louisiana) summer harvest, in the absence of
subsequent post-harvest mitigation, excluding oysters containing
\>10,000 *Vibrio parahaemolyticus*/g at the time of harvest is predicted
to prevent approximately 16% of illnesses with an impact of
approximately 3% of the oyster harvest(Summary Figure 6). However,
excluding oysters containing \>10,000 *Vibrio parahaemolyticus*
at-retail reduced predicted illness by 99% but would require
approximately 43% of the oyster harvest to be diverted from the raw
market consumption (or subjected to preventive controls). The impact of
compliance with different "at-harvest" and "at-retail" (i.e., after
refrigeration) control levels was also evaluated. As might be expected,
the effectiveness of a specific (or hypothetical) control level to
reduce illnesses depend was proportional to the extent of compliance
with that level.

**Summary Figure 6. Potential Effect of Control of Total *Vibrio
parahaemolyticus* Bacterium per gram At-Harvest for the Gulf Coast
Region (Louisiana) Summer Harvest**

**Summary Figure 7. Potential Effect of Control of Total *Vibrio
parahaemolyticus* per gram At-Retail for the Gulf Coast Region
(Louisiana) Summer Harvest**

**CONCLUSIONS**

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 *Vibrio parahaemolyticus* in raw oysters.
The assessment focuses on comparing the relative risk among different
geographic regions, seasons, and harvest practices. The scientific data
and the mathematical models developed during the risk assessment
facilitate a systematic evaluation of strategies to reduce the public
health impact of pathogenic *Vibrio parahaemolyticus* associated with
the consumption of raw oysters.

Although the risk assessment modeled sporadic *Vibrio 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
*Vibrio 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 *Vibrio parahaemolyticus* and illnesses?**

-   Although an individual may become ill from consumption of low levels
    of *Vibrio 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 *Vibrio
    parahaemolyticus* cells/serving (equivalent to about 50 cells/gram
    oysters). Consumption of about 100 million *Vibrio parahaemolyticus*
    cells/serving (500 thousand cells/gram oysters) increases the
    probability of illness to about 50%.

-   Anyone exposed to *Vibrio 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 *Vibrio 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 *Vibrio
    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 *Vibrio parahaemolyticus* O3:K6 are required to
    cause illness compared to other strains.

**What is the frequency and extent of pathogenic strains of *Vibrio
parahaemolyticus* in shellfish waters and in oysters?**

-   Levels of pathogenic *Vibrio parahaemolyticus* usually occur at low
    levels in shellfish waters.

-   Levels of pathogenic *Vibrio parahaemolyticus* in oysters at the
    time of harvest are only a small fraction of the total *Vibrio
    parahaemolyticus* levels.

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

-   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 *Vibrio
    parahaemolyticus* in oysters at harvest among regions and seasons.
    For all regions, the highest levels of *Vibrio parahaemolyticus*
    were predicted in the warmer months of summer and spring and the
    lowest levels in the fall and winter.

-   Overall, the highest levels of total and pathogenic *Vibrio
    parahaemolyticus* were predicted for the Gulf Coast (Louisiana) and
    the lowest levels in the Pacific Northwest (Dredged) harvested
    oysters.

-   After harvest, air temperature is also an important determinant of
    the levels of *Vibrio 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. 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.

**How do levels of *Vibrio parahaemolyticus* in oysters at harvest
compare to levels at consumption?**

-   With no mitigation treatments, levels of *Vibrio parahaemolyticus*
    are higher in oysters at consumption than at harvest. The difference
    between *Vibrio 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 *Vibrio parahaemolyticus* in oysters vary by region and
    season and are highest during the summer.

-   During intertidal harvest, oysters are exposed to higher
    temperatures for longer times, allowing additional growth of *Vibrio
    parahaemolyticus* in oysters and leading to higher predicted risk of
    illness.

-   Preventing growth of *Vibrio parahaemolyticus* in oysters after
    harvest (particularly in the summer) will lower the levels of
    *Vibrio 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 *Vibrio
parahaemolyticus* in** **oysters?**

-   Post-harvest measures aimed at reducing the *Vibrio
    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.]{.underline} The most influential factor affecting
    predicted risk of illness is the level of total *Vibrio
    parahaemolyticus* in oysters at the time of harvest. Intervention
    strategies should be aimed at reducing levels of *Vibrio
    parahaemolyticus* and/or preventing its growth in oysters after
    harvest. These strategies, either at-harvest or post-harvest, may
    need to consider regional/seasonal differences. For example, the use
    of ice on harvest boats to cool oysters to the no-growth temperature
    of *Vibrio parahaemolyticus* will have a larger impact on reducing
    illnesses in the summer than in the winter when air temperatures are
    cooler and *Vibrio parahaemolyticus* levels are lower.

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-   [Regional/Seasonal Differences]{.underline}. The risk of *Vibrio
    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 air temperature) 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 *Vibrio parahaemolyticus* may be more
    important in warmer seasons and regions.

> The risk of *Vibrio 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 *Vibrio 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 *Vibrio parahaemolyticus*. Overnight submersion for
> a single tidal cycle, reduces levels of *Vibrio parahaemolyticus* in
> oysters and the risk of illness.

-   [Post-Harvest Treatments]{.underline}. Post-harvest treatments that
    reduce levels of *Vibrio 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 *Vibrio 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 *Vibrio 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 *Vibrio parahaemolyticus* and *Vibrio vulnificus*
    respond similarly to control measures such as ultra high pressure,
    mild heat treatment, and freezing. Therefore, mitigations aimed at
    decreasing levels of *Vibrio parahaemolyticus* will also likely
    decrease levels of *Vibrio vulnificus*.

> The model also demonstrated that if oysters are not refrigerated soon
> after harvest, *Vibrio 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 *Vibrio parahaemolyticus* growth and as
> such refrigeration has little additional impact on levels of *Vibrio
> parahaemolyticus*.

-   [At-Harvest and At-Retail Controls]{.underline}. Controlling the
    levels of *Vibrio 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 *Vibrio parahaemolyticus*/g at-harvest would reduce
    illness by approximately 16% at a loss of approximately 3% of the
    total harvest from the raw consumption market; and this same control
    level at-retail would reduce illness by about 99% with a 43% loss
    from the raw oyster market (or subjected to preventive controls).
    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.

> In a sample-based control strategy, a reasonable surrogate for
> pathogenic *Vibrio 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 *Vibrio 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 *Vibrio 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
> *Vibrio parahaemolyticus* in an outbreak situation.

In conclusion, the risk assessment illustrates that the levels of
*Vibrio parahaemolyticus* at the time of harvest play an important role
in causing human illness. However, other factors that either reduce or
allow growth of *Vibrio 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
*Vibrio parahaemolyticus* in oysters and subsequently reduces illnesses
associated with this microorganism.

The results of this risk assessment are influenced by the assumptions
and data sets that were used to develop the Exposure Assessment and
Dose-Response models. The predicted risk of illness among consumers of
raw oysters could change as a result of future data obtained from
continuing surveillance studies. It is anticipated that periodic updates
to the model will continue to reduce the degree of uncertainty
associated with the factors that influence the risk. This risk
assessment provides an understanding of the relative importance of and
interactions among the factors influencing risk. It can be used to
facilitate the formulation of effective guidance and requirements for
the industry and in the evaluations of risk mitigation strategies. This
Interpretive Summary provides a brief, non-technical description of the
materials covered, but a full understanding requires the reader to
consider the complete risk assessment.