**Interpretive Summary:**

**Quantitative Assessment of the**

**Relative Risk to Public Health from**

**Foodborne *Listeria monocytogenes***

**Among Selected Categories of**

**Ready-to-Eat Foods**

Center for Food Safety and Applied Nutrition

Food and Drug Administration

U.S. Department of Health and Human Services

Food Safety and Inspection Service

U.S. Department of Agriculture

September 2003

![](media/image1.png){width="1.4in" height="0.9541666666666667in"}

**Quantitative Assessment of the Relative Risk to Public Health from
Foodborne *Listeria monocytogenes* Among Selected Categories of**

**Ready-to-Eat Foods**

## Preface

This Interpretive Summary provides an overview of the revised *Listeria
monocytogenes* risk assessment. Its purpose is to briefly describe, in
non-technical language, the material covered in the complete risk
assessment (also referred to as the 'technical document'), including
background information on *L. monocytogenes* and listeriosis, the
techniques and data used to develop the risk assessment, the results of
the risk assessment, and limitations and implications of those findings.
In order to achieve its purpose, much of the material covered
quantitatively in the complete risk assessment is described
qualitatively in the Interpretive Summary. A full understanding of the
risk assessment requires the reader to consider the technical document.
It is recommended that those who wish to do an in-depth evaluation of
the assessment read the technical document. The technical document may
be found on the web at [www.cfsan.fda.gov](http://www.cfsan.fda.gov/),
[www.fsis.usda.gov](http://www.fsis.usda.gov/),
[www.foodsafety.gov](http://www.foodsafety.gov/), and
[www.foodriskclearinghouse.umd.edu](http://www.foodriskclearinghouse.umd.edu/).
A printed copy will be provided upon request (fax request to the CFSAN
Outreach and Information Center at 1-877-366-3322).

## Introduction

The United States Department of Health and Human Services, Food and Drug
Administration's Center for Food Safety and Applied Nutrition
(DHHS/FDA/CFSAN) conducted this risk assessment in collaboration with
the U.S. Department of Agriculture's Food Safety and Inspection Service
(USDA/FSIS) and in consultation with the DHHS Centers for Disease
Control and Prevention (CDC).

The Healthy People 2010 goals for national health promotion and disease
prevention called on federal food safety agencies to reduce foodborne
listeriosis by 50% by the end of the year 2005. Preliminary FoodNet data
on the incidence of foodborne illnesses for the United States in 2001
indicated that the incidence of infection from *Listeria monocytogenes*
decreased between 1996 and 2001 from 0.5 to 0.3 cases per 100,000 people
per year. The level then reached a plateau. In order to reduce further
the incidence to a level of 0.25 cases per 100,000 people by the end of
2005, it became evident that additional targeted measures were needed.
The *Listeria monocytogenes* risk assessment was initiated as an
evaluation tool in support of this goal.

The purpose of the assessment is to examine systematically the available
scientific data and information and to estimate the relative risks of
serious illness and death associated with consumption of different types
of ready-to-eat (RTE) foods that may be contaminated with *Listeria
monocytogenes*. This examination of the current science and the models
developed from it are among the tools that food safety regulatory
agencies will consider when evaluating the effectiveness of current and
future policies, programs, and regulatory practices to minimize the
public health impact of this pathogen. Listeriosis (the most significant
illness induced by this pathogen) has serious public health consequences
to certain susceptible groups of people. The illness occurs rarely
(i.e., currently approximately 3.4 cases per million people annually),
but when it does occur, it can be life threatening. In healthy people,
the microorganism usually causes only a non-invasive gastrointestinal
illness, with symptoms including fever, vomiting, and/or diarrhea. For
the purposes of this risk assessment, a distinction is made between the
mild non-invasive illness (referred to as listerial gastroenteritis) and
the severe, sometimes life-threatening, disease (referred to as
listeriosis). This risk assessment only considers listeriosis.

**Modifications Made to the Draft 2001 Risk Assessment**

A draft risk assessment was issued for public comment in January 2001.
This version of the risk assessment incorporates, as appropriate,
changes in response to the submitted comments and newly available data.
It also contains improvements in the modeling techniques, as well as
changes to the model inputs and the use of 'what-if' scenarios. These
changes are listed in Summary Table 1. The combination of these changes
reduced the overall uncertainty ranges for the predicted cases in most
of the food categories.

## Scope and General Approach

This risk assessment provides analyses and models that (1) estimate the
potential level of exposure of three age-based population groups and the
total United States population to *L. monocytogenes* contaminated foods
for 23 food categories and (2) relate this exposure to public health
consequences. The food categories consist of foods with a documented
history of *L. monocytogenes* contamination. The models provide a means
of predicting the likelihood that severe illness or death will result
from consuming foods contaminated with this pathogen. These predictions
are interpreted and used to estimate the relative risks among the food
categories. The foods considered in this risk assessment are
ready-to-eat foods that are eaten without being cooked or reheated just
prior to consumption (e.g., cheese). One food, frankfurters, may or may
not be reheated prior to consumption and was considered as two separate
food categories.

The models developed in the risk assessment provide predictions in two
forms. The first measure is the estimated rate of a fatal infection from
*L. monocytogenes* on an individual serving basis for a particular food
category. This value can be considered the inherent risk associated with
the manufacture, distribution, marketing, and use of the food in the
category, and is reflective of the degree of *L. monocytogenes* control
achieved. Examples of factors related to food that influence the "per
serving" risk include the frequency of contamination, the extent of that
contamination, the ability of foods in the category to support the
growth of *L. monocytogenes*, the duration and temperature of
refrigerated storage, and the size of the serving.

**Summary Table 1. Modifications Made to the 2001 Draft *Listeria
monocytogenes* Risk Assessment**

+-----------+-----------------------------------------------------------+
| **Topic** | **Modifications**                                         |
+-----------+-----------------------------------------------------------+
| Food      | -   Reorganized cheeses based on percentage of moisture   |
| category  |                                                           |
| changes   | -   Removed pickled, dried, and processed vegetables; and |
|           |     canned fruits and nuts                                |
|           |                                                           |
|           | -   Moved vegetable and fruit salads (with dressings) to  |
|           |     Deli-type Salad food category                         |
|           |                                                           |
|           | -   Split frankfurters into 2 categories---reheated and   |
|           |     not reheated                                          |
|           |                                                           |
|           | -   Split miscellaneous dairy foods into 2                |
|           |     categories---Cultured Milk Products and High Fat and  |
|           |     Other Dairy Products                                  |
|           |                                                           |
|           | -   Increased the number of food categories from 20 to 23 |
+-----------+-----------------------------------------------------------+
| New data/ | -   New contamination data (approximately 40 studies,     |
| in        |     various food categories)                              |
| formation |                                                           |
|           | -   New growth data (e.g., Deli-type Salads)              |
|           |                                                           |
|           | -   New information on home storage of deli meats and     |
|           |     frankfurters                                          |
|           |                                                           |
|           | -   Changed the storage intervals for Smoked Seafood and  |
|           |     Fresh Soft Cheese                                     |
+-----------+-----------------------------------------------------------+
| Modeling  | -   Contamination data were weighted according to         |
| t         |     geographic area, age of the study, and size of the    |
| echniques |     data set                                              |
|           |                                                           |
|           | -   A correction factor was developed for food categories |
|           |     for which no recent, large-scale contamination        |
|           |     surveys were available                                |
|           |                                                           |
|           | -   Default shape parameters were used to determine the   |
|           |     spread of the contamination distribution for foods    |
|           |     with few quantitative data                            |
|           |                                                           |
|           | -   Separate mortality to hospitalization ratios were     |
|           |     calculated for each subpopulation                     |
|           |                                                           |
|           | -   An additional year of FoodNet surveillance            |
|           |     data (2000) was included                              |
|           |                                                           |
|           | -   A 'scaling factor' was selected to adjust the         |
|           |     uncertainty distribution for the predicted number of  |
|           |     cases to the FoodNet estimates                        |
|           |                                                           |
|           | -   Model was rewritten in Visual Basic for Applications  |
|           |     to speed up the computation time required for each    |
|           |     run of the model and to facilitate review             |
+-----------+-----------------------------------------------------------+

The predicted relative risk per serving can be viewed as the relative
risk faced by individual consumers when they consume a single serving of
the various foods considered in this risk assessment. The "per serving"
risk is typically the value upon which risk management decisions related
to a specific product are based.

The second measure is the 'per annum risk,' which is the predicted
number of fatal infections per year in the United States for each food
category. This value can be considered a "risk multiplier" value that
takes into account the number of servings of the food category that are
consumed. The predicted per annum risk of serious illnesses for each
food category can be considered the predicted relative total public
health impact for each food category. Since the "per annum" risk is
derived from the "per serving" risk, there is generally a higher degree
of uncertainty associated with the former. Both values are needed to
adequately interpret the risk assessment and evaluate the potential
public health impact of different risk management strategies.

The predicted per serving and per annum relative risks were used to
develop risk rankings to compare the various food categories. In
addition to presenting the "most likely" relative risk rankings for the
different population groups and food categories, the risk assessment
provides the inherent variability and the uncertainty associated with
these rankings.

The disease end point in the risk assessment is patient fatalities,
which is the most definitive measurement for this foodborne pathogen.
The number of cases of listeriosis is then estimated by multiplying the
predicted number of deaths by a correction factor determined from
FoodNet data from 1997 to 2000. For the total United States population,
the correction factor was approximately five, based on data indicating
that for each death there were four additional cases of listeriosis. A
specific correction factor to convert the predicted number of deaths to
cases of listeriosis is used for the three subpopulations evaluated in
the model.

Evaluation of sources of contamination, possible intervention steps, and
potential mitigation strategies for individual foods are outside the
scope of this assessment. However, the assessment and the models may
serve as the basis for these types of analyses. Some examples of such
applications (referred to as 'what-if' scenarios) are provided in the
technical document.

**Background**

*Listeria monocytogenes* is a bacterium that occurs widely in both
agricultural (soil, plants and water) and food processing environments.
Ingestion of *L. monocytogenes* can cause listeriosis, which can be a
serious human illness. In 1998, among populations tracked by FoodNet,
listeriosis resulted in higher rates of hospitalization than illnesses
caused by any other foodborne pathogen and caused over a third of the
reported deaths associated with foodborne illnesses. In 2000, the CDC
reported that of all the foodborne pathogens tracked by CDC, *L.
monocytogenes* had the second highest case fatality rate (21%) and the
highest hospitalization rate (90.5%). Serious illness almost always
occurs in people considered to be at higher risk, such as the elderly
and those who have a pre-existing illness that reduces the effectiveness
of their immune system. Perinatal listeriosis results from foodborne
exposure of the pregnant mother leading to *in utero* exposure of the
fetus, resulting in fetal infection that leads to fetal death, premature
birth, or neonatal illness and death. *Listeria monocytogenes* also
causes listerial gastroenteritis, a syndrome typically associated with
mild gastrointestinal symptoms in healthy individuals. This risk
assessment focuses on the severe public health consequences of
listeriosis.

*Listeria monocytogenes* has only recently been recognized as causing
foodborne disease with major public health consequences. The pathogen
gained prominence in the 1980's after several large outbreaks in the
United States, Canada, and Europe established conclusively that
contaminated foods are the primary means by which this microorganism
affects human populations. CDC's baseline estimate (using 1996-1997
surveillance data) is that foods contaminated with *L. monocytogenes*
caused approximately 2,500 cases of illness, including approximately 500
fatalities, in the United States each year. At the recommendation of
CDC, the risk assessment used 2,078 cases of listeriosis and 390
fatalities as the baseline values for the national annual public health
impact of the disease. These values are projected from an average of
CDC's surveillance data (1997 to 2000). Nearly all cases of listeriosis
are foodborne, however, in very few cases is the specific food that
caused the disease identified. Identification of a specific food vehicle
usually requires first that an outbreak be recognized as such and then,
that the outbreak is extensively investigated. Most cases of listeriosis
do not appear to be part of recognized outbreaks.

While outbreak investigations provide valuable information about
potential modes of *L. monocytogenes* transmission, they do not always
identify the contaminated food source. Thus, the full range of potential
food vehicles of *L. monocytogenes* has most likely not been identified,
and the assessment of risks among different foods cannot be based solely
on the current epidemiological record. The current risk assessment
relies on consumption and contamination data to estimate the United
States consumer's relative risk of exposure and then to estimate the
likely serious public health consequences among the various food
categories.

Efforts by industry and regulatory agencies during the early 1990's
resulted in approximately a 44% reduction in the incidence of
listeriosis. This reduction was an outcome of various factors including
research, surveillance, outbreak response, and regulatory oversight.
These efforts included identification of niches, better sanitation,
equipment redesign, and continual surveillance. However, further
reductions in the incidence of listeriosis often eluded the industry's
food safety efforts, in part because of the unique challenges associated
with controlling this pathogen and changes in the way foods are
distributed, prepared, and consumed today. Consumers continue to seek
convenience as reflected in their food purchasing, preparation, and
consumption practices. Use of ready-to-eat foods, including those with
extended refrigerated shelf lives, continues to increase. Foods are
increasingly bought already prepared from retail establishments, grocery
stores, and deli counters where food safety measures may not be
sufficient to meet the high level of sanitation needed to control or
prevent *L. monocytogenes* contamination. In addition, the increased
consumption of foods prepared outside the home and ready-to-eat foods
presents unique challenges in food handling and storage practices by
food manufacturers, food distributors, food preparers, and consumers to
minimize microbial contamination. Not only is *L. monocytogenes*
commonly found in food processing, distribution, and retail environments
and in the home, it is more resistant than most foodborne pathogens to
the treatments and conditions generally used to control microorganisms.
In particular, *L. monocytogenes* can grow in many foods when stored at
refrigeration temperatures.

This risk assessment focuses on the overall burden of listeriosis on
public health and includes the occurrence of both sporadic illnesses
(i.e., illnesses not associated with a documented outbreak) and outbreak
illnesses. Illnesses attributed to documented outbreaks were a small
proportion of the total estimated annual cases of listeriosis. Outbreaks
frequently represent a breakdown in food production, manufacturing, or
distribution systems that have been put in place to prevent *L.
monocytogenes* contamination. For example, outbreaks of listeriosis have
been linked to failure to protect a frankfurter processing line from
environmental contamination caused by plant renovations (1998-99), use
of defective processing equipment in the production of chocolate milk
(1994), and inadequate pasteurization of milk used to make Mexican-style
soft cheese (1985). Conversely, sporadic cases may reflect the
occasional, chance contamination of the food in a food safety system
that is otherwise "under control." In both cases, continued improvements
in food safety control systems through targeted initiation of new or
enhanced controls may be needed to achieve further reductions in the
incidence of listeriosis.

A significant difference between this risk assessment and the approach
taken by other attempts to evaluate the risks associated with
ready-to-eat foods was the complexity of factors considered in the
hazard characterization. In addition to establishing a general
dose-response relation, models were developed for three distinct
age-based subpopulations and for assessing the full range of virulence
potential that is likely to occur among *L. monocytogenes* isolates.
Including human susceptibility variability and pathogen virulence
variability substantially reduces the estimates of risk in relation to a
particular dose. It also emphasizes the fact that exposures to *L.
monocytogenes* seldom lead to listeriosis, even among highly susceptible
segments of the population.

**Risk Assessment Framework**

This risk assessment document includes an evaluation of the current
scientific data and information on listeriosis and it estimates the
relationship between exposure to *L. monocytogenes* and human
susceptibility to illness or death from listeriosis. It follows the
widely accepted framework that separates the assessment activities into
four components: hazard identification, exposure assessment, hazard
characterization (dose-response assessment), and risk characterization.
This framework allows organization of a highly complex array of varied
data, characterization of the predicted consequences, definition of
uncertainties, and identification of data gaps. The following definition
of terms is used:

-   [Hazard identification]{.underline}. Identifies known or potential
    health effects associated with *L. monocytogenes* by establishing
    the general relationship between the pathogen, its presence in
    foods, and the adverse outcome (illness or death) associated with
    consumption of contaminated foods.

-   [Exposure assessment]{.underline}. Estimates the frequency and level
    of *L. monocytogenes* ingestion from contaminated foods. Evaluates
    the probability that the pathogen will be present, the frequency of
    various levels of contamination at the time of sale, the frequency
    of consumption, and the impact of food handling and storage
    conditions on the overall potential exposure.

-   [Hazard characterization]{.underline}. Estimates the relationship
    between the exposure level (dose) and frequency of severe illness or
    mortality (response).

-   [Risk characterization]{.underline}. The components of the model are
    integrated to estimate the likelihood of an adverse outcome from
    exposure to the pathogen. The exposure assessment and hazard
    characterizations are integrated to express mathematically the
    probability of adverse effects on given population groups as well as
    to provide a qualitative or quantitative estimate of the uncertainty
    associated with the predicted risk values. An important part of this
    step is determining the range of uncertainty of the predictions made
    and distinguishing this from the variation that is inherent in any
    biological system. Risk characterization includes an interpretation
    of the results based on both the model's quantitative predictions
    and the qualitative knowledge base. This includes attributes such as
    the production and consumption characteristics of the various food
    categories and the epidemiological record associated with various
    foods.

The risk characterization includes an additional step wherein the
relative risk rankings are determined. Conceptually, an individual risk
assessment was performed for each of the 23 food categories using the
four-step process described above. The results and models for each food
category were obtained and then combined and contrasted to establish the
relative risk among the food categories, including an evaluation of the
uncertainty associated with the risk comparison.

#### Population Groups Evaluated

The three population groups modeled in this risk assessment are based on
the FoodNet[^1] surveillance data.

-   [Perinatal]{.underline}: The perinatal group includes fetuses and
    neonates from 16 weeks after fertilization to 30 days postpartum.
    The neonatal cases are assumed to be pregnancy-associated cases
    where exposure occurs *in utero* as a result of foodborne *L.
    monocytogenes* infections of the mothers during pregnancy and
    correspond to the FoodNet data for the age category of less than one
    month. Data from the County of Los Angeles Department of Health
    Services were used to determine mortality rates for the perinatal
    group. Manifestations of listeriosis for this population group
    include spontaneous abortions, stillbirths, and neonatal infections.

-   [Elderly]{.underline}: Individuals who were 60 or more years of age.
    This group was considered to have increased susceptibility to
    listeriosis due, in part, to physiological changes associated with
    the natural aging process.

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-   [Intermediate-age group]{.underline}: Because there are insufficient
    data to separate the remaining population into discrete subgroups,
    this group includes the remaining population, both healthy
    individuals (with a substantially lower risk of severe illness or
    death from *L. monocytogenes*) and certain susceptible subpopulation
    groups. These latter groups include individuals at higher risk due
    to underlying conditions; such as AIDS patients or individuals
    taking drugs that suppress the immune systems (e.g., cancer or
    transplant drugs), which increase their susceptibility to *L.
    monocytogenes*. Individuals within these susceptible subpopulations
    account for most of the cases of listeriosis within the
    intermediate-age group.

    The number of predicted cases of listeriosis is estimated for the
    total United States population on a per serving and per annum basis
    for each food category.

### Sources and Types of Data

The published scientific literature, government food intake surveys,
health statistics, epidemiological information, unpublished food product
surveys acquired from state and federal public health officials and
trade associations, and surveys specifically designed to augment the
data available for the risk assessment are the primary sources of data
used in this document. Expert advice on scientific assumptions was
actively sought from leading scientists from academia, industry, and
government. This included two formal reviews of the underlying model
structure and assumptions by the United States National Advisory
Committee on Microbiological Criteria for Foods. In addition, the risk
assessment was initially published in draft form and public comments
sought for six months. The following types of data are used in this risk
assessment:

-   Consumption surveys of the kinds and quantities of foods consumed;

-   Contamination data based on "presence/absence" studies and from
    studies involving quantitative determination of *L. monocytogenes*
    in foods;

-   Growth and survival data for *L. monocytogenes* during refrigerated
    storage and thermal inactivation data under cooking or reheating
    conditions;

-   Animal studies that address the range of virulence of *L.
    monocytogenes* strains and the range of susceptibility found in
    vulnerable population groups; and

-   Information from epidemiological investigations and surveillance on
    the incidence of listeriosis in the three age-based human
    populations.

In general, there are insufficient data to model individual foods.
However, even if more data were available, a model that attempts to
consider individually the ready-to-eat foods would be impractical due to
the increased model complexity introduced as the number of foods
increases. Therefore, 23 food categories were created based on primary
origin (seafood, produce, dairy, and meat), composition and processing
(moisture, raw vs. cooked, pH, and salt content), available data on the
prevalence of *L. monocytogenes* in the foods, and epidemiological
information. The 23 food categories are listed in Summary Table 2. In a
few instances, insufficient data on the frequency and extent of
contamination are available for some minor foods that have been
associated with *L. monocytogenes* contamination (e.g., steak tartar and
bakery products) but which did not fit well in any of the food
categories. These foods are not included in the risk assessment.

Summary Table 2. Food Categories Used in the *Listeria monocytogenes*
Risk Assessment

  --------------- --------------------------------------------------------------------
                  

  **SEAFOOD**     

                  Smoked Seafood (finfish and mollusks)

                  Raw Seafood (finfish, mollusks, and crustaceans)

                  Preserved Fish (dried, pickled, and marinated finfish)

                  Cooked Ready-to-Eat Crustaceans (shrimp and crab)

  **PRODUCE**     

                  Vegetables (raw)

                  Fruits (raw, dried)

  **DAIRY**       

                  Fresh Soft Cheese (queso fresco, queso de Crema, Queso de Puna)

                  Soft Unripened Cheese, \>50% moisture (cottage cheese, cream cheese,
                  ricotta)

                  Soft Ripened Cheese, \>50% moisture (brie, camembert, feta,
                  mozzarella)

                  Semi-soft Cheese, 39-50% moisture (blue, brick, Monterey, muenster)

                  Hard Cheese, \<39% moisture (cheddar, Colby, parmesan)

                  Processed Cheese (cheese foods, spreads, slices)

                  Pasteurized Fluid Milk

                  Unpasteurized Fluid Milk

                  Ice Cream and Other Frozen Dairy Products

                  Cultured Milk Products (yogurt, sour cream, buttermilk)

                  High Fat and Other Dairy Products (butter, cream, other
                  miscellaneous milk products)

  **MEAT**        

                  Frankfurters (reheated)

                  Frankfurters (not reheated)

                  Dry/Semi-Dry Fermented Sausages

                  Deli Meats (cooked, ready-to-eat)

                  Pâté and Meat Spreads

  **COMBINATION   
  FOODS**         

                  Deli-type Salads (fruit, vegetable, meat, pasta, egg, or seafood
                  salads)
  --------------- --------------------------------------------------------------------

The exposure assessment uses nutritional surveys to estimate the
frequency and quantities consumed of over 640 ready-to-eat foods.
Contamination data from published and unpublished sources are taken, for
the most part, from food samples collected at retail. These data are
assembled into a database of contamination levels in food samples; the
database includes 387 studies with over 346,000 samples. The
determination of the growth rate of *L. monocytogenes* in the 23 food
categories is calculated from 94 studies with a total of approximately
226 individual samples. The likely contamination levels in the foods
within the food categories when consumed (after storage and, in one
case, reheating) are subsequently estimated using predictive
microbiology models. In keeping with the goal of the Interpretive
Summary, the quantitative data used in the risk assessment are provided
in Summary Table 3 using qualitative terms to help clarify the type of
data used in the model.

An underlying assumption of the risk assessment is that all the cases of
listeriosis are attributed to the foods in these 23 categories. This
allows the risk assessment to be 'anchored' to the U.S. public health
statistics. However, it is recognized that additional foods or
cross-contamination from raw foods before cooking to other RTE foods may
also contribute additional cases.

The hazard characterization uses data from epidemiological studies,
health statistics, outbreaks, and scientific investigations with animal
models. Due to the lack of data on the rates of symptomatic versus
asymptomatic *L. monocytogenes* infections in humans, the dose-response
models used in the risk assessment are based on the likelihood that a
foodborne *L. monocytogenes* infection would lead to death. The rate of
severe cases of listeriosis requiring hospitalization is then estimated
based on medical records of the number of additional cases of
listeriosis for every death for the specific subpopulations. The
dose-response model used in the risk assessment is based on integrating
epidemiological data with the characteristics of the pathogen in an
animal model.

### Exposure Assessment

The purpose of the exposure assessment is to estimate how often
consumers eat food contaminated with *L. monocytogenes* and the number
of the bacteria likely to be in that food. The contamination data used
were primarily from food samples collected in the marketplace (retail).
In the few instances where data collected at the processing or
manufacturing plant are employed, the data are adjusted to reflect
anticipated *L. monocytogenes* levels at retail. *Listeria
monocytogenes* can grow during refrigerated storage and is destroyed by
heating. Therefore, it is necessary to estimate levels of contamination
at consumption. The exposure assessment model estimates the changes in
contamination levels during refrigerated storage and reheating in the
home. The exposure assessment assumes that the food categories
contribute to listeriosis through direct consumption. While
epidemiologic investigations suggest in some instances that cross
contamination could play a role in the transmission of *L.
monocytogenes* during preparation or storage, insufficient data are
available to consider this possibility.

*[Food Consumption.]{.underline}* Data from two large, nationwide U.S.
food consumption surveys are used to estimate exposure to *L.
monocytogenes*. The Continuing Survey of Food Intakes by Individuals
(CSFII 1994-96) is the latest survey conducted by the Agricultural
Research Service of the USDA. Data from the food intake component of the
Third National Health and Nutrition Examination Survey (NHANES III)
(1988-94), conducted by the National Center for Health Statistics in the
Centers for Disease Control and Prevention (CDC/NCHS) of the Department
of Health and Human Services (DHHS), are also used. The CSFII and NHANES
III surveys gather information on dietary intake by surveying a sample
of consumers over one or two days. Based on the survey data, the
consumption patterns for a larger population over one or two days can be
estimated, but this information may not predict consumption patterns of
individuals or the larger group over an entire year. This contributes to
the uncertainty in the model.

Summary Table 3. Summary of Data Used to Model *Listeria monocytogenes*
Exposure for each Food Relative to Other Food Categories

+---+---------------+-------+-------+-------+--------+-------+--------+
| * |               | **N   | **Med | **Con | **C    | **G   | **Most |
| * |               | umber | ian** | tamin | ontami | rowth | Likely |
| F |               | of    |       | ation | nation | Rate  | Home   |
| o |               | A     | **A   | F     | Level  | D     | Sto    |
| o |               | nnual | mount | reque | at     | uring | rage** |
| d |               | Servi | Consu | ncy** | Retail | Home  |        |
| C |               | ngs** | med** |       | (% of  | St    | **     |
| a |               |       |       | Low   | se     | orage | Time** |
| t |               | Low = | Low   | =≤2%  | rvings | (log  |        |
| e |               | ≤1x   | =≤40g |       | at**   | cfu/d | Short  |
| g |               | 10^9^ |       | Mod   |        | ay)** | =≤2    |
| o |               |       | Mod   | =\>2  | *      |       | days   |
| r |               | Mod = | =\>40 | to    | *10^3^ | Low   |        |
| y |               | \>1x  | to    | \<5%  | to     | =≤0.1 | Mod    |
| * |               | 10^9^ | \<90  |       | 10^6^  |       | =\>2   |
| * |               | to    | g     | High= | cfu)** | Mod   | to 5   |
|   |               | \<1x1 |       | ≥5%   |        | =     | days   |
|   |               | 0^10^ | High  |       | Low    | \>0.1 |        |
|   |               |       | =     |       | =\<0.1 | to    | Long = |
|   |               | High  | ≥90g  |       |        | \<0.2 | ≥      |
|   |               | =     |       |       | Mod    |       | range  |
|   |               | ≥1x1  |       |       | =\>0.1 | High  | 6 to   |
|   |               | 0^10^ |       |       | to     | =     | 10     |
|   |               |       |       |       | \<0.6  | ≥0.2  | days   |
|   |               |       |       |       |        |       |        |
|   |               |       |       |       | High=  |       |        |
|   |               |       |       |       | ≥0.6   |       |        |
+---+---------------+-------+-------+-------+--------+-------+--------+
| * |               |       |       |       |        |       |        |
| * |               |       |       |       |        |       |        |
| S |               |       |       |       |        |       |        |
| E |               |       |       |       |        |       |        |
| A |               |       |       |       |        |       |        |
| F |               |       |       |       |        |       |        |
| O |               |       |       |       |        |       |        |
| O |               |       |       |       |        |       |        |
| D |               |       |       |       |        |       |        |
| * |               |       |       |       |        |       |        |
| * |               |       |       |       |        |       |        |
+---+---------------+-------+-------+-------+--------+-------+--------+
|   | Smoked        | Low   | Mod   | High  | High   | Mod   | Mo     |
|   | Seafood       |       | erate |       |        | erate | derate |
+---+---------------+-------+-------+-------+--------+-------+--------+
|   | Raw Seafood   | Low   | Low   | High  | High   | Mod   | Short  |
|   |               |       |       |       |        | erate |        |
+---+---------------+-------+-------+-------+--------+-------+--------+
|   | Preserved     | Low   | Mod   | High  | Mo     | -     | ^---a^ |
|   | Fish          |       | erate |       | derate | --^a^ |        |
+---+---------------+-------+-------+-------+--------+-------+--------+
|   | Cooked        | Low   | Mod   | Mod   | Mo     | High  | Short  |
|   | Ready-to-Eat  |       | erate | erate | derate |       |        |
|   | Crustaceans   |       |       |       |        |       |        |
+---+---------------+-------+-------+-------+--------+-------+--------+
| * |               |       |       |       |        |       |        |
| * |               |       |       |       |        |       |        |
| P |               |       |       |       |        |       |        |
| R |               |       |       |       |        |       |        |
| O |               |       |       |       |        |       |        |
| D |               |       |       |       |        |       |        |
| U |               |       |       |       |        |       |        |
| C |               |       |       |       |        |       |        |
| E |               |       |       |       |        |       |        |
| * |               |       |       |       |        |       |        |
| * |               |       |       |       |        |       |        |
+---+---------------+-------+-------+-------+--------+-------+--------+
|   | Vegetables    | High  | Low   | Mod   | Low    | Low   | Mo     |
|   |               |       |       | erate |        |       | derate |
+---+---------------+-------+-------+-------+--------+-------+--------+
|   | Fruits        | High  | High  | High  | Low    | Low   | Mo     |
|   |               |       |       |       |        |       | derate |
+---+---------------+-------+-------+-------+--------+-------+--------+
| * |               |       |       |       |        |       |        |
| * |               |       |       |       |        |       |        |
| D |               |       |       |       |        |       |        |
| A |               |       |       |       |        |       |        |
| I |               |       |       |       |        |       |        |
| R |               |       |       |       |        |       |        |
| Y |               |       |       |       |        |       |        |
| * |               |       |       |       |        |       |        |
| * |               |       |       |       |        |       |        |
+---+---------------+-------+-------+-------+--------+-------+--------+
|   | Fresh Soft    | Low   | Low   | Low   | Low    | Low   | Mo     |
|   | Cheese        |       |       |       |        |       | derate |
+---+---------------+-------+-------+-------+--------+-------+--------+
|   | Soft          | Mod   | Low   | Mod   | Mo     | L     | Long   |
|   | Unripened     | erate |       | erate | derate | ow^b^ |        |
|   | Cheese, \>50% |       |       |       |        |       |        |
|   | moisture      |       |       |       |        |       |        |
+---+---------------+-------+-------+-------+--------+-------+--------+
|   | Soft Ripened  | Mod   | Low   | Mod   | Low    | L     | Long   |
|   | Cheese, \>50% | erate |       | erate |        | ow^b^ |        |
|   | moisture      |       |       |       |        |       |        |
+---+---------------+-------+-------+-------+--------+-------+--------+
|   | Semi-soft     | Mod   | Low   | Mod   | Low    | L     | Long   |
|   | Cheese,       | erate |       | erate |        | ow^b^ |        |
|   | 39-50%        |       |       |       |        |       |        |
|   | moisture      |       |       |       |        |       |        |
+---+---------------+-------+-------+-------+--------+-------+--------+
|   | Hard Cheese,  | Mod   | Low   | Low   | Low    | L     | Long   |
|   | \<39%         | erate |       |       |        | ow^b^ |        |
|   | moisture      |       |       |       |        |       |        |
+---+---------------+-------+-------+-------+--------+-------+--------+
|   | Processed     | High  | Low   | Low   | Low    | L     | Long   |
|   | Cheese        |       |       |       |        | ow^b^ |        |
+---+---------------+-------+-------+-------+--------+-------+--------+
|   | Pasteurized   | High  | High  | Low   | Low    | High  | Mo     |
|   | Fluid Milk    |       |       |       |        |       | derate |
+---+---------------+-------+-------+-------+--------+-------+--------+
|   | Unpasteurized | Low   | High  | Mod   | Mo     | High  | Mo     |
|   | Fluid Milk    |       |       | erate | derate |       | derate |
+---+---------------+-------+-------+-------+--------+-------+--------+
|   | Ice Cream and | High  | High  | Low   | Low    | -     | ---^a^ |
|   | Frozen Dairy  |       |       |       |        | --^a^ |        |
|   | Products      |       |       |       |        |       |        |
+---+---------------+-------+-------+-------+--------+-------+--------+
|   | Cultured Milk | Mod   | High  | Low   | Low    | L     | Long   |
|   | Products      | erate |       |       |        | ow^b^ |        |
+---+---------------+-------+-------+-------+--------+-------+--------+
|   | High Fat and  | High  | Low   | Low   | Low    | Mod   | Long   |
|   | Other Dairy   |       |       |       |        | erate |        |
|   | Products      |       |       |       |        |       |        |
+---+---------------+-------+-------+-------+--------+-------+--------+
| * |               |       |       |       |        |       |        |
| * |               |       |       |       |        |       |        |
| M |               |       |       |       |        |       |        |
| E |               |       |       |       |        |       |        |
| A |               |       |       |       |        |       |        |
| T |               |       |       |       |        |       |        |
| S |               |       |       |       |        |       |        |
| * |               |       |       |       |        |       |        |
| * |               |       |       |       |        |       |        |
+---+---------------+-------+-------+-------+--------+-------+--------+
|   | Frankfurters, | Mod   | Mod   | Mod   | High   | Mod   | Mo     |
|   | reheated      | erate | erate | erate |        | erate | derate |
+---+---------------+-------+-------+-------+--------+-------+--------+
|   | Frankfurters, | Low   | Mod   | Mod   | High   | Mod   | Mo     |
|   | not reheated  |       | erate | erate |        | erate | derate |
+---+---------------+-------+-------+-------+--------+-------+--------+
|   | Dry/Semi-dry  | Mod   | Mod   | High  | Mo     | L     | Long   |
|   | Fermented     | erate | erate |       | derate | ow^b^ |        |
|   | Sausages      |       |       |       |        |       |        |
+---+---------------+-------+-------+-------+--------+-------+--------+
|   | Deli Meats    | High  | Mod   | Mod   | Low    | High  | Long   |
|   |               |       | erate | erate |        |       |        |
+---+---------------+-------+-------+-------+--------+-------+--------+
|   | Pâté and Meat | Low   | Mod   | High  | Mo     | High  | Long   |
|   | Spreads       |       | erate |       | derate |       |        |
+---+---------------+-------+-------+-------+--------+-------+--------+
| * |               |       |       |       |        |       |        |
| * |               |       |       |       |        |       |        |
| C |               |       |       |       |        |       |        |
| O |               |       |       |       |        |       |        |
| M |               |       |       |       |        |       |        |
| B |               |       |       |       |        |       |        |
| I |               |       |       |       |        |       |        |
| N |               |       |       |       |        |       |        |
| A |               |       |       |       |        |       |        |
| T |               |       |       |       |        |       |        |
| I |               |       |       |       |        |       |        |
| O |               |       |       |       |        |       |        |
| N |               |       |       |       |        |       |        |
| F |               |       |       |       |        |       |        |
| O |               |       |       |       |        |       |        |
| O |               |       |       |       |        |       |        |
| D |               |       |       |       |        |       |        |
| S |               |       |       |       |        |       |        |
| * |               |       |       |       |        |       |        |
| * |               |       |       |       |        |       |        |
+---+---------------+-------+-------+-------+--------+-------+--------+
|   | Deli-type     | High  | High  | Mod   | Low    | Low   | Mo     |
|   | Salads        |       |       | erate |        |       | derate |
+---+---------------+-------+-------+-------+--------+-------+--------+

^a^ A nongrowth food category; growth rates and storage times are not
applicable. ^b^ Includes probabilities that *L. monocytogenes* numbers
will decline during storage

*[Food Contamination]{.underline}.* There is no systematic, quantitative
survey for levels of *L. monocytogenes* in the U.S. food supply. As a
result, contamination data were primarily gathered from published
scientific literature, and published and unpublished government and
industry documents. Most of these data were obtained from samples of
foods collected at retail or during storage before sale. Two types of
data on the levels of *L. monocytogenes* contamination in foods were
found for most of the food categories. The first type is
presence/absence data (whether the sample contains *L. monocytogenes* or
not). These data were converted to numerical data and included in the
model by assigning the lowest possible contamination level that can be
detected by laboratory methods (generally 0.04 organisms per gram of
food). So, if *L. monocytogenes* was detected in a food, but the number
of organisms was not measured, the sample is assumed to contain at least
0.04 organisms per gram of the food. The second type of data is
enumeration data, in which the actual numbers of *L. monocytogenes* were
measured in a food sample. Enumeration studies with multiple
observations that could be used to characterize the *L. monocytogenes*
distribution over a wide range of concentrations are not available for
many food categories. The shape of the distribution for these foods is
derived by analogy using data from other similar foods.

Most of the contamination studies were conducted in the industrialized
countries of Western Europe and North America. It is possible that the
studies of foods from other countries, even other industrialized ones,
may not be fully representative of the U. S. food supply. A three-tiered
system of weighting is used based on the geographical location of the
study. While all data are included data from developing countries are
given less weight for the purpose of constructing an *L. monocytogenes*
distribution for the United States.

Studies used in the risk assessment were published from 1986 to 2002.
However, many of the contamination data were collected from 1986 through
1991 (and reported through 1993), before substantial worldwide efforts
were made to reduce *L. monocytogenes* contamination. It is possible
that older data may unduly bias an estimate of the current rate of
contamination. Therefore, the contamination data are also weighted based
on the age of the study.

*[Post-Retail Growth of Listeria]{.underline}.* Even when *L.
monocytogenes* is initially present at a low level in a food, the
microorganism can multiply during storage, including storage at
refrigeration temperatures. Some foods are known to support growth of
*L. monocytogenes* better than others. In addition, consumer storage
practices and conditions vary. Some foods are subject to cooking or
reheating before consumption, which can greatly reduce contamination
levels. This risk assessment addresses, to the extent possible, each of
those factors.

Each food category has a "growth module" appropriate for its
characteristics to consider post-retail growth. These modules are used
to estimate the numbers of *L. monocytogenes* in the foods at the time
of consumption. Examples of factors considered in the growth modules are
refrigerator temperatures during storage, the rate of *L. monocytogenes*
growth that a food will support, the estimated length of time that food
is stored, and the maximum number of microorganisms that a food can
support.

##### Hazard Characterization

Hazard characterization describes the adverse health effects of a
particular substance or microorganism. It is the portion of the risk
assessment where a dose-response relationship is described, usually as a
percentage of the population that will become ill after being exposed to
a particular dose or level of contamination. In this risk assessment,
the dose-response relationship is based on the number of *L.
monocytogenes* consumed, their virulence, and the relative
susceptibility of an individual to listeriosis.

For *L. monocytogenes*, the overall incidence of severe illness and the
differences in risk to susceptible groups (e.g., the elderly vs.
intermediate-age groups) are well characterized. However, the
relationship between the number of *L. monocytogenes* consumed (the
dose) and the likelihood of illness occurring (response), in combination
with the severity of the illness resulting from that dose are not well
understood. As a result, the dose-response modules are derived from
laboratory experiments with rodents and then modified to fit human
epidemiological data.

The occurrence of death is modeled in the risk assessment as a clear,
distinct biological endpoint for which there is definitive
epidemiological data. The number of serious cases (i.e., those requiring
hospitalization) is then estimated from the mortality rate using the CDC
data. However, extrapolation from animal data to the human disease
response still required several factors to be taken into consideration.
These factors are related to the inherent differences between animal
species and humans, and how these differences interact with specific
aspects of listeriosis. This was accomplished by applying a 'scaling'
parameter to tie the dose-response relationship from the animal data to
the human epidemiological data from CDC. Thus, the dose-response curves
include adjustment factors that assure that the predicted numbers of
fatalities are in reasonable agreement with the public health statistics
available for listeriosis.

*[Human Susceptibility]{.underline}.* Immunological and physiological
factors in humans play a role in determining the susceptibility that may
be found throughout a population. The probability of death is described
for the three different age-based groups of people. As indicated above,
epidemiological information is used to adjust the dose-response model to
fit the number of listeriosis fatalities observed in FoodNet and to
develop the dose-response model for each of these groups.

### *[Virulence.]{.underline}* Epidemiological information and laboratory studies indicate that different strains of *L. monocytogenes* vary in their ability to cause illness. This variability influences the number of organisms required to produce illness and possibly the severity of symptoms of illness. A large number of potential human virulence factors have been described that seem to occur in essentially all of the human and environmental isolates that have been studied. Data from mouse lethality studies are used to model the range of strain virulence that may be encountered.  {#virulence.-epidemiological-information-and-laboratory-studies-indicate-that-different-strains-of-l.-monocytogenes-vary-in-their-ability-to-cause-illness.-this-variability-influences-the-number-of-organisms-required-to-produce-illness-and-possibly-the-severity-of-symptoms-of-illness.-a-large-number-of-potential-human-virulence-factors-have-been-described-that-seem-to-occur-in-essentially-all-of-the-human-and-environmental-isolates-that-have-been-studied.-data-from-mouse-lethality-studies-are-used-to-model-the-range-of-strain-virulence-that-may-be-encountered. .unnumbered}

##### Risk Characterization {#risk-characterization .unnumbered}

Risk characterization combines the results of the exposure assessment
and the hazard characterization to produce an estimate of the likelihood
of adverse health effects associated with the hazard. It also provides
estimates of the variability and uncertainty associated with the
predictions of relative risk and the contributing factors. This
information is critical to the correct interpretation of the results of
a risk assessment. In a quantitative risk assessment, such as this, the
risk characterization is developed using computer simulation modeling
techniques and then interpreted in relation to available scientific and
medical data and expert scientific judgment.

In this assessment, the risk characterization was developed using a
two-step computer modeling process. In the first step, the results of
the exposure assessment and hazard characterization are combined to
provide estimates of risk on a per serving basis and on a per annum
basis. This is done using a technique referred to as a two-dimensional
Monte Carlo simulation. This involves running the model 30,000 times and
integrating the results to produce a "simulation." The process is then
repeated to obtain 300 simulations. In addition to providing "most
likely" results, this technique allows the variability and uncertainty
of the estimates to be evaluated. Consideration of variability and
uncertainty associated with the exposure assessment and hazard
characterization is a critical component in establishing and
interpreting the relative risk of an individual food category causing
listeriosis relative to the other food categories.

The second step of the risk characterization is the development of a
'relative risk ranking.' This ranks the foods in relation to each other.
The relative risk ranking values range from 1 to 23, with 1 being the
food category with the greatest predicted relative risk and 23 the food
category with the lowest predicted relative risk. This is again
accomplished using computer simulation techniques where the 'most
likely' relative risk ranking for each of the food categories is
generated for 4,000 simulations. This allows the predicted relative risk
ranking for a food category to be accompanied by a measure of the
uncertainty associated with that ranking. This provides a means of
interpreting the ranking in relation to the degree of confidence
associated with the predicted ranking.

**Reliability of Results and the Role of Uncertainty and Variability**

The risk assessment evaluates available data to describe, as accurately
as possible, the current state of scientific knowledge about human
listeriosis associated with different types of ready-to-eat foods. In
doing so, the assessment attempts to capture both the variability
inherent in the incidence of foodborne listeriosis and the uncertainty
associated with the data analysis. Two examples of factors that result
in risk estimates with wide confidence intervals are the propensity for
*L. monocytogenes* to affect only a small segment of the population and
the extreme range of levels of *L. monocytogenes* that occur in
different foods. Likewise, limits in the availability of data for
certain foods and the need to consider how this could affect the final
risk estimates increase the associated confidence intervals. Thus, to
depict the current knowledge about the risks associated with *L.
monocytogenes* contamination of the foods included in this risk
assessment, the models developed portray the predicted variability in
the occurrence of illness and death and the uncertainty associated with
those predictions. The impact of the uncertainties captured by the risk
assessment is evaluated by examining the ranges of predicted values or
the levels of certainty associated with different estimates. If data
used in the model reflect consistency among numerous studies, the
degrees of uncertainty associated with the risk estimates are relatively
small even if the biological phenomenon might be highly variable.
Conversely, if data are meager or inconsistent, the resulting
uncertainty estimates have wide ranges. In addition to these
quantitative estimates, the variability and uncertainty associated with
each food category was evaluated and is discussed in the interpretation
of the results.

The nature of the data and the structure of the model used in this risk
assessment lead to varying degrees of certainty in the results for each
food category and subpopulation. In general, the additional data and
modeling techniques employed in revising the risk assessment has lead to
a substantial reduction in uncertainty for most food categories. Whether
or not the degree of certainty in the risk assessment is sufficient to
justify a particular risk management decision will likely depend upon
the food and course of action being considered. The accumulation of
additional data from research and surveillance, as well as
industry-collected data, could potentially reduce the uncertainty
associated with the risk assessment's predictions. However, acquisition
of additional or better data may or may not change the estimated risk
values because of the high degree of inherent variability. The inherent
variability associated with this microorganism and its disease potential
in humans will always be a critical part of translating the results of
this risk assessment into effective risk management strategies.

#### Results

The relative risk rankings, along with the corresponding risk estimates
expressed in terms of both the predicted number of cases per serving and
per annum, are provided in Summary Table 4. Both measures are important
in understanding and interpreting the risks associated with foodborne
listeriosis. The per serving estimate measures the inherent risk faced
by the consumer when he/she consumes an individual serving of that
product. This estimate reflects the degree of control achieved by
current food safety risk management systems for that product and, as
such, is the primary value for assessing which foods are likely to be
amenable to further risk reduction efforts. The per annum value can be
viewed as a "risk multiplier" which takes the per serving value and
accounts for the number of servings that are consumed in the United
States each year. This value can be viewed as the predicted total public
health burden. Thus, a food that is inherently risky but is consumed
only to a highly limited degree would have a high per serving risk but a
low per annum risk. Conversely, a food that is manufactured under a
rigorous control program may have a low per serving risk but, if
consumed daily by most consumers, the food may have a significant per
annum risk.

The interpretation of the results also requires an appreciation of the
fact that the values being compared are the median values of
distributions that may be highly skewed (i.e., not evenly distributed).
The use of median values was selected as being the appropriate method
for comparing the overall relative risks among the different food
categories. However, these values must be considered in relation to the
associated variability and uncertainty (i.e., the confidence intervals
surrounding the median). These analyses are provided in detail within
the body and appendices of the risk assessment.

The results clearly predict that the risk of listeriosis on both per
serving and per annum basis varies greatly among the various food
categories. For example, the differential between per serving risks
associated with Deli Meats (relative risk rank of 1) and Hard Cheeses
(relative risk rank of 23) is almost 10,000,000-fold.

**Summary Table 4. Relative Risk Ranking and Predicted Median Cases of
Listeriosis for the Total United States Population on a per Serving and
per Annum Basis**

+--------+---+----------------+-------+---+----------------+-------+
| **Re   | # |                |       |   |                |       |
| lative | # |                |       |   |                |       |
| Risk   | # |                |       |   |                |       |
| Ran    |   |                |       |   |                |       |
| king** | P |                |       |   |                |       |
|        | r |                |       |   |                |       |
|        | e |                |       |   |                |       |
|        | d |                |       |   |                |       |
|        | i |                |       |   |                |       |
|        | c |                |       |   |                |       |
|        | t |                |       |   |                |       |
|        | e |                |       |   |                |       |
|        | d |                |       |   |                |       |
|        |   |                |       |   |                |       |
|        | M |                |       |   |                |       |
|        | e |                |       |   |                |       |
|        | d |                |       |   |                |       |
|        | i |                |       |   |                |       |
|        | a |                |       |   |                |       |
|        | n |                |       |   |                |       |
|        |   |                |       |   |                |       |
|        | C |                |       |   |                |       |
|        | a |                |       |   |                |       |
|        | s |                |       |   |                |       |
|        | e |                |       |   |                |       |
|        | s |                |       |   |                |       |
|        |   |                |       |   |                |       |
|        | o |                |       |   |                |       |
|        | f |                |       |   |                |       |
|        |   |                |       |   |                |       |
|        | L |                |       |   |                |       |
|        | i |                |       |   |                |       |
|        | s |                |       |   |                |       |
|        | t |                |       |   |                |       |
|        | e |                |       |   |                |       |
|        | r |                |       |   |                |       |
|        | i |                |       |   |                |       |
|        | o |                |       |   |                |       |
|        | s |                |       |   |                |       |
|        | i |                |       |   |                |       |
|        | s |                |       |   |                |       |
|        |   |                |       |   |                |       |
|        | f |                |       |   |                |       |
|        | o |                |       |   |                |       |
|        | r |                |       |   |                |       |
|        |   |                |       |   |                |       |
|        | 2 |                |       |   |                |       |
|        | 3 |                |       |   |                |       |
|        |   |                |       |   |                |       |
|        | F |                |       |   |                |       |
|        | o |                |       |   |                |       |
|        | o |                |       |   |                |       |
|        | d |                |       |   |                |       |
|        |   |                |       |   |                |       |
|        | C |                |       |   |                |       |
|        | a |                |       |   |                |       |
|        | t |                |       |   |                |       |
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|        | i |                |       |   |                |       |
|        | e |                |       |   |                |       |
|        | s |                |       |   |                |       |
+--------+---+----------------+-------+---+----------------+-------+
|        | * |                |       | * |                |       |
|        | * |                |       | * |                |       |
|        | P |                |       | P |                |       |
|        | e |                |       | e |                |       |
|        | r |                |       | r |                |       |
|        | S |                |       | A |                |       |
|        | e |                |       | n |                |       |
|        | r |                |       | n |                |       |
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|        | i |                |       | ^ |                |       |
|        | s |                |       | b |                |       |
|        | ^ |                |       | ^ |                |       |
|        | a |                |       | * |                |       |
|        | ^ |                |       | * |                |       |
|        | * |                |       |   |                |       |
|        | * |                |       |   |                |       |
+--------+---+----------------+-------+---+----------------+-------+
|        |   | **Food**       | **Ca  |   | **Food**       | **Ca  |
|        |   |                | ses** |   |                | ses** |
+--------+---+----------------+-------+---+----------------+-------+
| 1      | > | Deli Meats     | 7.7x1 | V | Deli Meats     | 1     |
|        |   |                | 0^-8^ | e |                | 598.7 |
|        | H |                |       | r |                |       |
|        | i |                |       | y |                |       |
|        | g |                |       | H |                |       |
|        | h |                |       | i |                |       |
|        | > |                |       | g |                |       |
|        |   |                |       | h |                |       |
|        | R |                |       |   |                |       |
|        | i |                |       |   |                |       |
|        | s |                |       |   |                |       |
|        | k |                |       |   |                |       |
+--------+---+----------------+-------+---+----------------+-------+
| 2      |   | Frankfurters,  | 6.5x1 | > | Pasteurized    | 90.8  |
|        |   | not reheated   | 0^-8^ |   | Fluid Milk     |       |
|        |   |                |       | H |                |       |
|        |   |                |       | i |                |       |
|        |   |                |       | g |                |       |
|        |   |                |       | h |                |       |
|        |   |                |       | > |                |       |
|        |   |                |       |   |                |       |
|        |   |                |       | R |                |       |
|        |   |                |       | i |                |       |
|        |   |                |       | s |                |       |
|        |   |                |       | k |                |       |
+--------+---+----------------+-------+---+----------------+-------+
| 3      |   | Pâté and Meat  | 3.2x1 |   | High Fat and   | 56.4  |
|        |   | Spreads        | 0^-8^ |   | Other Dairy    |       |
|        |   |                |       |   | Products       |       |
+--------+---+----------------+-------+---+----------------+-------+
| 4      |   | Unpasteurized  | 7.1x1 |   | Frankfurters,  | 30.5  |
|        |   | Fluid Milk     | 0^-9^ |   | not reheated   |       |
+--------+---+----------------+-------+---+----------------+-------+
| 5      |   | Smoked Seafood | 6.2x1 | > | Soft Unripened | 7.7   |
|        |   |                | 0^-9^ |   | Cheese         |       |
|        |   |                |       | M |                |       |
|        |   |                |       | o |                |       |
|        |   |                |       | d |                |       |
|        |   |                |       | e |                |       |
|        |   |                |       | r |                |       |
|        |   |                |       | a |                |       |
|        |   |                |       | t |                |       |
|        |   |                |       | e |                |       |
|        |   |                |       | > |                |       |
|        |   |                |       |   |                |       |
|        |   |                |       | R |                |       |
|        |   |                |       | i |                |       |
|        |   |                |       | s |                |       |
|        |   |                |       | k |                |       |
+--------+---+----------------+-------+---+----------------+-------+
| 6      |   | Cooked         | 5.1x1 |   | Pâté and Meat  | 3.8   |
|        |   | Ready-to-Eat   | 0^-9^ |   | Spreads        |       |
|        |   | Crustaceans    |       |   |                |       |
+--------+---+----------------+-------+---+----------------+-------+
| 7      | > | High Fat and   | 2.7x1 |   | Unpasteurized  | 3.1   |
|        |   | Other Dairy    | 0^-9^ |   | Fluid Milk     |       |
|        | M | Products       |       |   |                |       |
|        | o |                |       |   |                |       |
|        | d |                |       |   |                |       |
|        | e |                |       |   |                |       |
|        | r |                |       |   |                |       |
|        | a |                |       |   |                |       |
|        | t |                |       |   |                |       |
|        | e |                |       |   |                |       |
|        | > |                |       |   |                |       |
|        |   |                |       |   |                |       |
|        | R |                |       |   |                |       |
|        | i |                |       |   |                |       |
|        | s |                |       |   |                |       |
|        | k |                |       |   |                |       |
+--------+---+----------------+-------+---+----------------+-------+
| 8      |   | Soft Unripened | 1.8x1 |   | Cooked         | 2.8   |
|        |   | Cheese         | 0^-9^ |   | Ready-to-Eat   |       |
|        |   |                |       |   | Crustaceans    |       |
+--------+---+----------------+-------+---+----------------+-------+
| 9      |   | Pasteurized    | 1.0x1 |   | Smoked Seafood | 1.3   |
|        |   | Fluid Milk     | 0^-9^ |   |                |       |
+--------+---+----------------+-------+---+----------------+-------+
| 10     | > | Fresh Soft     | 1     | > | Fruits         | 0.9   |
|        |   | Cheese         | .7x10 |   |                |       |
|        | L |                | ^-10^ | L |                |       |
|        | o |                |       | o |                |       |
|        | w |                |       | w |                |       |
|        | > |                |       | > |                |       |
|        |   |                |       |   |                |       |
|        | R |                |       | R |                |       |
|        | i |                |       | i |                |       |
|        | s |                |       | s |                |       |
|        | k |                |       | k |                |       |
+--------+---+----------------+-------+---+----------------+-------+
| 11     |   | Frankfurters,  | 6     |   | Frankfurters,  | 0.4   |
|        |   | reheated       | .3x10 |   | reheated       |       |
|        |   |                | ^-11^ |   |                |       |
+--------+---+----------------+-------+---+----------------+-------+
| 12     |   | Preserved Fish | 2     |   | Vegetables     | 0.2   |
|        |   |                | .3x10 |   |                |       |
|        |   |                | ^-11^ |   |                |       |
+--------+---+----------------+-------+---+----------------+-------+
| 13     |   | Raw Seafood    | 2     |   | Dry/Semi-dry   | \<0.1 |
|        |   |                | .0x10 |   | Fermented      |       |
|        |   |                | ^-11^ |   | Sausages       |       |
+--------+---+----------------+-------+---+----------------+-------+
| 14     |   | Fruits         | 1     |   | Fresh Soft     | \<0.1 |
|        |   |                | .9x10 |   | Cheese         |       |
|        |   |                | ^-11^ |   |                |       |
+--------+---+----------------+-------+---+----------------+-------+
| 15     |   | Dry/Semi-dry   | 1     |   | Semi-Soft      | \<0.1 |
|        |   | Fermented      | .7x10 |   | Cheese         |       |
|        |   | Sausages       | ^-11^ |   |                |       |
+--------+---+----------------+-------+---+----------------+-------+
| 16     |   | Semi-soft      | 6     |   | Soft Ripened   | \<0.1 |
|        |   | Cheese         | .5x10 |   | Cheese         |       |
|        |   |                | ^-12^ |   |                |       |
+--------+---+----------------+-------+---+----------------+-------+
| 17     |   | Soft Ripened   | 5     |   | Deli-type      | \<0.1 |
|        |   | Cheese         | .1x10 |   | Salads         |       |
|        |   |                | ^-12^ |   |                |       |
+--------+---+----------------+-------+---+----------------+-------+
| 18     |   | Vegetables     | 2     |   | Raw Seafood    | \<0.1 |
|        |   |                | .8x10 |   |                |       |
|        |   |                | ^-12^ |   |                |       |
+--------+---+----------------+-------+---+----------------+-------+
| 19     |   | Deli-type      | 5     |   | Preserved Fish | \<0.1 |
|        |   | Salads         | .6x10 |   |                |       |
|        |   |                | ^-13^ |   |                |       |
+--------+---+----------------+-------+---+----------------+-------+
| 20     |   | Ice Cream and  | 4     |   | Ice Cream and  | \<0.1 |
|        |   | Other Frozen   | .9x10 |   | Other Frozen   |       |
|        |   | Dairy Products | ^-14^ |   | Dairy Products |       |
+--------+---+----------------+-------+---+----------------+-------+
| 21     |   | Processed      | 4     |   | Processed      | \<0.1 |
|        |   | Cheese         | .2x10 |   | Cheese         |       |
|        |   |                | ^-14^ |   |                |       |
+--------+---+----------------+-------+---+----------------+-------+
| 22     |   | Cultured Milk  | 3     |   | Cultured Milk  | \<0.1 |
|        |   | Products       | .2x10 |   | Products       |       |
|        |   |                | ^-14^ |   |                |       |
+--------+---+----------------+-------+---+----------------+-------+
| 23     |   | Hard Cheese    | 4     |   | Hard Cheese    | \<0.1 |
|        |   |                | .5x10 |   |                |       |
|        |   |                | ^-15^ |   |                |       |
+--------+---+----------------+-------+---+----------------+-------+

^a^Food categories were classified as high risk (\>5 cases per billion
servings), moderate risk (≤5 but ≥1 case per billion servings), and low
risk (\<1 case per billion servings).

^b^Food categories were classified as very high risk (\>100 cases per
annum), high risk (\>10 to 100 cases per annum), moderate risk (≥1 to 10
cases per annum), and low risk (\<1 cases per annum).

### 'What-If' Scenarios {#what-if-scenarios .Normal+Bold}

While the risk assessment purposely did not look into the pathways for
the manufacture of individual foods, the risk assessment model developed
can be used to estimate the likely impact of control strategies by
changing one or more input parameters and measuring the change in the
model outputs. This process, referred to as conducting 'what-if'
scenarios, can be used to explore how the components of a complex model
interact. Scenarios were run to allow comparison of baseline
calculations to new situations that might arise as a result of potential
risk reduction strategies. Several 'what-if' scenarios are detailed
within the risk assessment.

-   [Refrigerator Temperature Scenario]{.underline}. One example of a
    'what-if' scenario considered was the impact that assuring that home
    refrigerators do not operate above either 45 ºF or 41 ºF would have
    on the total number of cases of listeriosis. In this example, the
    predicted number of cases of listeriosis would be reduced
    approximately 69% (from 2105 to 656) by assuring that all home
    refrigerators temperatures operated at 45 ºF or less. The predicted
    number of cases was further reduced to 28 per year (\>98%) when the
    distribution of home refrigerator temperatures did not exceed 41 ºF.

-   [Storage Times Scenario]{.underline}. Storage affects the amount of
    possible growth in a food. The impact of reducing the maximum
    storage time (e.g., by labeling food with "consume-by" dates) is
    evaluated. Limiting the storage time for Deli Meat, for example,
    from the maximum 28 days estimated to 14 days reduces the median
    number of cases of listeriosis in the elderly population from 228 to
    197 (13.6% reduction). Shortening storage time to 10 days further
    reduces the cases to 154 (32.5% reduction). Another 'what-if'
    scenario explores how extended shelf life affects the predicted risk
    of listeriosis in Smoked Seafood. Increasing the storage time from a
    maximum of 30 days to a maximum of 45 days more than doubled the
    predicted median number of cases of listeriosis for the elderly
    subpopulation (0.8 vs. 2.1). Storage times and temperatures interact
    to affect the amount of growth that could occur in a food.

-   [Reduction of the Number of Microorganisms Scenario]{.underline}.
    Interventions designed to reduce the number of *L. monocytogenes* in
    food before it is sold could impact risk. For example, the cooking
    or pasteurization of a product in its final package will reduce the
    level of contamination and if the magnitude of the reduction is
    large enough, it will also reduce the frequency of contaminated
    packages. The effects of such interventions are typically expressed
    in relation to the extent of the surviving microbial population as
    measured by the number of log reductions achieved. Thus, there is
    10% survival with a 1-log reduction, 1% survival with a 2-log
    reduction, etc. To model the effect of such interventions, scenarios
    were run to calculate the predicted reduction in the number of cases
    in the elderly population attributable to Deli Meats after different
    levels of treatment (e.g., 1-log, 2-log). The storage times and
    temperatures were not truncated so re-growth in Deli Meats, foods
    that typically support the rapid growth of *L. monocytogenes* and
    have extended refrigerated storage times, could occur in packages
    that contained surviving cells. The modeling predicts that the
    inclusion of a treatment that produced a 1-log reduction in
    contamination at retail would reduce the number of predicted cases
    in the elderly population by 50%, from 227 to 120, and a 2-log
    treatment would result in a 74% reduction. Further analysis of the
    results of the scenario (see the "Contamination Level Scenario"
    below) suggests that these reductions are largely attributable to
    those instances where either the initial levels in the product were
    low enough or the extent of the treatment was greatest such that
    there was an effective reduction in the frequency of contaminated
    packages. This reflects the fact that in packages where there were
    surviving *L. monocytogenes*, the pathogens were likely to re-grow
    during the normal shelf life of the product. Even greater reductions
    in relative risk would be expected with food categories that do not
    support the growth of *L. monocytogenes.* In those products the lack
    of potential re-growth would result in both a reduction in the
    frequency of contaminated packages and a reduction in the levels of
    the pathogen in packages still containing viable *L. monocytogenes*
    at the time of consumption.

-   [Contamination Level Scenario]{.underline}. The effect on
    interventions that reduce the frequency and extent of contamination
    on the risk of foodborne listeriosis was further characterized by a
    second 'what-if' scenario that considered the impact of limiting the
    maximum levels of *L. monocytogenes* that occur in retail foods.
    Deli Meats, which support the rapid growth of the pathogen and can
    have extended refrigerated storage times, was again examined. The
    scenario was performed by using single contamination levels instead
    of a distribution of contamination levels without altering the
    distribution of storage times and temperatures. This 'what-if'
    scenario predicts that reducing *L. monocytogenes* in this food
    category from 1000 to 100 and even down to 1 cfu/g reduces the risk
    about 30% for each log decrease in contamination level. When the
    contamination level was decreased to less than one *L.
    monocytogenes* per serving of Deli Meats, the risk decreased in
    proportion to the frequency of contamination. This is interpreted as
    indicating that for foods, such as Deli Meats, that can support the
    rapid growth of *L. monocytogenes* to high levels during their
    normal shelf-life, reducing contamination levels by itself would not
    have a major effect on reducing risk until reductions in
    contamination levels are large enough to affect the frequency of
    contamination. Thus, the risk reductions observed in the "Reduction
    of the Number of Organisms Scenario" above are largely attributable
    to reductions in the frequency of contaminated servings. This
    scenario further suggests that, in order for Deli Meats treatments
    that reduce the levels of *L. monocytogenes* without reducing the
    frequency of contamination to be effective in reducing the risk of
    listeriosis, the treatments would have to be coupled with other
    interventions that limit re-growth (e.g., inclusion of inhibitors,
    reducing the time and temperature of refrigerated storage). Again,
    the effect of reducing the levels of contamination in foods that do
    not support the growth of the pathogen would be expected to have a
    more substantial impact on reducing the relative risk associated
    with those food categories since re-growth cannot occur.

The following two scenarios were developed to serve as focused
sensitivity analyses directed towards two particular issues.

-   [Fresh Soft Cheese Made from Unpasteurized Milk
    Scenario]{.underline}. Unlike the 2001 draft risk assessment, the
    revised risk assessment indicates that the risk from Fresh Soft
    Cheese is low. This change is largely attributable to the inclusion
    of additional retail data indicating a very low prevalence rate in
    this food category. However, there is a strong epidemiological
    correlation between Hispanic-style fresh soft cheese (Queso Fresco)
    and listeriosis. A likely explanation for this discrepancy is that
    the data collected for this category represents commercially
    produced cheese whereas the cheeses linked to the disease have often
    been associated with non-commercially produced cheese, and in some
    cases illicitly distributed fresh soft cheese made from raw,
    unpasteurized milk. To characterize the risk from Queso Fresco made
    from raw milk, the exposure model is constructed using contamination
    data representative of raw milk. The estimated risk per serving was
    43 times greater for the neonatal population and 36 times greater
    for the elderly population when these cheeses were assumed to be
    made from unpasteurized milk compared to manufacture with
    pasteurized milk. This suggests that decreasing the consumption of
    Fresh Soft Cheese made from unpasteurized milk could reduce the risk
    associated with this food category.

-   [Pasteurized Fluid Milk Scenarios]{.underline}. The primary
    intervention for fluid milk is pasteurization. Requirements for
    pasteurization can vary from country to country. As with other food
    categories, the available exposure data were weighted both
    geographically and temporally. To better understand the possible
    impact of including contamination data from other countries and data
    for chocolate milk, scenarios were run using domestic milk only with
    and without the inclusion of chocolate milk. Excluding non-U.S. milk
    and chocolate milk had little impact on the predicted number of
    cases of listeriosis attributed to Pasteurized Fluid Milk on both a
    per serving and per annum basis.

**Evaluation and Interpretation**

The scientific evaluations and the mathematical models developed during
the risk assessment provide a systematic assessment of the scientific
knowledge needed to assist both in reviewing the effectiveness of
current policies, programs, and practices, and identifying new
strategies to minimize the public health impact of foodborne *L.
monocytogenes*. This systematic assessment provides a foundation to
assist future evaluations of the potential effectiveness of new
strategies for controlling foodborne listeriosis. The risk assessment
provides a means of comparing the relative risks associated with these
foods on a per serving and a per annum basis. However, overall
interpretation of the risk assessment requires more than just a simple
consideration of only the relative risk rankings associated with the
various food categories. As discussed above, the results must also be
evaluated in relation to the degree of variability and uncertainty
inherent in the predicted relative risk, and interpreted in relation to
available scientific knowledge of the production, marketing, and
consumption of the various food categories. Likewise, the results must
be evaluated in relation to the available epidemiological record. A
detailed consideration of the quantitative and qualitative findings for
each food category is provided in the technical risk assessment
document.

As part of the evaluation and interpretation of the predicted risk
estimates and the accompanying relative risk rankings, the risk
assessment considered various qualitative and quantitative methods of
grouping the results that may be useful for risk management or risk
communication purposes. For example, Summary Table 4 includes an
arbitrary grouping of the per serving and per annum results into very
high, high, moderate, and low risk categories based on the criteria
provided in the table's footnotes. In this instance, six food categories
were considered to be high risk on a per serving basis: Deli Meats,
Frankfurters (not reheated), Pâté and Meat Spreads, Unpasteurized Fluid
Milk, Smoked Seafood, and Cooked Ready-to-Eat Crustaceans. Three food
categories are considered to be moderate risk and the remaining 14 food
categories are considered to be low risk on a per serving basis. On a
per annum basis, the majority of the cases are predicted to be
attributable to Deli Meats. The high risk food categories include
Pasteurized Fluid Milk, High Fat and Other Dairy Products, and
Frankfurters (not reheated). Five food categories are considered to be
moderate risks and the remaining 14 food categories are considered to be
low risk on a per annum basis.

A number of methods for objectively grouping the results were evaluated,
and are discussed in detail within the risk assessment. One approach
that appears to be very useful for risk management/communication
purposes is the evaluation of the relative risk ranking results using
cluster analysis. When performed at the 90% confidence level, this
analysis groups the per serving rankings into four clusters and the per
annum rankings into five (see Summary Table 5). These clusters are used,
in turn, to develop a two-dimensional matrix of per serving vs. per
annum rankings (see Summary Figure 1) of the food categories. In this
approach, the four per serving clusters are arrayed against three groups
of per annum clusters (A and B, C and D, and E). The matrix is then used
to depict five overall risk designations: Very High, High, Moderate,
Low, and Very Low. For example, as shown in Summary Table 5, Deli Meats
is included in the 'per serving' Cluster 1 and in the 'per annum'
Cluster A, so it is placed in the two-dimensional matrix cell, Very High
Risk, Cluster 1-A (see Summary Figure 1). Frankfurters (not reheated) is
in the 'per serving' Cluster 1 and in the 'per annum' Cluster B, so it
is also placed in the Very High Risk cell, representing Cluster 1-B. No
food categories are in the Moderate Risk cell for Clusters 3-A and 3-B
because there are no foods in the 'per serving' Cluster 3 that match
with the 'per annum' Cluster A or Cluster B.

The risk characterization combines the exposure and dose-response models
to predict the relative risk of illness attributable to each food
category. While the risk characterization must be interpreted in light
of both the inherent variability and uncertainty associated with the
extent of contamination of ready-to-eat foods with *L. monocytogenes*
and the ability of the microorganism to cause disease, the results
provide a means of comparing the relative risks among the different food
categories and population groups considered in the assessment and should
prove to be a useful tool in focusing control strategies and ultimately
improving public health through effective risk management. As described
above, cluster analysis techniques are employed as a means of discussing
the food categories within a risk analysis framework. The food
categories are divided into five overall risk designations (see Summary
Figure 1), which are likely to require different approaches to
controlling foodborne listeriosis.

###### Summary Table 5. Results of Cluster Analysis at the 0.1 Level

+-----------------------------------+----------------------------------+
| **Risk per Serving**              | **Risk per Annum**               |
+-----------------------------------+----------------------------------+
| ######## CLUSTER 1                | # **CLUSTER A**                  |
|                                   |                                  |
| > Deli Meats                      | # Deli Meats                     |
| >                                 |                                  |
| > Frankfurters, not reheated      |                                  |
| >                                 |                                  |
| > Pâté and Meat Spreads           |                                  |
| >                                 |                                  |
| > Unpasteurized Fluid Milk        |                                  |
| >                                 |                                  |
| > Smoked Seafood                  |                                  |
+-----------------------------------+----------------------------------+
| ######## CLUSTER 2                | ######## CLUSTER B               |
|                                   |                                  |
| > Cooked RTE Crustaceans          | > High Fat and Other Dairy       |
| >                                 | > Products                       |
| > High Fat and Other Dairy        | >                                |
| > Products                        | > Frankfurters, not reheated     |
| >                                 | >                                |
| > Pasteurized Fluid Milk          | > Pasteurized Fluid Milk         |
| >                                 | >                                |
| > Soft Unripened Cheese           | > Soft Unripened Cheese          |
+-----------------------------------+----------------------------------+
| ######### CLUSTER 3               | ######## CLUSTER C               |
|                                   |                                  |
| > Deli-type Salads                | > Cooked RTE Crustaceans         |
| >                                 | >                                |
| > Dry/Semi-dry Fermented Sausages | > Fruits                         |
| >                                 | >                                |
| > Fresh Soft Cheese               | > Pâté and Meat Spreads          |
| >                                 | >                                |
| > Frankfurters, reheated          | > Unpasteurized Fluid Milk       |
| >                                 | >                                |
| > Fruits                          | > Smoked Seafood                 |
| >                                 |                                  |
| > Preserved Fish                  |                                  |
| >                                 |                                  |
| > Raw Seafood                     |                                  |
|                                   |                                  |
| # Semi-soft Cheese                |                                  |
|                                   |                                  |
| > Soft Ripened Cheese             |                                  |
| >                                 |                                  |
| > Vegetables                      |                                  |
+-----------------------------------+----------------------------------+
| ######## CLUSTER 4                | ######## CLUSTER D               |
|                                   |                                  |
| > Cultured Milk Products          | > Deli-type Salads               |
| >                                 | >                                |
| > Ice Cream and Other Frozen      | > Dry/Semi-dry Fermented         |
| > Dairy Products                  | > Sausages                       |
| >                                 | >                                |
| > Processed Cheese                | > Frankfurters, reheated         |
| >                                 | >                                |
| > Hard Cheese                     | > Fresh Soft Cheese              |
|                                   | >                                |
|                                   | > Soft Ripened Cheese            |
|                                   | >                                |
|                                   | > Semi-Soft Cheese               |
|                                   | >                                |
|                                   | > Vegetables                     |
+-----------------------------------+----------------------------------+
|                                   | ######## CLUSTER E               |
|                                   |                                  |
|                                   | > Cultured Milk Products         |
|                                   | >                                |
|                                   | > Hard Cheese                    |
|                                   | >                                |
|                                   | > Ice Cream and Other Frozen     |
|                                   | > Dairy Products                 |
|                                   | >                                |
|                                   | > Preserved Fish                 |
|                                   | >                                |
|                                   | > Processed Cheese               |
|                                   | >                                |
|                                   | > Raw Seafood                    |
+-----------------------------------+----------------------------------+

+---+---+------------------+---+--------------------+---+-------------+-------+
|   |   | ### Decrease     |   |                    |   |             |       |
|   |   | d Risk per Annum |   |                    |   |             |       |
+---+---+------------------+---+--------------------+---+-------------+-------+
| > |   | **Clusters A and |   | **Clusters C and   | # | ##          |       |
|   |   | B**              |   | D**                | # | # Cluster E |       |
| * |   |                  |   |                    | # |             |       |
| * |   |                  |   |                    |   |             |       |
| D |   |                  |   |                    |   |             |       |
| e |   |                  |   |                    |   |             |       |
| c |   |                  |   |                    |   |             |       |
| r |   |                  |   |                    |   |             |       |
| e |   |                  |   |                    |   |             |       |
| a |   |                  |   |                    |   |             |       |
| s |   |                  |   |                    |   |             |       |
| e |   |                  |   |                    |   |             |       |
| d |   |                  |   |                    |   |             |       |
| > |   |                  |   |                    |   |             |       |
|   |   |                  |   |                    |   |             |       |
| R |   |                  |   |                    |   |             |       |
| i |   |                  |   |                    |   |             |       |
| s |   |                  |   |                    |   |             |       |
| k |   |                  |   |                    |   |             |       |
| > |   |                  |   |                    |   |             |       |
|   |   |                  |   |                    |   |             |       |
| p |   |                  |   |                    |   |             |       |
| e |   |                  |   |                    |   |             |       |
| r |   |                  |   |                    |   |             |       |
| > |   |                  |   |                    |   |             |       |
|   |   |                  |   |                    |   |             |       |
| S |   |                  |   |                    |   |             |       |
| e |   |                  |   |                    |   |             |       |
| r |   |                  |   |                    |   |             |       |
| v |   |                  |   |                    |   |             |       |
| i |   |                  |   |                    |   |             |       |
| n |   |                  |   |                    |   |             |       |
| g |   |                  |   |                    |   |             |       |
| * |   |                  |   |                    |   |             |       |
| * |   |                  |   |                    |   |             |       |
+---+---+------------------+---+--------------------+---+-------------+-------+
|   |   | **[Very High     |   | **[High            |   | **[Moderate | ##    |
|   |   | Ris              |   | R                  |   | Risk]{.u    | ##### |
|   |   | k]{.underline}** |   | isk]{.underline}** |   | nderline}** |  Clus |
|   |   |                  |   |                    |   |             | ter 1 |
|   |   | **[(Clusters     |   | **[(Clusters 1-C,  |   | **[(Cluster |       |
|   |   | 1-A,             |   | 1                  |   | 1-E)]{.u    |       |
|   |   | 1-B              |   | -D)]{.underline}** |   | nderline}** |       |
|   |   | )]{.underline}** |   |                    |   |             |       |
|   |   |                  |   | Pâté and Meat      |   | No food     |       |
|   |   | Deli Meats       |   | Spreads            |   | categories  |       |
|   |   |                  |   |                    |   |             |       |
|   |   | Frankfurters     |   | Unpasteurized      |   |             |       |
|   |   | (not reheated)   |   | Fluid Milk         |   |             |       |
|   |   |                  |   |                    |   |             |       |
|   |   |                  |   | Smoked Seafood     |   |             |       |
+---+---+------------------+---+--------------------+---+-------------+-------+
|   |   |                  |   |                    |   |             | ###   |
|   |   |                  |   |                    |   |             | ####  |
+---+---+------------------+---+--------------------+---+-------------+-------+
|   |   | **[High          |   | **[Moderate        |   | **[Moderate | **Cl  |
|   |   | Ris              |   | R                  |   | Risk]{.u    | uster |
|   |   | k]{.underline}** |   | isk]{.underline}** |   | nderline}** | 2**   |
|   |   |                  |   |                    |   |             |       |
|   |   | **[(Clusters     |   | **[(Clusters 2-C,  |   | **[(Cluster |       |
|   |   | 2-A,             |   | 2                  |   | 2-E)]{.u    |       |
|   |   | 2-B              |   | -D)]{.underline}** |   | nderline}** |       |
|   |   | )]{.underline}** |   |                    |   |             |       |
|   |   |                  |   | Cooked RTE         |   | No food     |       |
|   |   | High Fat and     |   | Crustaceans        |   | categories  |       |
|   |   | Other Dairy      |   |                    |   |             |       |
|   |   | Products         |   |                    |   |             |       |
|   |   |                  |   |                    |   |             |       |
|   |   | Pasteurized      |   |                    |   |             |       |
|   |   | Fluid Milk       |   |                    |   |             |       |
|   |   |                  |   |                    |   |             |       |
|   |   | Soft Unripened   |   |                    |   |             |       |
|   |   | Cheese           |   |                    |   |             |       |
+---+---+------------------+---+--------------------+---+-------------+-------+
|   |   |                  |   |                    |   |             |       |
+---+---+------------------+---+--------------------+---+-------------+-------+
|   |   | **[Moderate      |   | **[Moderate        |   | **[Low      | **Cl  |
|   |   | Ris              |   | R                  |   | Risk]{.u    | uster |
|   |   | k]{.underline}** |   | isk]{.underline}** |   | nderline}** | 3**   |
|   |   |                  |   |                    |   |             |       |
|   |   | **[(Clusters     |   | **[(Clusters 3-C,  |   | **[(Cluster |       |
|   |   | 3-A,             |   | 3                  |   | 3-E)]{.u    |       |
|   |   | 3-B              |   | -D)]{.underline}** |   | nderline}** |       |
|   |   | )]{.underline}** |   |                    |   |             |       |
|   |   |                  |   | Deli-type Salads   |   | Preserved   |       |
|   |   | No food          |   |                    |   | Fish        |       |
|   |   | categories       |   | Dry/Semi-dry       |   |             |       |
|   |   |                  |   | Fermented          |   | Raw Seafood |       |
|   |   |                  |   |                    |   |             |       |
|   |   |                  |   | Sausages           |   |             |       |
|   |   |                  |   |                    |   |             |       |
|   |   |                  |   | Frankfurters       |   |             |       |
|   |   |                  |   | (reheated)         |   |             |       |
|   |   |                  |   |                    |   |             |       |
|   |   |                  |   | Fresh Soft Cheese  |   |             |       |
|   |   |                  |   |                    |   |             |       |
|   |   |                  |   | Fruits             |   |             |       |
|   |   |                  |   |                    |   |             |       |
|   |   |                  |   | Semi-soft Cheese   |   |             |       |
|   |   |                  |   |                    |   |             |       |
|   |   |                  |   | Soft Ripened       |   |             |       |
|   |   |                  |   | Cheese             |   |             |       |
|   |   |                  |   |                    |   |             |       |
|   |   |                  |   | Vegetables         |   |             |       |
+---+---+------------------+---+--------------------+---+-------------+-------+
|   |   |                  |   |                    |   |             |       |
+---+---+------------------+---+--------------------+---+-------------+-------+
|   |   | **[Moderate      |   | **[Low             |   | **[Very Low | **Cl  |
|   |   | Ris              |   | R                  |   | Risk]{.u    | uster |
|   |   | k]{.underline}** |   | isk]{.underline}** |   | nderline}** | 4**   |
|   |   |                  |   |                    |   |             |       |
|   |   | **[(Clusters     |   | **[(Clusters 4-C,  |   | **[(Cluster |       |
|   |   | 4-A,             |   | 4                  |   | 4-E)]{.u    |       |
|   |   | 4-B              |   | -D)]{.underline}** |   | nderline}** |       |
|   |   | )]{.underline}** |   |                    |   |             |       |
|   |   |                  |   | No food categories |   | Cultured    |       |
|   |   | No food          |   |                    |   | Milk        |       |
|   |   | categories       |   |                    |   | Products    |       |
|   |   |                  |   |                    |   |             |       |
|   |   |                  |   |                    |   | Hard Cheese |       |
|   |   |                  |   |                    |   |             |       |
|   |   |                  |   |                    |   | Ice Cream   |       |
|   |   |                  |   |                    |   | and Other   |       |
|   |   |                  |   |                    |   | Frozen      |       |
|   |   |                  |   |                    |   | Dairy       |       |
|   |   |                  |   |                    |   | Products    |       |
|   |   |                  |   |                    |   |             |       |
|   |   |                  |   |                    |   | Processed   |       |
|   |   |                  |   |                    |   | Cheese      |       |
+---+---+------------------+---+--------------------+---+-------------+-------+

**Summary Figure 1. Two-Dimensional Matrix of Food Categories Based on
Cluster Analysis of Predicted per Serving and per Annum Relative
Rankings**

\[The matrix was formed by the interception of the four per serving
clusters vs. three groups of per annum clusters (A and B, C and D, and
E). For example, Cluster 3-E (Low Risk) refers to the food categories
that are in both Cluster level 3 for the risk per serving and Cluster
level E for the risk per annum. See Summary Table 5.\]

[Risk Designation Very High]{.underline}. This designation includes two
food categories, Deli Meats and Frankfurters, Not Reheated. These are
food categories that have high predicted relative risk rankings on both
a per serving and per annum basis, reflecting the fact that they have
relatively high rates of contamination, support the relatively rapid
growth of *L. monocytogenes* under refrigerated storage, are stored for
extended periods, and are consumed extensively. These products have also
been directly linked to outbreaks of listeriosis. This risk designation
is one that is consistent with the need for immediate attention in
relation to the national goal for reducing the incidence of foodborne
listeriosis. Likely activities include the development of new control
strategies and/or consumer education programs suitable for these
products.

[Risk Designation High]{.underline}. This designation includes six food
categories, High Fat and Other Dairy Products, Pasteurized Fluid Milk,
Pâté and Meat Spreads, Soft Unripened Cheeses, Smoked Seafood, and
Unpasteurized Fluid Milk. These food categories all have in common the
ability to support the growth of *L. monocytogenes* during extended
refrigerated storage. However, the foods within this risk designation
appear to fall into two distinct groups based on their rates of
contamination and frequencies of consumption.

-   Pâté and Meat Spreads, Smoked Seafood, and Unpasteurized Fluid Milk
    have relatively high rates of contamination and thus high predicted
    per serving relative risks. However, these products are generally
    consumed only occasionally in small quantities and/or are eaten by a
    relatively small portion of the population, which lowers the per
    annum risk. All three products have been associated with outbreaks
    or sporadic cases, at least internationally.

These foods appear to be priority candidates for new control measures
(i.e., Smoked Seafood, Pâté and Meat Spreads) or continued avoidance
(i.e., Unpasteurized Fluid Milk).

-   High Fat and Other Dairy Products, Pasteurized Fluid Milk, and Soft
    Unripened Cheeses have low rates of contamination and corresponding
    relatively low predicted per serving relative risks. However, these
    products are consumed often by a large percentage of the population,
    resulting in elevated predicted per annum relative risks. In
    general, the predicted per annum risk is not matched with an
    equivalent U.S. epidemiologic record. However, the low frequency of
    recontamination of individual servings of these products in
    combination with their broad consumption makes it likely that these
    products are primarily associated with sporadic cases and normal
    case control studies would be unlikely to lead to the identification
    of an association between these products and cases of listeriosis.

These products (High Fat and Other Dairy Products, Pasteurized Fluid
Milk, and Soft Unripened Cheeses) appear to be priority candidates for
advanced epidemiologic and scientific investigations to either confirm
the predictions of the risk assessment or identify the factors not
captured by the current models that would reduce the predicted relative
risk.

[Risk Designation Moderate.]{.underline} This risk designation includes
nine food categories (Cooked Ready-to-Eat Crustaceans, Deli-type Salads,
Dry/Semi-Dry Fermented Sausages, Frankfurters-Reheated, Fresh Soft
Cheese, Fruits, Semi-soft Cheese, Soft Ripened Cheese, and Vegetables)
that encompass a range of contamination rates and consumption profiles.
A number of these foods include effective bactericidal treatments in
their manufacture or preparation (e.g., Cooked Ready-to-Eat Crustaceans,
Frankfurters-Reheated, Semi-soft Cheese) or commonly employ conditions
or compounds that inhibit the growth of *L. monocytogenes* (e.g., Deli
Salads, Dry/Semi-dry Fermented Sausages). The risks associated with
these products appear to be primarily associated with product
recontamination, which in turn, is dependent on continued, vigilant
application of proven control measures.

It is worth noting that two food categories, Fresh Soft Cheese and Soft
Ripened Cheese, were previously classified as higher risk products in
the draft 2001 version of the risk assessment. This change reflects the
acquisition of extensive new exposure data that indicate a significant
reduction in contamination rates. The changes in contamination rates, in
turn, appear to be the result of increased use of pasteurized or
otherwise heat-treated milk, and reflect how relative risk can change as
a result of effective food safety control programs.

[Risk Designation Low]{.underline}. This risk designation includes two
food categories, Preserved Fish and Raw Seafood. Both products have
moderate contamination rates, but include conditions (e.g.,
acidification) or consumption characteristics (e.g., short shelf-life)
that limit *L. monocytogenes* growth and thus limit predicted per
serving risks. The products are generally consumed in small quantities
by a small portion of the population on an infrequent basis, which
results in low predicted per annum relative risks. Exposure data for
these products are limited so there is substantial uncertainty in the
findings. However, the current results predict that these products, when
manufactured consistent with current good manufacturing practices, are
not likely to be a major source of foodborne listeriosis.

[Risk Designation Very Low]{.underline}. This risk designation includes
four food categories, Cultured Milk Products, Hard Cheese, Ice Cream and
Other Frozen Dairy Products, and Processed Cheese. These products all
have in common the characteristics of being subjected to a bactericidal
treatment, having very low contamination rates, and possessing an
inherent characteristic that either inactivates *L. monocytogenes*
(e.g., Cultured Milk Products, Hard Cheese) or prevents its growth
(e.g., Ice Cream and Other Frozen Dairy Products, Processed Cheese).
This results in a very low predicted per serving relative risks. The
predicted per annum relative risks are also low despite the fact that
these products are among the more commonly consumed ready-to-eat
products considered by the risk assessment. The results of the risk
assessment predict that unless there was a gross error in their
manufacture, these products are highly unlikely to be a significant
source of foodborne listeriosis.

**Conclusions**

The following conclusions are provided as an integration of the results
derived from the models, the evaluation of the variability and
uncertainty underlying the results, and the impact that the various
qualitative factors identified in the hazard identification, exposure
assessment, and hazard characterization have on the interpretation of
the risk assessment.

-   The risk assessment reinforces past epidemiological conclusions that
    foodborne listeriosis is a moderately rare although severe disease.
    United States consumers are exposed to low to moderate levels of *L.
    monocytogenes* on a regular basis.

-   The risk assessment supports the findings of epidemiological
    investigations of both sporadic illness and outbreaks of listeriosis
    that certain foods are more likely to be vehicles for *L.
    monocytogenes*.

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-   Three dose-response models were developed that relate the exposure
    to different levels of *L. monocytogenes* in three age-based
    subpopulations \[i.e., perinatal (fetuses and newborns), elderly,
    and intermediate-age\] with the predicted number of fatalities.
    These models were used to describe the relationship between levels
    of *L. monocytogenes* ingested and the incidence of listeriosis. The
    dose of *L. monocytogenes* necessary to cause listeriosis depends
    greatly upon the immune status of the individual.

1.  Susceptible subpopulations (such as the elderly and perinatal) are
    more likely to contract listeriosis than the general population.

2.  Within the intermediate-age subpopulation group, almost all cases of
    listeriosis are associated with specific subpopulation groups with
    increased susceptibility (e.g., individuals with chronic illnesses,
    individuals taking immunosuppressive medication).

3.  The strong association of foodborne listeriosis with specific
    population groups suggests that strategies targeted to these
    susceptible population groups, i.e., perinatal (pregnant women),
    elderly, and susceptible individuals within the intermediate-age
    group, would result in the greatest reduction in the public health
    impact of this pathogen.

-   The dose-response models developed for this risk assessment
    considered, for the first time, the range of virulence observed
    among different isolates of *L. monocytogenes*. The dose-response
    curves suggest that the relative risk of contracting listeriosis
    from low dose exposures could be less than previously estimated.

-   The exposure models and the accompanying 'what-if' scenarios
    identify five broad factors that affect consumer exposure to *L.
    monocytogenes* at the time of food consumption.

1.  Amounts and frequency of consumption of a ready-to-eat food

2.  Frequency and levels of *L. monocytogenes* in a ready-to-eat food

3.  Potential of the food to support growth of *L. monocytogenes* during
    refrigerated storage

4.  Refrigerated storage temperature

5.  Duration of refrigerated storage before consumption

Any of these factors can affect potential exposure to *L. monocytogenes*
from a food category. These factors are 'additive' in the sense that
foods where multiple factors favor high levels of *L. monocytogenes* at
the time of consumption are typically more likely to be riskier than
foods where a single factor is high. These factors also suggest several
broad control strategies that could reduce the risk of foodborne
listeriosis such as reformulation of products to reduce their ability to
support the growth of *L. monocytogenes* or encouraging consumers to
keep refrigerator temperatures at or below 40 ºF and reduce refrigerated
storage times. For example, the 'what-if' scenarios using Deli Meats
predict that consumer education and other strategies aimed at
maintaining home refrigerator temperatures at 40 ºF could substantially
reduce the risks associated with this food category. Combining this with
pre-retail treatments that decrease the contamination levels in Deli
Meats would be expected to reduce the risk even further.

The models generated as the basis for this risk assessment can be used
to further evaluate the impact of listeriosis on the public health. For
example, the Food and Agricultural Organization/World Health
Organization (FAO/WHO) risk assessment on *L. monocytogenes*, which is
largely based on the approaches used in the current risk assessment, is
being developed to consider several risk management questions posed by
Codex Alimentarius. It is anticipated that additional risk assessments
on individual foods within specific food categories will be conducted to
help answer specific questions about how individual steps in their
production and processing impact public health, including the likely
effectiveness of different preventative strategies. The models may be
used to evaluate the expected public health impact of preventive
controls such as storage limits, sanitation improvements, or new
processing technologies. Sources of contamination during food production
and retail conditions can also be added to the model to provide more
detailed examination of factors contributing to the risk of listeriosis
from the final product. For example, the FSIS *Listeria* Risk Assessment
in Deli Meats, used portions of the exposure and dose-response models
from the current risk assessment to develop information about the
effects of combining testing, sanitation and post-lethality processing
interventions to reduce cases of listeriosis.

The models may also be used to evaluate the impact of hypothetical
changes in a process, such as limits on storage time or temperature, to
provide insight to how the different components of the model interact.
The 'what-if' scenarios modeled in this risk assessment provide insight
to the impact on public health of limiting storage times, avoiding high
temperature refrigeration storage, and reducing contamination levels.
Scenario testing emphasizes that the results of any risk assessment are
influenced by the assumptions and data sets that were used to develop
the exposure assessment and hazard characterization. The results of this
revised *L.* *monocytogenes* risk assessment, particularly the predicted
relative risk ranking values, could change as a result of the
availability of new information, changes in scientific approaches, or
data.

This risk assessment significantly advances our ability to describe our
current state of knowledge about this important foodborne pathogen,
while simultaneously providing a framework for integrating and
evaluating the impact of new scientific knowledge on public health
enhancement.

[^1]: FoodNet is the Foodborne Diseases Active Surveillance Network,
    which conducts active surveillance for foodborne diseases and
    related epidemiological studies designed to help public health
    officials better understand the epidemiology of foodborne diseases
    in the United States.