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  1. Risk and Safety Assessments (Food)

Listeria monocytogenes Risk Assessment Questions and Answers

October 21, 2003

Q: What is Listeria monocytogenes?

A: Listeria monocytogenes is a bacterium that can cause listeriosis, a serious bacterial infection in humans.  L. monocytogenes is commonly found in soil and water and on plant material, particularly decaying plant material. These environments are regarded as the natural habitat of this organism. The bacterium often moves through the animal and human intestinal tract without causing illness, and has been found in many domestic and wild animals, including birds and fish.  L. monocytogenes may enter a food processing environment through incoming food materials and on the shoes and clothing of personnel.  Once L. monocytogenes is in a food processing environment, it may become established on food contact surfaces and non-food contact surfaces (e.g. floors and floor drains).  

Q: What is listeriosis?

A: Listeriosis is a serious and potentially fatal infection caused by eating food contaminated with the bacterium L. monocytogenes.  The disease affects primarily pregnant women, older adults and persons with weakened immune systems due to a disease (e.g., diabetes, organ transplant, cancer, age) or due to medications (e.g., steroids).  A person with listeriosis initially has fever, muscle aches, and sometimes gastrointestinal symptoms, such as nausea or diarrhea.  If infection spreads to the nervous system, symptoms such as headache, stiff neck, confusion, loss of balance, or convulsions can occur.  In some instances other organ systems within the body can become infected.  Infected pregnant women may experience only a mild, flu-like illness; however, their unborn children are at risk.  Infections during pregnancy can lead to premature delivery, infection of the newborn, or even stillbirth.

Listeriosis is a significant public health concern because it is life-threatening.  Death occurs in about 20 percent of the cases.  The Centers for Disease Control and Prevention estimates that approximately 2500 persons become seriously ill and 500 persons die each year from listeriosis.

Q: What is the incubation period for Listeria infection?

A: There can be a substantial delay between the time of ingestion of contaminated food and the onset of serious symptoms. The average time from exposure to illness is approximately 30 days, but symptoms can appear as long as 90 days after exposure. It is important for consumers to know that the infection can occur as much as 90 days later, so that they can seek appropriate treatment if they have symptoms of Listeria infection.

Q: What foods have been associated with cases of listeriosis?

A: Cases of listeriosis in the U.S. have been associated with frankfurters (hot dogs), deli/luncheon meats, pâté, salami, brie cheese, mexican-style soft cheese, shrimp, butter, raw vegetables, and pasteurized milk. Additionally, outside the U.S., shellfish, raw fish, smoked seafood, pork tongue, cream, rice salad, coleslaw, soft cheeses, pâté, rillettes, unpasteurized milk, and butter were associated with outbreaks of listeriosis.

Q:  How does L. monocytogenes survive in ready-to-eat foods?

A:  L. monocytogenes is a remarkably tough organism. It resists heat, salt, nitrite and acidity much better than many organisms. This bacterium survives on cold surfaces and also can multiply slowly at 0°C (32°F), defeating one traditional food safety defense--refrigeration. Optimal refrigeration temperatures of 40°F (4.4°C) or below stops the multiplication of most other foodborne bacteria.  Refrigeration does not kill most bacteria. The risk from L. monocytogenes in foods that support its growth is increased substantially when such foods are stored in refrigerators that are too warm (>40°F) or when foods are stored for extended periods.

Commercial freezer temperatures of 0°F (-18°C) stop L. monocytogenes from multiplying. 

Q: What advice do you give to consumers to reduce their risk of contracting listeriosis?

A: Because Listeria monocytogenes can grow at refrigerator temperatures FDA and FSIS advise all consumers to reduce the risk of illness by:

  • Using a refrigerator thermometer to make sure that the refrigerator always stays at 40 degrees F or below.
  • Using perishable items that are precooked or ready-to-eat as soon as possible;

The following additional advice is provided for pregnant women, older adults, and people with weakened immune systems who are at a higher risk for foodborne disease, including listeriosis:

Do not eat hot dogs and luncheon meats, unless they are reheated until steaming hot.

Do not eat soft cheese such as Feta, Brie, and Camembert cheeses, blue-veined cheeses, queso blanco, queso fresco, and Panela unless it is labeled as made with pasteurized milk.

Do not eat refrigerated smoked seafood, unless it is contained in a cooked dish, such as a casserole.  Refrigerated smoked seafood, such as salmon, trout, whitefish, cod, tuna, or mackerel, is most often labeled as "nova-style," "lox," "kippered," "smoked," or "jerky."  The fish is found in the refrigerator section or sold at deli counters of grocery stores and delicatessens.  Canned or shelf-stable smoked seafood may be eaten.

Do not drink raw (unpasteurized) milk or eat foods that contain unpasteurized milk.

Q: What has been the trend in the incidence of foodborne listeriosis over the past several years.

A: The Centers for Disease Control and Prevention has been evaluating the incidence of foodborne listeriosis since 1996 through its FoodNet active foodborne disease surveillance program.  The program has shown a decline in the incidence of Listeria monocytogenes infections of approximately 40% during the period from 1996 to 2002, and further improvements are anticipated in 2003.  The improvement in public health appears to be the direct result of changes instituted by the food industry and new regulatory initiatives developed by the FDA and the FSIS.  With this progress, the regulatory agencies are on target for meeting the Administration's Healthy People 2010 goal of reducing the incidence of foodborne listeriosis by 50%. 

Q: Where can I purchase a refrigerator thermometer and are they expensive?

A: Refrigerator thermometers are available for purchase at grocery stores, department stores, kitchen stores, on the internet and a variety of other places.  The cost generally ranges from $2 to $20 depending on the type of thermometer purchased.

 The Listeria Risk Assessment  

Q: Why did FDA and FSIS conduct a risk assessment of L. monocytogenes?

A: FDA's risk assessment, conducted with FSIS and the Centers for Disease Control and Prevention, was designed to predict the potential relative risk of listeriosis from eating certain ready-to-eat foods among three age-based groups of people - perinatal (16 weeks after fertilization to 30 days after birth), elderly (60 years of age and older), and intermediate-age (general population, less than 60 years of age). This assessment evaluated foods within 23 categories considered to be principal potential sources of Listeria.  The results of this assessment will assist both FDA and FSIS in the evaluation of the adequacy and focus of current programs, help in the development of new programs to ensure that these programs protect the public health and to evaluate the effectiveness of new strategies to minimize the public health impact of food-borne Listeria. 

Q:   How were the food categories selected for the risk assessment?

A:  Scientific and medical literature were reviewed to identify foods associated with Listeria monocytogenes.  The published and unpublished literature included studies on outbreaks, sporadic cases, and food surveys.  The identified foods were grouped into categories within five commodity groups including seafood, produce, dairy, meats, and combination foods (mainly salads that are composed of a mixture of ingredients, such as meat, poultry, seafood, egg and pasta).   In keeping with the FDA data quality guidelines, all data used in the risk assessment is available for review.

Q: What are the conclusions of the risk assessment?

A: The main findings from the risk assessment are:

  1. The risk assessment reinforces past conclusions that foodborne listeriosis is a moderately rare although severe disease.
  2. The risk assessment supports the findings of epidemiological investigations of both sporadic illness and outbreaks of listeriosis, e.g., pâté, fresh soft cheeses, smoked seafood, frankfurters, and foods typically purchased from deli counters as potential vehicles of listeriosis for susceptible populations.
  3. New case control studies and other advanced epidemiological investigations are needed to reflect changes in food processing, distribution patterns, preparation, and consumption practices.
  4. From the exposure models and "what-if scenarios", it is apparent that five factors affect consumer exposure to L. monocytogenes at the time of food consumption.
    • Amount and frequency of consumption of a food
    • Frequency and levels of L. monocytogenes in ready-to-eat food
    • Potential to support growth of L. monocytogenes in food during refrigerated storage
    • Refrigerated storage temperature.
    • Duration of refrigerated storage before consumption.

Q: The assessment presents the results on a per serving and a per annum basis.  Why?

A: The predicted risk on a per serving basis is the risk to an individual consumer.  This is the probability that consuming one serving of a food will cause listeriosis in a single individual.  On the other hand, the predicted risk on a per annum basis is the risk for the entire population.  The per annum risk estimates take into account the total number of servings of a food consumed each year.  In general, the uncertainty and variability in consumption rates can magnify the uncertainty of the predicted risk on a per annum basis.  

Q: Why has there been change in the advice to consumers relative to soft cheeses.

A: The risk assessment includes substantial new data from surveys of soft cheeses at retail and from outbreaks of listeriosis associated with these food categories.  This new information indicates that the consumers' risk of listeriosis from soft cheeses, such as queso blanco, queso fresco, Panela, Camembert, Feta, Brie, and blue-veined cheeses, is primarily associated with cheeses made from unpasteurized milk.  Thus, the consumer who wants to eat soft cheeses should make a healthy food choice by purchasing only those soft cheeses that are clearly labeled as being made from pasteurized milk.  

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