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Listeria monocytogenes--questions and answers.

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 people with immune systems that have been weakened by a disease (e.g., diabetes, organ transplant, cancer, age) or by 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, flulike 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 (CDC) estimate that approximately 2,500 people become seriously ill and 500 people 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 United States have been associated with frankfurters (hot dogs), deli/luncheon meats, pate, salami, Brie cheese, Mexican-style soft cheese, shrimp, butter, raw vegetables, and pasteurized milk. Outside the United States, shellfish, raw fish, smoked seafood, pork tongue, cream, rice salad, coleslaw, soft cheeses, pate, rillettes, unpasteurized milk, and butter have been 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 do. This bacterium survives on cold surfaces and also can multiply slowly at 0[degrees]C (32[degrees]F), defeating one traditional food safety defense--refrigeration. Optimal refrigeration temperatures of 40[degrees]F (4.4[degrees]C) or below stop the multiplication of most other foodborne bacteria, although 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[degrees]F) or when foods are stored for extended periods. Commercial freezer temperatures of 0[degrees]F (-18[degrees]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, the Food and Drug Administration (FDA) and the Food Safety and Inspection Service (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 and

* using precooked or ready-to-eat perishable items 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 cheeses such as feta, Brie, and Camembert, or blue-veined cheeses, queso blanco, queso fresco, or Panela unless the cheese 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: CDC has been evaluating the incidence of foodborne listeriosis since 1996 through its FoodNet foodborne-disease surveillance program. The program has shown a decline in the incidence of Listeria monocytogenes infections of approximately 40 percent 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 FDA and 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 percent.

For more information on Listeria monocytogenes, visit

(Adapted from "Listeria monocytogenes Risk Assessment Questions and Answers," FDA/Center for Food Safety and Applied Nutrition and USDA/FSIS, October 21, 2003,
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Title Annotation:Technical Briefs
Publication:Journal of Environmental Health
Geographic Code:1USA
Date:Sep 1, 2004
Previous Article:Letters to the editor.
Next Article:Contaminated produce is top food-poisoning culprit.

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