Listeriosis is a serious infection caused by eating food contaminated with the bacterium Listeria monocytogenes. Listeriosis primarily affects pregnant women, newborns, and adults with weakened immune systems.

In the United States, an estimated 2,500 persons become seriously ill with listeriosis each year. Of these, 500 die. Pregnant women are about 20 times more likely than other healthy adults to get listeriosis. About one-third of listeriosis cases happen during pregnancy. Other people at risk include newborns (who suffer the serious effects of infection in pregnancy rather than the pregnant women themselves), people with cancer, diabetes, or kidney disease, people with AIDS (almost 300 times more likely to get listeriosis than people with normal immune systems), people who take glucocorticosteroid medications, and the elderly.

Listeriosis Symptoms

Symptoms of listeriosis include fever, muscle aches, and sometimes gastrointestinal symptoms (nausea or diarrhea). If listeriosis spreads to the nervous system, symptoms such as headache, stiff neck, confusion, loss of balance, or convulsions can occur.

Infected pregnant women may experience only a mild flu-like illness; however infections during pregnancy can lead to miscarriage or stillbirth, premature delivery, or infection of the newborn. Newborns rather than the pregnant women themselves suffer the serious effects of infection in pregnancy.

How does Listeria get into food?

Listeria monocytogenes is found in soil and water. Vegetables can become contaminated from the soil or from manure used as fertilizer.

Animals can carry Listeria without appearing ill and can contaminate foods of animal origin such as meats and dairy products. Listeria has been found in a variety of raw foods, such as uncooked meats and vegetables, as well as in processed foods that become contaminated after processing, such as soft cheeses and cold cuts at the deli counter. Unpasteurized (raw) milk or foods made from unpasteurized milk may contain Listeria.

Listeria is killed by pasteurization and cooking. However, in certain ready-to-eat foods such as hot dogs and deli meats, contamination may occur after cooking but before packaging.

How do you get listeriosis?

You get listeriosis by eating food contaminated with Listeria. Babies can be born with listeriosis if their mothers eat contaminated food during pregnancy. Although healthy persons may consume contaminated foods without becoming ill, those at increased risk for infection can probably get listeriosis after eating food contaminated with even a few bacteria. Persons at risk can prevent listeriosis by avoiding certain high-risk foods and by handling food properly.

The general guidelines recommended for the prevention of listeriosis are similar to those used to help prevent other foodborne illnesses, such as salmonellosis.

Reducing the Risk of Listeriosis

General recommendations:

• Thoroughly cook raw food from animal sources, such as beef, pork, or poultry
• Wash raw vegetables thoroughly before eating
• Keep uncooked meats separate from vegetables and from cooked foods and ready-to-eat foods
• Avoid unpasteurized (raw) milk or foods made from unpasteurized milk
• Wash hands, knives, and cutting boards after handling uncooked foods
• Consume perishable and ready-to-eat foods as soon as possible

Recommendations for persons at high risk (in addition to the above general recommendations):

• Do not eat hot dogs, luncheon meats, or deli meats, unless they are reheated until steaming hot
• Avoid getting fluid from hot dog packages on other foods, utensils, and food preparation surfaces, and wash hands after handling hot dogs, luncheon meats, and deli meats.
• Do not eat soft cheeses such as feta, Brie, and Camembert, blue-veined cheeses, or Mexican-style cheeses such as queso blanco, queso fresco, and Panela, unless they have labels that clearly state they are made from pasteurized milk.
• Do not eat refrigerated pâtés or meat spreads. Canned or shelf-stable pâtés and meat spreads may be eaten.
• 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.

Diagnosing Listeriosis

There is no routine screening test for susceptibility to listeriosis during pregnancy, as there is for rubella and some other congenital infections. If you have symptoms such as fever or stiff neck, consult your doctor. A blood or spinal fluid test (to cultivate Listeria) will show if you have listeriosis. During pregnancy, a blood test is the most reliable way to find out if your symptoms are due to listeriosis.

What should I do if I have eaten a food recalled because of Listeria contamination?

The risk of an individual person developing Listeria infection after consumption of a contaminated product is very small. If you have eaten a contaminated product and do not have any symptoms, it is actually recommended that you do not undergo any tests or treatment, even if you are in a high-risk group. However, if you are in a high-risk group, and have eaten the contaminated product, and within two months become ill with fever or signs of serious illness, then you should contact your physician and inform him or her about this exposure.

Treating Listeriosis

When infection occurs during pregnancy, antibiotics given promptly to the pregnant woman can often prevent infection of the fetus or newborn. Babies with listeriosis receive the same antibiotics as adults, although a combination of antibiotics is often used until physicians are certain of the diagnosis. Even with prompt treatment, some infections result in death. This is particularly likely in the elderly and in persons with other serious medical problems.