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Listeriosis is a serious infection caused by eating food contaminated with the bacterium called Listeria monocytogenes. Although there are other types of Listeria, most cases of listeriosis are caused by Listeria monocytogenes. Listeria is found in soil and water. Vegetables can become contaminated from the soil or from manure used as fertilizer. Animals can carry the bacterium 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 unpasteurized (raw) milk or foods made from unpasteurized milk. Listeria is killed by pasteurization and cooking; however, in certain ready-to-eat foods, like hot dogs and cold cuts from the deli counter, contamination may occur after cooking but before packaging.
Adults can get listeriosis by eating food contaminated with Listeria, but babies can be born with listeriosis if their mothers eat contaminated food during pregnancy. Infections during pregnancy can cause premature delivery, miscarriage, stillbirth, or serious health problems for the newborn. 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.
Approximately 2,500 illnesses and 500 deaths are attributed to listeriosis in the United States annually. Neonatal infections are often severe, with a mortality (death) rate of 25-50%. Recent surveillance data estimate that the incidence of listeriosis is 0.29 cases per 100,000 population. These data from 2008 represent a decrease of 36% compared with 1996−1998. Listeriosis-related mortality rates also decreased annually by 10.7% from 1990 through 1996 and by 4.3% from 1996 through 2005. Seasonal trends show a distinct peak in cases overall and in deaths from listeriosis from July through October.
Symptoms of Listeria infection
The incubation period (time between ingestion and the onset of symptoms) for Listeria ranges from three to 70 days and averages 21 days. A person with listeriosis may develop 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 immune-deficient individuals, Listeria can invade the central nervous system, causing meningitis and/or encephalitis (brain infection). Infected pregnant women ordinarily experience only a mild, flu-like illness; however, infection during pregnancy can lead to miscarriage, infection of the newborn or even stillbirth. Newborns may present clinically with early-onset (less than 7 days) or late-onset forms of infection (7 or more days). Those with the early-onset form are often diagnosed in the first 24 hours of life with sepsis (infection in the blood). Early-onset listeriosis is most often acquired from the mother through transplacental transmission. Late-onset neonatal listeriosis is less common than the early-onset form. Clinical symptoms may be subtle and include irritability, fever and poor feeding. The mode of acquisition of late-onset listeriosis is poorly understood.
Diagnosis and treatment of Listeria infection
If you have symptoms such as fever or stiff neck, consult your doctor. A blood or spinal fluid test (to cultivate the bacteria) 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. There is no routine screening test for listeriosis during pregnancy, as there is for rubella and some other congenital infections.
Invasive infections with Listeria can be treated with antibiotics. 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.
Preventing Listeria infection
- 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, such as pregnant women and persons with weakened immune systems, in addition to the recommendations listed above include:
- 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.