What is Listeria?

Listeria monocytogenes (Listeria) is a foodborne disease-causing bacteria; the disease is called listeriosis. Listeria can invade the body through a normal and intact gastrointestinal tract. Once in the body, Listeria can travel through the blood stream but the bacteria are often found inside cells. Listeria also produces toxins that damage cells. Listeria invades and grows best in the central nervous system among immune compromised persons, causing meningitis and/or encephalitis (brain infection). In pregnant women, the fetus can become infected, leading to spontaneous abortion, stillbirths, or sepsis (blood infection) in infancy.

Approximately 2,500 cases of listeriosis are estimated to occur in the U.S. each year. About 200 in every 1000 cases result in death. Certain groups of individuals are at greater risk for listeriosis, including pregnant women (and their unborn children) and immunocompromised persons. Among infants, listeriosis occurs when the infection is transmitted from the mother, either through the placenta or during the birthing process. These host factors, along with the amount of bacteria ingested and the virulence of the strain, determine the risk of disease. Human cases of listeriosis are, for the most part, sporadic and treatable. Nonetheless, Listeria remains an important threat to public health, especially among those most susceptible to this disease.

Listeria is often isolated in cattle, sheep, and fowl, and is also found in dairy products, fruits, and vegetables.

Symptoms of Listeria Infection

It is thought that ingestion of as few as 1,000 cells of Listeria bacteria can result in illness. After ingestion of food contaminated with Listeria, incubation periods (from time of exposure to onset of illness) are in the range of one to eight weeks, averaging about 31 days. Five days to three weeks after ingestion, Listeria has access to all body areas and may involve the central nervous system, heart, eyes, or other locations.

A person with listeriosis usually has fever, muscle aches, and gastrointestinal symptoms such as nausea or diarrhea. If infection spreads to the nervous system, symptoms such as headache, stiff neck, loss of balance, confusion, obtundation (decreased consciousness) or convulsions can occur. With brain involvement, listeriosis may mimic a stroke. Infected pregnant women will ordinarily experience only a mild, flu-like illness; however, infection during pregnancy can lead to miscarriage, infection of the newborn or even stillbirth. Pregnant women are about 20 times more likely than other healthy adults to get listeriosis; about one-third of listeriosis cases happen during pregnancy. The incidence of listeriosis in the newborn is 8.6 cases per 100,000 live births. The perinatal and neonatal mortality rate (stillbirths and early infant deaths) from listeriosis is 80%.

Diagnosis and treatment of Listeria Infection

If you have symptoms of listeriosis, a health care provider can have a blood or spinal fluid test done to detect the infection. During pregnancy, a blood test is the most reliable way to find out if your symptoms are due to listeriosis. If you are in a high-risk group, have eaten the contaminated product, and within 2 months become ill with fever or signs of serious illness, you should contact your health care provider and inform him or her about this exposure.

There are several antibiotics with which Listeria may be treated. When infection occurs during pregnancy, antibiotics given promptly to the pregnant woman can often prevent infection of the fetus. 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

General recommendations include: thoroughly cook raw food from animal sources; keep uncooked meats separate from vegetables and from cooked 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; wash raw vegetables thoroughly before eating; and 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 or deli meats, unless they are reheated until steaming hot, and wash hands after handling those products; do not eat soft cheeses (such as feta, Brie, Camembert, blue-veined, or Mexican-style cheese), unless they have labels that clearly state they are made from pasteurized milk; and do not eat meat spreads or smoked seafood from the refrigerated or deli section of the store (canned or shelf-stable products may be eaten).

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