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.
What are the 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%.
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. The mode of transmission of Listeria to the fetus is either transplacental via the maternal blood stream or ascending from a colonized genital tract (Silver, 1998). 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.
The Centers for Disease Control and Prevention (CDC), U.S. Department of Agriculture’s Food Safety and Inspection Service (USDA-FSIS) and Food and Drug Administration (FDA) Center for Food Safety and Applied Nutrition established the Foodborne Diseases Active Surveillance Network (FoodNet) to monitor the effectiveness of food safety interventions in 1996. In a study of FoodNet laboratory-confirmed invasive cases (where infection is detected in blood, cerebrospinal fluid, amniotic fluid, placenta or products of conception), listeriosis decreased by 24% from 1996 through 2003; pregnancy-associated disease decreased by 37%; and cases among those ≥50 years old decreased by 23% (Voetsch et al., 2007). The highest incidence was reported among Hispanic persons from 1997 through 2001. Differences in incidence by age group and ethnicity may be explained by dietary preferences. More recently, FoodNet surveillance data revealed that the incidence of listeriosis in FoodNet active surveillance sites was 0.27 cases per 100,000 population in 2007, representing a decrease of 42% compared with 1996—1998 (CDC, 2009). The incidence did not change significantly in 2007 compared with 2004—2006 but remained higher than at its lowest point in 2002 (CDC, 2008).
Listeriosis-related mortality rates also decreased annually by 10.7% from 1990 through 1996 and by 4.3% from 1996 through 2005 (Bennion et al., 2008). Seasonal trends show a distinct peak in cases overall and in deaths from listeriosis from July through October. The most recent data suggest that about 2,500 illnesses and 500 deaths are attributed to listeriosis in the United States annually (CDC website, 2009). Neonatal infections are often severe, with a mortality rate of 25-50% (Bortolussi, 2008).
Treatment for Listeriosis
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.
Who is most susceptible to Listeria monocytogenes infection?
Healthy adults and children occasionally get infected with Listeria, but they rarely become seriously ill. Several segments of the population are at increased risk and need to be informed so that proper precautions can be taken. The body’s defense against Listeria is called “cell-mediated immunity” because it depends on our cells (as opposed to our antibodies), especially lymphocytes called “T-cells.” Therefore, individuals whose cell-mediated immunity is suppressed are more susceptible to the devastating effects of listeriosis. Pregnant women naturally have a depressed cell-mediated immune system. In addition, the systems of fetuses and newborns are very immature and are extremely susceptible to these types of infections. Other adults, especially transplant recipients and lymphoma patients, are given necessary therapies with the specific intent of depressing T-cells, and these individuals become especially susceptible to Listeria as well.
Individuals at increased risk include (CDC website, 2009):
• Pregnant women: They are about 20 times more likely than other healthy adults to get listeriosis. About one-third of listeriosis cases happen during pregnancy.
• Newborns: Newborns rather than the pregnant women themselves suffer the serious effects of infection in pregnancy.
• Persons with weakened immune systems
• Persons with cancer, diabetes, or kidney disease
• Persons with AIDS: They are almost 300 times more likely to get listeriosis than people with normal immune systems.
• Persons who take glucocorticosteroid medications (such as cortisone)
• The elderly
How to prevent Listeriosis?
The CDC (CDC website, 2009) provides several recommendations to avoid infection with Listeria:
• 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.
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