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).
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
1. Symptoms of listeriosis
The incubation period (time between ingestion and the onset of symptoms) for Listeria ranges from three to 70 days and averages 21 days (Bryan, 1999).
A person with listeriosis may develop fever, muscle aches, and sometimes gastrointestinal symptoms such as nausea or diarrhea (CDC website, 2009). 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) (Bortolussi, 2008). 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.
2. 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.
3. 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.
Symptoms of listeriosis, treatment and prevent listeriosis