National Toxicology Program

Overview

Listeria are bacteria which can cause an infection known as listeriosis. The bacteria are very resistant to common food preservation agents such as heat, salt, nitrite, and acids. It can also multiply in refrigerated foods. Listeria is often present in the intestines of seemingly healthy animals. The bacteria can contaminate milk and meat products produced from infected animals and can also contaminate vegetables fertilized with contaminated manure. Since the early 1980s Listeria infections have been traced to food products such as coleslaw, milk, soft cheeses, hot dogs, and luncheon meats. Pregnant women are advised to avoid foods, such as soft cheeses, that are easily contaminated with Listeria. Taking precautions such as thoroughly cooking foods, eating only pasteurized milk products, washing fruits and vegetables, and washing hands after contact with raw meat also reduces the chances of contracting listeriosis. The federal government has established programs to test for Listeria in ready-to-eat foods and to recall food containing the bacteria. Healthy people are generally resistant to listeriosis but pregnant women are very susceptible to the infection. Listeria infections in pregnant women may result in miscarriages or stillbirths. Meningitis (brain infections) and septicemia (bacteria in the bloodstream) may occur in infants born to women with listeriosis.

Description of Listeria

According to the U.S. Department of Agriculture (USDA, April 1992), "Listeria refers to a genus (related group) of bacteria. One species in this genus, Listeria monocytogenes, can cause a serious bacterial infection called listeriosis. Usually when public health officials refer to Listeria, they are referring specifically to Listeria monocytogenes."

The USDA (USDA, April 1992) goes on to say, "L. monocytogenes is a remarkable tough organism. It resists heat, salt, nitrite and acidity much better than many organisms. The bacteria survive on cold surfaces and also can multiply slowly at 24 degrees Fahrenheit, defeating one traditional food safety defense--refrigeration. (Refrigeration at 40 degrees Fahrenheit or below stops the multiplication of many other food borne bacteria. Refrigeration does not kill most bacteria.) Commercial freezer temperatures of 0 degrees Fahrenheit, however, will stop L. monocytogenes from multiplying."

Sources of Listeria

The Centers for Disease Control and Prevention (CDC 2003) has stated, "Listeria monocytogenes 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. The bacterium 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 the bacterium. Listeria is killed by pasteurization, and heating procedures used to prepare ready-to-eat processed meats should be sufficient to kill the bacterium; however, unless good manufacturing practices are followed, contamination can occur after processing." It is important to note that "contaminated food may not look, smell or taste different from uncontaminated food."  (FDA July 1997)

Recent cases of listeriosis have been traced to food products. The USDA (USDA, April 1992) states that:

• An outbreak in 1981 in Nova Scotia resulted in 41 cases of listeriosis including 18 deaths; 83 percent of the cases were perinatal (occurring near the time of birth). The outbreak was traced to L. monocytogenes on coleslaw that had been made from cabbage grown in field fertilized with manure from Listeria-infected sheep.
• An outbreak in 1983 in Boston resulted in 49 cases of listeriosis including 14 deaths; 14 percent of the cases were perinatal, the remainder in immunocompromised adults. Although pasteurized milk from Listeria-infected dairy cows was linked to the outbreak, L. monocytogenes was not found in the suspected brand of milk.
• An outbreak in 1985 in Los Angeles resulted in 142 cases of listeriosis including 46 deaths; 85 percent of the cases were perinatal. The outbreak was traced to L. monocytogenes on soft, Mexican-style cheese, manufactured with contaminated milk.
• From the 1986-1987 study results, as well as from findings in studies conducted form 1989-1990, CDC determined that sporadic (non-outbreak) individual cases of listeriosis were associated with soft cheese, undercooked poultry, hot dogs not thoroughly reheated and food purchased from delicatessen counters.

Prevention of Listeria infections

Preventative measures can be taken by pregnant women in order to avoid contracting Listeria infections. The U.S. Food and Drug Administration (FDA July 1997) has stated,

To protect your unborn baby, eat hard cheeses instead of soft cheeses while you are pregnant. Soft cheeses that can easily become contaminated include:

Mexican-Style Soft Cheeses

• queso blanco
• queso fresco
• queso de hoja
• queso de crema
• asadero

Other Soft Cheeses

• feta (goat cheese)
• brie
• Camembert
• blue-veined cheeses, like Roquefort.

The FDA (FDA July 1997) cautions,  "Listeria can also contaminate other foods. Contaminated food may not look, smell or taste different from uncontaminated food. Although Listeria bacteria are killed with thorough cooking, these 'tough bugs' can grow in the refrigerator and survive in the freezer."  The FDA also recommends the following precautions to avoid Listeria infections:

• Use hard cheeses, like cheddar, instead of soft cheeses during pregnancy.
• If you do use soft cheeses during pregnancy, cook them until they are boiling (bubbling).
• Use only pasteurized dairy products. It will state "pasteurized" on the label.
• If you do use hard cheeses made from unpasteurized milk, use only those marked "aged 60 days" (or longer).
• Eat only thoroughly cooked meat, poultry or seafood.
• Thoroughly reheat all meats purchased at deli counters, including cured meats like salami, before eating them.
• Wash all fruits and vegetables with water.
• Follow label instructions on products that must be refrigerated or that have a "use by" date.
• Keep the inside of the refrigerator, counter tops, and utensils clean.
• After handling raw foods, wash your hands with warm soapy water, and wash the utensil you used with hot soapy water before using them again.

The USDAs Food Safety and Inspection Service web site provides the following precautions to prevent listeriosis:

• Do not eat hot dogs, luncheon meats or deli meats unless they are reheated until steaming hot.
• Do not eat soft cheeses such as feta, Brie, Camembert, blue-veined cheeses and Mexican-style cheeses such as "queso blanco fresco." Hard cheeses, semi-soft cheeses such as mozzarella, pasteurized processed cheese slices and spreads, cream cheese, and cottage cheese can be safely consumed.
• Do not eat refrigerated pâté or meat spreads. Canned or shelf-stable pâté and meat spreads can be eaten.
• Do not eat refrigerated smoked seafood unless it is an ingredient in a cooked dish such as a casserole. Examples of refrigerated smoked seafood include salmon, trout, whitefish, cod, tuna, and mackerel which are most often labeled as "nova-style," "lox," "kippered," "smoked," or "jerky." This fish is found in the refrigerated section or sold at deli counters of grocery stores and delicatessens. Canned fish such as salmon and tuna or shelf-stable smoked seafood may be safely eaten.
• Do not drink raw (unpasteurized) milk or eat foods that contain unpasteurized milk.

The Food Safety and Inspection Service (FSIS) and the FDA have established food testing and recall programs to protect the public from Listeria contaminated foods. According to the USDA (USDA, April 1992),

Neither FSIS nor FDA will accept any detectable L. monocytogenes on cooked, ready-to-eat food. This is called "zero tolerance" for the bacteria. Both agencies have testing programs for L. monocytogenes. The goals of these programs is to help government and industry identify the causes of contamination in processing plants and to make permanent changes that will reduce Listeria monocytogenes contamination, prevent problems and ensure a safe food supply. Both agencies can hold or detain products at the food processing plant, request a voluntary recall of the product or seize products through court order if necessary.

Initially, FSIS regulatory testing programs included selected cooked meat products. Following a CDC report that traced the first case of listeria meningitis to incompletely heated turkey franks consumed by a cancer patient, FSIS expanded the L. monocytogenes monitoring program to further prevent the sale of any cooked and ready-to-eat meat or poultry products from which L. monocytogenes is isolated, such as cooked sausages (including frankfurters and bologna), cooked roast beef, cooked corned beef, sliced canned ham, sliced canned luncheon meat, jerky, cooked poultry, and poultry and meat salads and spreads.

When ready-to-eat meat or poultry product is found to contain L. monocytogenes, the plant is notified and the product is subject to detention at the plant, voluntary recall or court-ordered seizure. From 1987 through March 1992, 27 FSIS-regulated cooked products from 27 firms have been recalled, including frankfurters, bologna and other luncheon meat, chicken salad, ham salad, sausages, chicken, sliced turkey breast and sliced roast beef.

FDA's monitoring programs initially concentrated on cheese and dairy products both domestic and imported. Later, FDA expanded coverage to include other ready-to-eat foods such as sandwiches, prepared salads and smoked fish. From 1987 to March 1992, 516 products from 105 firms have been recalled.

The agencies' stepped up monitoring and surveillance programs for L. monocytogenes, and food industry efforts have helped identify intervention measures aimed at controlling the organism.

Listeria infections during pregnancy

Eating contaminated food is the most common way that listeriosis is contracted. The CDC has stated that (CDC 2003), pregnant women "are about 20 times more likely than other healthy adults to get listeriosis. About one-third of listeriosis cases happen during pregnancy."

According to the U.S. Department of Agriculture (USDA, April 1992), "Healthy people rarely contract listeriosis, but the illness can be serious for some people, especially the elderly, newborns, pregnant women and those with weakened immune systems. The disease symptoms are variable and depend on the individual's susceptibility. Symptoms may be limited to fever, fatigue, nausea, vomiting and diarrhea. However, these symptoms can precede a more serious illness. The more serious forms of listeriosis can result in meningitis (brain infections) and septicemia (bacteria in the bloodstream). Pregnant women may contract flu-like symptoms of listeriosis; complications can result in miscarriage, stillbirth, or septicemia or meningitis in the newborn. Skin contact with L. monocytogenes can cause localized abscesses or skin lesions. It takes from one to six weeks for a serious case of listeriosis to develop, although flu-like symptoms may occur 12 hours after eating L. monocytogenes-contaminated food. Onset time probably depends on the health of the patient, the strain of L. monocytogenes and the dose--or amount of bacteria-- ingested."  Although there is a theoretical possibility that L. monocytogenes could be transmitted via mother’s milk, this has not been proven (USDA, Food Safety and Inspection Service).

Diagnosis and treatment of Listeriosis

According to the U.S. Department of Agriculture (CDC 2003), "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. 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."  You should contact your health care provider if you have questions, or believe you have eaten a contaminated product or have flu-like symptoms.

Food Safety Guidelines

The USDA, Food Safety and Inspection Service recommends the following basic guidelines for food handling:

1. Clean
   Wash hands and surfaces often
   
2. Separate
   Don't allow cross-contamination between raw and cooked foods
   
3. Cook
   Cook to proper temperatures and use a food thermometer
   
4. Chill
   Refrigerate or freeze promptly

Contacts for additional information

The following agencies can be contacted for additional information about Listeria:

FSIS
Media Inquires: 202-720-0314
Public Inquires: 202-720-9113, 1-800-535-4555
www.fsis.usda.gov

FDA
Media Inquires: 202-205-4144
Consumer Inquires (CFSAN): 202-205-5004; 1-888-SAFEFOOD
www.cfsan.fda.gov

CDC
Media Inquires: 404-639-3286
Technical questions about listeriosis: Meningitis and Special Pathogens Branch: 404-639-2215
www.cdc.gov/foodsafety

USDA's Meat and Poultry Hotline
1-800-535-4555
In the Washington, DC area, call 202-720-3333

Hotline hours are from 10 a.m. to 4 p.m., Eastern time.

Links to additional useful information

FDA Foodborne Pathogenic Microorganisms and Natural Toxins Handbook