What is Listeriosis?

Listeriosis, a serious infection usually caused by eating food contaminated with the bacteriumListeria monocytogenes 1, is an important public health problem in the United States 2. The disease primarily affects older adults, pregnant women, newborns, and adults with weakened immune systems 3. However, rarely, people without these risk factors can also be affected. The risk may be reduced by following a few simple recommendations.

What are the Symptoms of Listeriosis?

A person with listeriosis usually has fever and muscle aches, sometimes preceded by diarrhea or other gastrointestinal symptoms. Almost everyone who is diagnosed with listeriosis has "invasive" infection, in which the bacteria spread beyond the gastrointestinal tract. The symptoms vary with the infected person:

• Pregnant women: Pregnant women typically experience fever and other non-specific symptoms, such as fatigue and aches. However, infections during pregnancy can lead to miscarriage, stillbirth, premature delivery, or life-threatening infection of the newborn 3,4.
• People other than pregnant women: Symptoms, in addition to fever and muscle aches, can include headache, stiff neck, confusion, loss of balance, and convulsions.

Listeriosis can present in different ways depending on the type of infection. Manifestations of listeriosis are host-dependent. In older adults and people with immunocompromising conditions, septicemia and meningitis are the most common clinical presentations 2. Pregnant women may experience a fever and other non-specific symptoms, such as fatigue and aches, followed by fetal loss or bacteremia and meningitis in their newborns 4. Immunocompetent people may experience acute febrile gastroenteritis or no symptoms .

Key Facts for Those at Greatest Risk:

The following groups are at increased risk:

• Pregnant women: About one in seven (14%) cases ofListeria infection occurs during pregnancy 2. Infection during pregnancy can cause fetal loss (miscarriage or stillbirth), preterm labor, and illness or death in newborn infants.
• Pregnant women are about 10 times more likely than the general population to get Listeria infection 2.
• Pregnant Hispanic women are about 24 times more likely than the general population to get Listeria infection 2.
• Older adults: More than half (58%) of Listeria infections occur among adults 65 and older 2.
• Adults 65 years and older are about 4 times more likely than the general population to get Listeria infection 2.
• People with weakened immune systems: Individuals within this group are also considered high risk for Listeria infection due to underlying medical conditions such as cancer and  immunosuppresive therapy (i.e., steroids, chemotherapy, radiation),  liver or kidney disease, diabetes, alcoholism, and HIV/AIDS.

How can I reduce my risk for listeriosis?

The general guidelines recommended for the prevention of listeriosis are similar to those used to help prevent other foodborne illnesses, such as salmonellosis. In addition, there are specific recommendations for persons at higher risk for listeriosis. Recommendations related to Listeria in melons, including cantaloupes, are also included on this page.
There are some general recommendations on how to prevent an infection with Listeria, and some additional recommendations specifically for persons who are at higher risk.

General recommendations to prevent an infection with Listeria:

FDA recommendations for washing and handling food.

• Rinse raw produce, such as fruits and vegetables, thoroughly under running tap water before eating, cutting, or cooking. Even if the produce will be peeled, it should still be washed first.
• Scrub firm produce, such as melons and cucumbers, with a clean produce brush.
• Dry the produce with a clean cloth or paper towel.
• Separate uncooked meats and poultry from vegetables, cooked foods, and ready-to-eat foods.

Keep your kitchen and environment cleaner and safer.

• Wash hands, knives, countertops, and cutting boards after handling and preparing uncooked foods.
• Be aware that Listeria monocytogenes can grow in foods in the refrigerator. Use an appliance thermometer, such as a refrigerator thermometer, to check the temperature inside your refrigerator. The refrigerator should be 40°F or lower and the freezer 0°F or lower.
• Clean up all spills in your refrigerator right away–especially juices from hot dog and lunch meat packages, raw meat, and raw poultry.
• Clean the inside walls and shelves of your refrigerator with hot water and liquid soap, then rinse.

Cook meat and poultry thoroughly.

• Thoroughly cook raw food from animal sources, such as beef, pork, or poultry to a safe internal temperature. For a list of recommended temperatures for meat and poultry, visit thesafe minimum cooking temperatures chart at FoodSafety.gov.

Store foods safely.

• Use precooked or ready-to-eat food as soon as you can. Do not store the product in the refrigerator beyond the use-by date; follow USDA refrigerator storage time guidelines:
• Hot Dogs – store opened package no longer than 1 week and unopened package no longer than 2 weeks in the refrigerator.
• Luncheon and Deli Meat – store factory-sealed, unopened package no longer than 2 weeks. Store opened packages and meat sliced at a local deli no longer than 3 to 5 days in the refrigerator.  
• Divide leftovers into shallow containers to promote rapid, even cooling. Cover with airtight lids or enclose in plastic wrap or aluminum foil. Use leftovers within 3 to 4 days.

Choose safer foods.

• Do not drink raw (unpasteurized) milk, and do not eat foods that have unpasteurized milk in them.

Recommendations for persons at higher risk, such as pregnant women, persons with weakened immune systems, and older adults in addition to the recommendations listed above, include:
Meats

• Do not eat hot dogs, luncheon meats, cold cuts, other deli meats (e.g., bologna), or fermented or dry sausages unless they are heated to an internal temperature of 165°F or until steaming hot just before serving.
• Avoid getting fluid from hot dog and lunch meat packages on other foods, utensils, and food preparation surfaces, and wash hands after handling hot dogs, luncheon meats, and deli meats.
• Pay attention to labels. Do not eat refrigerated pâté or meat spreads from a deli or meat counter or from the refrigerated section of a store. Foods that do not need refrigeration, like canned or shelf-stable pâté and meat spreads, are safe to eat. Refrigerate after opening.

Cheeses

• Do not eat soft cheese such as feta, queso blanco, queso fresco, brie, Camembert, blue-veined, or panela (queso panela) unless it is labeled as made with pasteurized milk. Make sure the label says, "MADE WITH PASTEURIZED MILK."
• Be aware that Mexican-style cheeses made from pasteurized milk, such as queso fresco, likely contaminated during cheese-making, have caused Listeria infections.

Seafood

• Do not eat refrigerated smoked seafood, unless it is contained in a cooked dish, such as a casserole, or unless it is a canned or shelf-stable product.
• Refrigerated smoked seafood, such as salmon, trout, whitefish, cod, tuna, and mackerel, is most often labeled as "nova-style," "lox," "kippered," "smoked," or "jerky."
• These fish are typically found in the refrigerator section or sold at seafood and deli counters of grocery stores and delicatessens.
• Canned and shelf stable tuna, salmon, and other fish products are safe to eat.

Safety tips for eating melons

Get specific safety information about the Listeria outbreak in cantaloupes here.

Follow this general FDA advice for melon safety:

• Consumers and food preparers should wash their hands with warm water and soap for at least 20 seconds before and after handling any whole melon, such as cantaloupe, watermelon, or honeydew. 
• Scrub the surface of melons, such as cantaloupes, with a clean produce brush under running water and dry them with a clean cloth or paper towel before cutting. Be sure that your scrub brush is sanitized after each use, to avoid transferring bacteria between melons.
• Promptly consume cut melon or refrigerate promptly. Keep your cut melon refrigerated at, or less than 40 degrees F (32-34 degrees F is best), for no more than 7 days. 
• Discard cut melons left at room temperature for more than 4 hours.

Sources

How does someone get listeriosis?

People get listeriosis by eating food contaminated with Listeria monocytogenes 1. Babies can be born with listeriosis if their mothers eat contaminated food during pregnancy. However, healthy people may consume contaminated foods without becoming ill. People at risk can prevent listeriosis by avoiding certain high-risk foods and by handling and storing food properly.

Reservoir

Listeria monocytogenes is commonly found in soil and water. Animals can carry the bacterium without appearing ill and can contaminate foods of animal origin, such as meats and dairy products.

Transmission

Most human infections follow consumption of contaminated food 2. Rare cases of nosocomial transmission have been reported 3.
When Listeria bacteria get into a food processing factory, they can live there for years, sometimes contaminating food products 4. The bacterium has been found in a variety of raw foods, such as uncooked meats and vegetables, as well as in foods that become contaminated after cooking or processing, such as soft cheeses, processed meats such as hot dogs and deli meat (both products in factory-sealed packages and products sold at deli counters), and smoked seafood. Unpasteurized (raw) milk and cheeses and other foods made from unpasteurized milk are particularly likely to contain the bacterium.
Listeria is killed by pasteurization and cooking; however, in some ready-to-eat foods, such as hot dogs and deli meats, contamination may occur after factory cooking but before packaging. Unlike most bacteria, Listeria can grow and multiply in some foods in the refrigerator.

Diagnosis

How do I know if I have Listeriosis?

If you develop fever and chills while pregnant or if you are very sick with fever and muscle aches or stiff neck, consult your doctor immediately. A blood or spinal fluid test (to look for the bacteria) will show if you have listeriosis. 

Clinical Diagnosis

Listeria is found in the environment and all people are exposed to it regularly. Therefore, there is no clinical value in performing laboratory testing on asymptomatic patients, even if high risk.
For symptomatic patients, diagnosis is confirmed only after isolation of Listeria monocytogenesfrom a normally sterile site, such as blood, CSF (in the setting of nervous system involvement), or amniotic fluid/placenta (in the setting of pregnancy). Stool samples are of limited use and are not recommended. Listeria monocytogenes can be isolated readily on routine media, but care must be taken to distinguish this organism from other Gram-positive rods, particularly diphtheroids. Selective enrichment media improve rates of isolation from contaminated specimens. You can expect that that the cultures will take 1-2 days for growth. Importantly, a negative culture does not rule out infection in the presence of strong clinical suspicion. Serological tests are unreliable, and not recommended at the present time.

How is Listeriosis Treated?

• Listeriosis is treated with antibiotics. A person in a high-risk category who experiences fever and other non-specific symptoms, such as fatigue and aches, within 2 months of eating contaminated food should seek medical care and tell the physician or health care provider about eating the contaminated food.
• If a person has eaten food contaminated with Listeria and does not have any symptoms, most experts believe that no tests or treatment are needed, even for persons at high risk for listeriosis.

Outcomes
Even with prompt treatment, some listeriosis cases result in death. This is particularly likely in older adults and in persons with other serious medical problems.