Listeria monocytogenes (Listeriosis)

Medically Reviewed on 7/24/2023

What is Listeria monocytogenes (Listeriosis)?

Listeria (Listeriosis)
In general, listeria is not contagious from person to person (except in pregnancy); the disease spreads to humans mainly by contaminated food or fluids.

Listeriosis is an infection caused by a rod-shaped gram stain-positive motile bacterium named Listeria monocytogenes. The foodborne illness produces fever, muscle aches, and, in many people, diarrhea. Severe infections can cause headaches, meningitis, convulsions, and death. Most healthy people exposed to the bacteria have minor or no symptoms, but a few people, especially the elderly, pregnant females and their fetus, newborns, and anyone with a compromised immune system are especially susceptible to these organisms. Listeria bacteria are widespread throughout the world and are often associated with farm animals that may show no signs of infection. Research shows that many animals are uninfected carriers, and they suggest that some humans carry these organisms as part of their bowel flora. Except for pregnant women, fetuses, or newborns, there is no direct transfer of Listeria from human to human.

The organisms (Listeria monocytogenes) that cause listeriosis have probably been infecting humans for centuries. Listeria was first isolated from an infected WWI soldier in 1918 and had many different names until 1940 when the genus and species names were firmly established. However, people first recognized the bacteria as a food-borne (food poisoning) pathogen in 1979. The bacteria can penetrate human cells and can multiply inside them. People with altered or impaired immune systems have cells that are less able to control the spread of these organisms into the blood or other cells. In 2010, health researchers discovered that a known species, Listeria ivanovii, thought only to infect cattle, also infect humans -- so two Listeria species can infect humans.

Outbreaks of listeriosis can occur with some frequency. For example, in 2017, soft raw milk cheese was a source of an outbreak (Vulto Creamery). In this outbreak, eight people were infected and two died. Other recalls of products in 2017 include 3 tons of cheese for possible Listeria contamination (La Nica Products INC.), macadamia nuts (Simple Truth brand), Ava's Organic Cashews, and Queso Fresco cheese, for the same problem. In 2018, at least nine people have died from Listeria-contaminated frozen sweet corn. British authorities have recalled 43 products with possible Listeria contamination to halt this significant outbreak.

What causes listeria?

The majority of people who get listeriosis have consumed Listeria-contaminated foods. Because the bacteria exist in both soil and water, cultivated foods like vegetables can easily become contaminated, especially from fertilizer or animal waste. Listeria has been found in many types of raw food (even seafood) but especially in meats, vegetables, and cheeses. It has even been found in processed foods because of contamination during or after processing. After someone ingests the contaminated food or fluid, it may take up to three weeks for the organisms to cause symptoms.

The fetus may become infected after the mother ingests the organisms; the bacteria apparently reach the fetus via the bloodstream. Newborn infants can acquire the bacteria during a cesarean procedure or be exposed to them while traversing the vagina.

Listeriosis is mainly a food-borne disease; except in the situation in which a pregnant woman can transfer the bacteria to the fetus or newborn, the disease is not contagious from person to person.

Foods that have been associated with Listeria outbreaks are many (for example, soft cheeses, yogurt, apples, smoked seafood, deli meats, hot dogs, fruits, and vegetables). There have been many outbreaks of the disease over the world. The year 2015 had at least three outbreaks of listeriosis. Bidart Brothers of Bakersfield, Calif., produced apples contaminated with the bacteria. People first noticed the organisms in caramel-coated apples. Hummus produced by the Sabra Dipping Company was recalled (30,000 cases of hummus) due to Listeria contamination. Also in 2015, the very popular brand of ice cream, Blue Bell, caused a serious outbreak of listeriosis. The company shut down its facilities in Alabama, Oklahoma, and Texas to rid them of Listeria. During the outbreak, health care professionals hospitalized 10 individuals and three people died. In 2016, CRF Frozen Foods of Pasco, Washington, recalled 11 frozen vegetable products because of Listeria contamination. Health care providers admitted nine people to the hospital and three died during this outbreak to date. Unfortunately, this outbreak is complicated by the fact that some of the vegetable products in the recall date back as far as 2014. Individuals who have stored frozen CRF products that date back as far as 2014 need to get rid of the potentially contaminated frozen products, according to the U.S. Centers for Disease Control and Prevention (CDC).

The bacteria are not contagious from person to person in most instances. The one exception is that a pregnant woman can transfer the bacteria to the fetus or the newborn.


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What are the risk factors for listeria?

The major risk factor for getting listeriosis is eating or drinking foods and liquids contaminated with Listeria bacteria. Foods and liquids contaminated with animal feces or soil are the most frequently identified sources for these organisms. Drinking inadequately treated or unpasteurized liquids, especially milk products, is another source of infection.

Some individuals have an increased risk for getting listeriosis. In general, people with an altered or damaged immune system have a higher risk of getting listeriosis and its more severe complications. Specifically, people at much higher risk include pregnant females, newborns, the elderly, diabetics, cancer patients, AIDS patients, patients with kidney diseases, alcoholics, and those patients undergoing any immune-suppression therapy. Most individuals who get severe infections and/or die from listeriosis have one or more of the medical problems listed above.

What are the symptoms of listeria?

However, in some people, the organisms can spread to the brain.

What is the incubation period for listeria?

The incubation period between exposure and symptoms is quite variable (three to 70 days, with 21 days as average) and may extend up to about two months or more according to some reports.

Pregnant women who are otherwise healthy usually have only minor symptoms. However, Listeria infection during pregnancy often cause problems for the fetus:

Breastfeeding in humans does not transmit the bacteria to newborns; however, animal studies show the organisms are transferred in breast milk of other mammalian species. Researchers consider it theoretically possible for organism transmission in human breast milk.

Occasionally, localized skin infections may occur, especially in people who handle animals infected with Listeria. These skin infections rarely lead to further complications such as brain infection.

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Diagnosis of listeria

Although primary care physicians can treat listeriosis, other specialists may be involved, especially if the illness is serious. Other specialists such as infectious disease, critical care, and ob-gyn physicians, especially if a woman is pregnant, are likely to be consulted. In immunosuppressed patients, physicians who are treating the cause of the immunosuppression should also be consulted.

Physicians base their preliminary diagnosis on the patient's clinical history and physical exam, especially after the patient gives a history of likely exposure to a contaminated food source during a Listeria outbreak. Without this information, the diagnosis is difficult to sort out from many other diseases; this situation may result in a delay of treatment as the physician may do other tests to rule out other diseases such as salmonellosis, shigellosis, botulism, and E. coli infections. Definitive diagnosis of listeriosis is by culturing Listeria monocytogenes bacteria from the patient's blood, cerebrospinal fluid, or amniotic fluid, usually on a medium that is selective for Listeria (for example, RAPID'L mono agar). Currently, no reliable tests are available to detect the bacteria in the stool; also, there are no reliable serological tests available (blood tests that can identify specific proteins associated with the bacteria or antibodies to the bacteria) according to the CDC.

What is the treatment for listeria?

The majority of people with Listeria infections spontaneously clear the infection in about seven days. However, those patients at increased risk, especially pregnant women, usually require immediate IV antibiotic treatment to prevent, halt, or slow the development of more severe disease. For example, early effective antibiotic treatment of pregnant females may be lifesaving for the fetus.

In general, the length of antibiotic treatment increases with the severity of the infection. The treatment for meningitis lasts three weeks while brain abscess treatment lasts six weeks. The initial choice of antibiotics is usually IV ampicillin. Doctors have used Bactrim (trimethoprim-sulfamethoxazoleseizures

There are no over-the-counter diagnostic tests for listeriosis, so it will be difficult or impossible to know if you have a Listeria infection; that makes it difficult to decide if you can treat the problem at home. Because listeriosis can be fatal in about 20%-30% of those who develop the disease, home remedies may be a dangerous option without consulting a physician. However, there are suggestions to try, like activated charcoal, syrup of ipecac, garlic, and/or alcohol-free goldenseal to treat food poisoning in general. However, before trying any of these options, you should discuss them first with your doctor.

What is the prognosis for listeria?

Since most infections go unnoticed or only produce minor symptoms, the prognosis is excellent. However, the prognosis for people with altered or impaired immune systems range from good (with early appropriate therapy) to poor, depending on how debilitated the patient is when first infected.

Normal healthy people infected with Listeria usually recover from the infection. However, more serious infections that require antibiotic treatment may last longer. For example, if abscesses develop in the brain, the antibiotic therapy will be required for about six weeks.

The pregnant woman usually has a good prognosis. However, her fetus or newborn has a good to guarded prognosis, again depending how quickly the mother or newborn is effectively treated.

What are the complications of listeria?

The major complications of listeriosis include

Is it possible to prevent listeria?

Yes, it's possible to prevent listeriosis. The CDC recommends the following food safety measures:

General recommendations:

  • Thoroughly cook raw food from animal sources, such as beef, pork, or poultry.
  • Wash raw vegetables thoroughly (scrub with a clean produce brush in uncontaminated running water) 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.

Here are food safety recommendations for people at high risk, such as pregnant women and people with weakened immune systems, in addition to the recommendations listed above:

  • Do not eat hot dogs, luncheon meats, or deli meats, unless heated 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 refrigerated pâtés or meat spreads. It's OK to eat canned or shelf-stable pâtés and meat spreads.
  • 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.
  • Avoid cross-contaminating other foods, utensils, and food-preparation surfaces with fluid from hot-dog packages, and wash hands after handling hot dogs.
  • 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. Cheeses that are OK to eat include hard cheeses, semi-soft cheeses such as mozzarella, pasteurized processed cheeses such as slices and spreads, cream cheese, and cottage cheese.
  • Do not drink raw (unpasteurized) milk or eat foods that contain unpasteurized milk.

There is no commercially available vaccine to protect against infection by Listeria.

Medically Reviewed on 7/24/2023
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