What is Listeria?

What is Listeria monocytogenes?

Listeria monocytogenes is a bacterial species that was initially discovered in the 1920’s in rodents, rabbits, and guinea pigs, and is now recognized as a foodborne pathogen in humans. Although Listeria monocytogenes infection in humans is relatively rare, it is associated with a higher mortality rate than other foodborne diseases. Listeria is responsible for nearly 20% of deaths due to consumption of contaminated food in the United States.

Where is Listeria found?

Listeria monocytogenes is found throughout nature in soil, water, and foods. Listeria can grow in various environmental conditions including a wide range of temperatures and pH levels. Because Listeria can survive exposures to low temperatures, refrigeration does not prevent its growth. Listeria cannot grow or survive at low pH levels, so vinegar, sorbic acid, and other acidic ingredients are often added to refrigerated foods to increase acidity and prevent bacterial growth. Exposure to high levels of heat, including pasteurization, are effective in killing Listeria, but foods that are unpasteurized or minimally pasteurized can easily harbor the bacteria.

What foods can contain Listeria?

Raw vegetables, dairy products, meat, and soft cheeses are common sources of Listeria. Up to 70% of hot dogs contain Listeria. Refrigerated deli meats, frozen vegetables, and milk can also be affected by Listeria contamination.

What causes outbreaks of Listeria?

Because Listeria is present throughout our natural environment, it can easily be introduced into food processing environments through contaminated raw materials, equipment, or individual workers. If sanitation practices are not closely followed, the bacteria can spread from equipment and workers to food. Outbreaks of Listeria can occur after people consume contaminated processed, ready-to-eat, and refrigerated meat and dairy products. Listeria outbreaks have also been connected to fresh produce, caramel apples, smoked fish, and canned vegetables. 

How does Listeria  affect the body?

Listeria is absorbed through the gastrointestinal tract and spreads throughout the bloodstream to organs including the liver and spleen. Healthy individuals are typically able to clear the infection from their bodies without difficulty, resulting in self-limited disease. In susceptible individuals, including people with compromised immune systems, Listeria can undergo further spread through the bloodstream to other organs, causing a serious, invasive, and often fatal disease called “listeriosis”. In pregnant women, Listeria can spread through the bloodstream to the placenta and fetus and can cause both maternal and fetal disease.

What are Listeria  symptoms?

The signs and symptoms of Listeria illness typically occur within days of eating contaminated food, although in some cases the clinical presentation can be delayed for weeks. In healthy individuals, Listeria monocytogenes infection often causes gastrointestinal illness consisting of nausea, vomiting, diarrhea, and fever. Pregnant women, the elderly, young children, and fetuses are susceptible to a more serious type of Listeria infection called “listeriosis”. Listeriosis is characterized by meningitis, pneumonia, sepsis, and other serious infections. Pregnant women affected by listeriosis may also experience miscarriage or premature delivery, and infants born to mothers infected by Listeria can develop breathing difficulties, fever, meningitis, and other life-threatening infections.

What are symptoms of Listeria  in pregnancy?

Signs and symptoms of Listeria in pregnant women are often non-specific and include flu-like symptoms, muscle aches, and gastrointestinal distress. Some pregnant women may experience more severe symptoms, including infections of the heart, brain, or placenta. Unfortunately, nearly one-third of pregnant women affected by Listeria infection are asymptomatic, making the diagnosis difficult to establish in many cases. While Listeria infection can occur at any time during pregnancy, most cases occur during the third trimester (after the 28th week of pregnancy).

Can Listeria  be killed by cooking?

Heating meats, leftovers, and ready-to-eat foods to a temperature of 165 degrees Fahrenheit before eating them can kill Listeria bacteria that may be present in these foods. Listeria can also be killed by pasteurization. However, even with the use of pasteurization, food products can still contain Listeria if the bacteria is introduced through contaminated workers or processing equipment. 

Is there a treatment for Listeria?

Most healthy individuals who have self-limiting gastrointestinal symptoms related to Listeria will recover completely without specific treatment. In some cases, the gastrointestinal symptoms may be severe enough to cause dehydration that necessitates hospitalization for intravenous fluid administration. People with listeriosis typically require hospitalization and treatment with intravenous antibiotics for several weeks.

What do I do if I think I have Listeria?

For questions about food poisoning or adverse or unexpected symptoms related to food consumption, contact Poison Control for expert advice. There are two ways to contact Poison Control in the United States: online at www.poison.org or by phone at 1-800-222-1222. Both options are free, confidential, and available 24 hours a day. 

Kelly Johnson-Arbor, MD
Medical Toxicologist

For media inquiries, please contact Krista Osterthaler at osterthaler@poison.org.