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Get the Facts about Listeria

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You’ve probably heard of Salmonella and are familiar with the symptoms of salmonellosis. But you may not have heard much about the lesser known foodborne illness listeriosis caused by the bacteria Listeria monocytogenes. After going through several name changes since the bacteria were first described in 1911, the name was officially changed to Listeria monocytogenes in 1940 to honor Joseph Lister, the British surgeon who discovered that sterilizing surgical instruments before operations reduced the risk of infections.

In 1929, A. Nyfeldt reported the first confirmed cases of listeriosis in people. But L. monocytogenes weren’t identified as a major cause of foodborne illness in people until the 1980s when several large outbreaks occurred.

The Statistics at a Glance

Compared to other foodborne illnesses, listeriosis is rare but very serious. Even with adequate antibiotic treatment, the disease has a high mortality rate of 20 to 30 percent. Over 90 percent of people with listeriosis are hospitalized, often in intensive care units. In the U.S., the Centers for Disease Control and Prevention estimate that about 1,600 people get listeriosis each year, and about 260 die. The hospitalization rate is 94%, meaning that of the approximately 1,600 people who get the disease each year, about 1,500 will be hospitalized.

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Characteristics

L. monocytogenes is widespread in the environment, especially in soil and water. The bacteria can survive in soil for many months. Animals, particularly cattle, can carry L. monocytogenes without appearing sick and shed the bacteria in their feces.

The bacteria are able to live in a wide range of conditions and environments—they can tolerate both acidic and salty conditions, both high and low temperatures, and a fairly low moisture content. These characteristics allow L. monocytogenes to survive a long time in a variety of food products and food processing plants. Because the bacteria can multiply and persist in food processing plants for years—even more than 10 years in one documented case—L. monocytogenes is especially hard to control and can result in intermittent contamination of food.

Unlike most bacteria, L. monocytogenes can grow and multiply at low temperatures, making the bacteria a potential problem even in properly refrigerated food. One study also found a relatively high percentage of frozen raw beef products contaminated with L. monocytogenes. This finding highlights the ability of the bacteria to thrive in cold environments (Pao & Ettinger).

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How People Become Infected

People become infected with L. monocytogenes by eating contaminated food. People can also become infected by handling contaminated food, such as contaminated pet food, or touching contaminated surfaces and utensils and then accidentally transferring the bacteria from their hands to their mouths. Babies can become infected in utero or at birth if their mothers ate contaminated food during pregnancy.

The bacteria can contaminate a variety of food, such as:

  • Raw meat;
  • Ready-to-eat processed meat such as hot dogs and deli meat (both factory-sealed packages and products sold at deli counters);
  • Raw vegetables;
  • Refrigerated pates;
  • Ready-to-eat smoked seafood and raw seafood;
  • Prepared or stored salads, including coleslaw and fresh fruit salad;
  • Melons;
  • Soft cheeses made with unpasteurized milk; and
  • Unpasteurized milk and milk products.

Pasteurization, cooking, and most disinfecting agents kill L. monocytogenes. However, in some ready-to-eat food, such as hot dogs and deli meats, contamination may occur after the food is cooked in the factory but before it’s packaged. These products can be safely eaten if reheated until steaming hot.

The U.S. Department of Agriculture’s Food Safety and Inspection Service (FSIS) maintains a zero-tolerance policy for L. monocytogenes in ready-to-eat meat and poultry products. FSIS continues to strengthen programs and recommendations to reduce or eliminate the bacteria from ready-to-eat products.

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Listeriosis in People

Listeriosis occurs almost exclusively in pregnant women, newborns, the elderly, and people with weakened immune systems (such as those with cancer or other diseases). After a person ingests L. monocytogenes, the bacteria grow quickly in the liver and then move into the bloodstream and can invade many places in the body, including the:

  • Brain;
  • Membranes surrounding the brain and spinal cord (called the “meninges”);
  • Gastrointestinal, or digestive, tract (the stomach and intestines); and
  • Bloodstream.

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Pregnant Women and Newborns

Pregnant women are 18 times more likely to get listeriosis than other healthy adults, and 16 to 27% of all L. monocytogenes infections are in pregnant women. Scientists don’t know why pregnant women are so susceptible to the disease. It usually affects pregnant women who are healthy and don’t have other risk factors.

Diagnosing listeriosis in pregnant women is difficult because they usually don’t have the gastrointestinal symptoms—such as vomiting and diarrhea—that are normally seen with a foodborne illness. The most common, and sometimes only, symptom of listeriosis in pregnant women is fever. They often have a flu-like illness with non-specific symptoms, such as fatigue and muscle aches, which are often temporary and go away on their own. Some pregnant women show no symptoms. In a series of 191 cases of listeriosis in pregnancy, more than one-quarter (29%) of the women had no symptoms (Lamont et al). 

Compared to listeriosis in non-pregnant people, the disease in pregnant women has a longer incubation period—the length of time between when a person ingests the bacteria and first shows symptoms. In one study, the median incubation period in pregnant women was 27.5 days, with a range of 17 to 67 days (Goulet et al, 2013). This long incubation period adds to the difficulty in diagnosing listeriosis in pregnant women.

Listeria from Food Safety for Moms to Be (En español)
Food Safety for Pregnant Women (En español)

While listeriosis in the mother is mild, infection in the fetus and newborn can be severe. Listeriosis can develop at any time during pregnancy, and the disease causes miscarriage, stillbirth, premature birth, and life-threatening infection of the newborn, such as a blood infection, respiratory distress or pneumonia, and meningitis (inflammation of the membranes surrounding the brain and spinal cord). Listeriosis is one of the most common causes of meningitis in newborns.

With a mortality rate of 20 to 30%, newborns suffer the most serious consequences of listeriosis. They have either early- or late-onset disease depending on when their symptoms first appear.

Table 1: Comparison of Early- and Late-onset Listeriosis in Newborns
  Early-onset listeriosis Late-onset listeriosis
Baby Usually premature Usually full-term and healthy
Mother Recent flu-like illness before delivery No signs of illness before delivery
Source of infection In utero from L. monocytogenes bacteria crossing the placenta from mother to baby Often unclear—the baby is possibly infected at birth from contact with the mother’s birth canal or gastrointestinal tract (maternal feces), or after birth from the environment
Develops 0 to 7 days after birth (average is 36 hours) 5 to 30 days after birth (average is 14 days)
Commonly Causes Blood infection, pneumonia, and meningitis Blood infection and meningitis
Severity Up to one-third of newborns die despite adequate treatment with antibiotics Better prognosis than early-onset disease

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Non-pregnant People

Healthy children and adults occasionally get listeriosis, but rarely become seriously ill. In healthy, non-pregnant people, listeriosis can cause gastroenteritis—mainly diarrhea—accompanied by a fever. Other symptoms may include vomiting, joint pain, headache, and body pain. This form of listeriosis has a much shorter incubation period, with symptoms typically occurring within 24 hours after a person ingests a high number of the bacteria. The illness is usually mild and goes away on its own. The diarrhea can last up to 5 days.

The elderly and people with weakened immune systems have a higher risk of getting a more severe form of listeriosis, including:

  • Inflammation of the brain (encephalitis)
  • Inflammation of the membranes surrounding the brain and spinal cord (meningitis); and
  • Infection of the blood.

In a joint risk assessment report on L. monocytogenes in ready-to-eat food, the Food and Agriculture Organization of the United Nations/World Health Organization found that the elderly (people 60 years and older) were 2.6 times more likely to get listeriosis than the general healthy population. In the U.S. in 2011, the median age of non-pregnant people diagnosed with a severe form of listeriosis was 71 years.

Symptoms of listeriosis vary depending on the form and can come on suddenly. Symptoms may include fever, muscle aches, severe headache, nausea, vomiting, stiff neck, confusion, loss of balance, and convulsions.

In contrast to the incubation period for other, more common foodborne illnesses, like salmonellosis, the incubation period for listeriosis can be long and also varies depending on the form. Cases involving a blood infection have a fairly short incubation period, with a median of 2 days. For cases involving encephalitis or meningitis, the incubation period is longer, with a median of 9 days.

There is also a skin form of listeriosis that results from intact skin coming into direct contact with the bacteria. The skin form is rare and mostly seen in people with at-risk jobs, such as veterinarians or farmers, who are exposed to sick animals or diseased tissues, especially after an animal has a pregnancy loss caused by L. monocytogenes.

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Listeriosis in Animals

L. monocytogenes can infect many animal species. Signs of disease are mainly seen in ruminants, such as cattle, goats, and sheep. Sheep are particularly sensitive to the bacteria. Listeriosis is one of the most common causes of encephalitis (inflammation of the brain) in adult ruminants. Infected animals most commonly show severe neurologic signs, such as loss of balance, circling, and unusual body spasms. Fever, loss of appetite, and decreased activity level are also usually seen. Some ruminant herds have had large numbers of late-term pregnancy losses. Newborn ruminants typically develop a blood infection. Listeriosis in ruminants most often occurs in the winter and early spring and is likely caused by the animals eating spoiled silage.

Listeriosis is more common in rabbits and rodents than dogs and cats. Rabbits and chinchillas (South American rodents) are particularly sensitive to the bacteria. The disease usually causes a blood infection. Pregnancy loss and uterine inflammation are also common, especially in chinchillas. These reproductive problems are often associated with gastrointestinal signs, such as diarrhea or constipation. Large outbreaks of listeriosis have been seen in captive rabbits and rodents. The source of infection in these outbreaks was thought to be contaminated food.

L. monocytogenes rarely cause disease in dogs and cats and, even when infected, pets usually have only mild gastrointestinal signs, such as diarrhea and vomiting. However, more serious signs are possible, such as fever, muscle pain, breathing problems, pregnancy loss, and even death. After consuming contaminated pet food, dogs and cats often don’t show any signs of listeriosis but can become carriers of the bacteria. This means that even if the pets appear healthy, they can still shed L. monocytogenes in their stool and then spread the bacteria to the home environment and to people and other pets in the household. For example, cats can spread L. monocytogenes through shared litter boxes or when roaming throughout the house, such as on kitchen countertops. One way dogs can spread the bacteria is when they have stool accidents inside the home. Pet waste from both sick and healthy pets can be a source of infection for people. 

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Animal Feed and Pet Food Recalls due to Listeria Contamination

Animal feed and pet food are recalled for various reasons, including for the presence of Listeria. For a list of recalled animal feed and pet food products and the reason for the recall, please see FDA’s Recalls & Withdrawals webpage.

Reference List

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