Get the Facts about Listeria!

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. Listeriosis is caused by the bacteria Listeria monocytogenes and is a leading cause of hospitalization and death due to foodborne illness, especially in high-income, industrialized countries.

A Brief History

In 1911, G. Hülphers described bacteria that he isolated from the diseased liver of a rabbit. Hülphers named the bacteria Bacillus hepatis, and his description closely matched the findings of E.G.D. Murray 15 years later. In 1926, Murray isolated bacteria from the livers of sick rabbits and guinea pigs and named the bacteria Bacterium monocytogenes. The bacteria went through several more name changes, and in 1940, the name was officially changed to Listeria monocytogenes 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. Since the 1980s, there have been about 80 recognized outbreaks of listeriosis in people.¹

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.² Over 90 percent of people with listeriosis are hospitalized, often in intensive care units.^3-5

The World Health Organization estimated that worldwide in 2010, listeriosis sickened 23,150 people, and of these, 5,463 died.⁵ In the U.S., estimates range from 1,600 to 2,500 cases of listeriosis each year, with 260 to 500 deaths.^6-9 The European Union has similar numbers: in 2009, there were 1,645 reported cases of listeriosis, with an estimated 270 deaths.¹⁰

A 2011 outbreak of listeriosis in the U.S. from contaminated cantaloupes sickened 147 people in 28 states, resulting in 33 deaths and 1 miscarriage in a pregnant woman.¹¹ With a mortality rate of over 22%, this was the deadliest recorded outbreak of a foodborne illness in the U.S. since the Centers for Disease Control and Prevention began tracking outbreaks in the 1970s.⁵

In the U.S., the estimated total annual cost of listeriosis—this includes medical expenses, decreased productivity, and premature death—is $2.6 billion, making it the third most costly foodborne illness for Americans, behind salmonellosis and toxoplasmosis (a foodborne illness caused by ingesting a parasite).¹²

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.^{2, 5, 13}

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.¹⁴ This finding highlights the ability of the bacteria to thrive in cold environments.

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.

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.

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.¹⁵ 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, over one-quarter (29%) of the women had no symptoms.¹⁵

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.¹⁶ This long incubation period adds to the difficulty in diagnosing listeriosis in pregnant women.

Food Safety for Moms-to-Be: While You’re Pregnant – Listeria (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.¹⁵

With a mortality rate of 20 to 30%, newborns suffer the most serious consequences of listeriosis.¹⁵ They have either early- or late-onset disease depending on when their symptoms first appear.

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.^{2, 3, 13}

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, FAO/WHO found that the elderly (people 60 years and older) were 2.6 times more likely to get listeriosis than the general healthy population.²⁰ In the U.S. in 2011, the median age of non-pregnant people diagnosed with a severe form of listeriosis was 71 years.¹³

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.^{3, 19}

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.¹⁶

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.^{2, 13}