Listeria is the name of a genus of bacteria that includes different types or “strains” that can contaminate food and cause a potentially life-threatening medical condition known as listeriosis.

“Listeriosis is a serious infection usually caused by eating food contaminated with the bacterium Listeria monocytogenes,” explains Jennifer Hunter, MPH, DrPH, an epidemiologist with the Centers for Disease Control and Prevention (CDC).

While cases of listeriosis are relatively rare — roughly 1,600 people are diagnosed with it each year, Dr. Hunter says — listeriosis can be lethal. About 260 of those 1,600 people die as a result of the infection. And it’s important to note that some groups of people — including pregnant women, older adults, and those with weakened immune systems — are at a higher risk of listeriosis infection than others.

Listeria Makes You Sick When You Eat Foods Contaminated With the Bacteria

Listeria bacteria are found pretty much everywhere. They turn up in water, soil, and even dust. Most species of listeria are not a problem for people.

The type of listeria that poses a grave threat to humans is Listeria monocytogenes. It tends to make its way into our systems when we eat contaminated foods. “Listeria monocytogenes can grow at refrigeration temperatures [whereas] most pathogens cannot,” says Linda Harris, PhD, a department chair and specialist in food microbiology at the University of California in Davis.

While many foods may be contaminated with nonthreatening amounts of Listeria monocytogenes, the bacteria’s ability to withstand cold conditions can allow small communities to reproduce and form colonies large enough to cause illness or a serious infection, explains Robert Buchanan, PhD, the director of the University of Maryland’s Center for Food Safety and Security Systems in Baltimore.

Estimates vary, but data suggests that tens of thousands or even millions of Listeria monocytogenes organisms usually have to be present to make a person sick.Cooking kills listeria.Also, when stored at proper temperatures — 40 degrees or below — listeria bacteria replicate slowly, Dr. Buchanan says. But the longer foods are stored in the refrigerator, the more the bacteria has the chance to grow.
The greatest food-based listeria risks are associated with ready-to-eat foods — meaning foods that don’t require cooking — that have been sitting around for a while in a person’s home or in some other food-storage environment (like a convenience store cooler). Deli meats, soft cheeses, unpasteurized dairy products, smoked meats, sprouts, and melon have all been the sources of listeria outbreaks, and are all considered high-risk foods.

Many foods that are left sitting out for a few hours at room temperatures — especially meats, cut fruit, and dairy foods — are risky. “At room temperature, listeria will double every 20 minutes,” Buchanan says. “In a refrigerator at 40 degrees, they double about once a day.”

Taking that into account, it’s important to eat foods soon after pulling them out of the fridge, and to refrigerate them as soon as possible if you’re not going to eat them, he says.

Once swallowed, Listeria monocytogenes is able to survive the acidic environment in the stomach and small intestine — an environment that kills most other types of harmful bacteria.Listeria also possesses genes that allow it to penetrate the cells of the human GI tract, says Janet Donaldson, PhD, a professor of microbiology at the University of Southern Mississippi in Hattiesburg. This penetration can lead to listeriosis and the infection’s life-threatening complications.

Learn More About How and Why Listeria Makes You Sick

Causes and Risk Factors for Listeria Infection

The Listeria monocytogenes bacteria is typically found in soil, water, and animal feces. Therefore, you’re most likely to develop listeriosis by eating contaminated fruits, vegetables, meats, and other animal products.Past outbreaks of listeria were traced back to contaminated deli meats and hot dogs, whereas more recent outbreaks are often linked to unpasteurized milk and dairy products, as well as produce.
According to the CDC, the following foods are more likely to contain listeria:
  • Queso fresco and other soft cheeses (Brie, Camembert, feta, queso blanco, queso panela, blue cheese)
  • Raw or lightly cooked sprouts (alfalfa, clover, radish, mung bean)
  • Melons
  • Hot dogs and deli meats
  • Smoked fish (unless canned or shelf-stable, or part of a cooked dish like casserole)
  • Unpasteurized (raw) milk and dairy products (soft cheese, ice cream, and yogurt)
Anyone can get sick from these and other contaminated foods, but infants and young children, pregnant women, older adults, and people with weakened immune systems face a much higher risk of infection.

How Is Listeria Infection Diagnosed?

If your doctor suspects you may have listeriosis, he or she will typically take a sample of blood or spinal fluid for testing.In the case of pregnant women with a fever and other signs and symptoms of listeriosis, the doctor may also gather samples from the placenta in the event of delivery.
If your results indicate a listeria infection, your doctor will create an appropriate treatment for you; but some people with mild symptoms may not need to be treated at all.

Treatment and Medication Options for Listeria Infection

Most healthy patients who are exposed to listeria and develop symptoms will not require any sort of treatment. Their immune system will eradicate the bacteria, and symptoms will tend to go away within three days, though they may last as long as one week.

If symptoms do not go away in that amount of time, it’s a good idea to check with your doctor to make sure the infection is not spreading or to determine what else could be causing the problems.

But pregnant women, seniors, and immunocompromised patients who experience symptoms and suspect they’ve been exposed to listeria should see a doctor as soon as possible. If they test positive for a listeria infection, the most common treatment is a 14- to 21-day course of intravenous antibiotics.In many cases, these individuals will require hospitalization so that doctors can closely monitor their condition, says Rob Danoff, DO, a family physician and the program director of the family practice residency at Philadelphia’s Jefferson Health.
In some cases — such as when a pregnant woman develops a fever and other symptoms after eating a food known to be contaminated with listeria — doctors may prescribe intravenous antibiotics while they wait for test results to confirm the presence of the bacteria.
It’s important to note that eating a food implicated in a listeria outbreak does not mean a person should start taking antibiotics. Even for older or immunocompromised individuals, antibiotic treatment is usually not recommended unless symptoms emerge.

Learn More About Treatment for Listeria Infections

Listeria Infections Can Be Much More Dangerous if You’re Pregnant

Pregnant women are roughly 10 times more likely to develop listeriosis than the average person.Why? Pregnancy causes changes to a woman’s immune system that may make her more vulnerable to infections like listeriosis.While listeriosis can cause mild flu-like symptoms in pregnant women, it may not cause any symptoms at all, according to the CDC’s Hunter.

RELATED: How to Tell the Flu From a Common Cold or Something Else

But pregnant women can pass the infection to their unborn babies, Hunter says. And this can result in miscarriage, stillbirth, or preterm labor — as well as sepsis (a blood infection) or meningitis (inflammation of the membranes surrounding the brain and spinal cord) in a newborn.“Listeriosis during pregnancy results in fetal loss in about 20 percent of cases and newborn death in about 3 percent of cases,” she says.
Hispanic women are at especially high risk of developing pregnancy-associated listeriosis — a risk that may stem from the consumption of unpasteurized cheeses, such as queso fresco or queso blanco.

Learn More About Specific Listeria Risks in Pregnant Women

Prevention of Listeria Infection

To protect yourself from the bacteria, the CDC recommends avoiding foods that come with a high risk of Listeria monocytogenes contamination. These include dairy products made with unpasteurized milk, raw sprouts, cut melon that has been left out for several hours at room temperature, ready-to-eat meat products that have been sitting open in your fridge for more than five days (approximately), and refrigerated meat spreads and smoked seafoods.
Other important safety tips include:
  • Always cook or heat meat products before eating them.
  • Promptly refrigerate any foods you’re not going to eat right away.
  • If you handle or prepare raw meat, be sure to wash your hands and any food prep equipment before you work with other foods.

It’s important to pay attention to news of listeria outbreaks in your area. The U.S. Department of Health and Human Services offers info about all food-related recalls and health issues at FoodSafety.gov, and you can sign up for automatic alerts about listeria concerns in your region.

New technologies, like whole genome sequencing, link pathogens causing illnesses in people to their contaminated food sources, allowing those foods to be removed from store shelves and homes more quickly.
Related

How DNA Sequencing Has Changed the Way Listeria Outbreaks Are Tracked

Whole genome sequencing is the process of fully mapping a living organism’s genetic code.Public health experts today use this form of genetic testing to track listeria outbreaks and better understand how the bacteria make people sick. Eventually, it may help public health experts prevent outbreaks from happening in the first place.
“There are many, many different genetic variants of listeria,” says Donaldson. While some of these strains can cause very serious infections in humans — even when consumed in small numbers — others are harmless, she says. By mapping and studying the DNA of each of these strains, she and others can better predict where listeria may show up and also how to prevent it from contaminating food. “With whole genome sequencing, listeria outbreaks can be detected when as few as two people have gotten sick,” the CDC reports.

Learn More About How to Prevent Listeria Infection

People over 65 and those who are immunocompromised face a greater risk of severe complications from listeriosis, including:

  • Sepsis (infection of the blood)
  • Meningitis (inflammation of the membranes surrounding the brain and spinal cord)
  • Encephalitis (inflammation of the brain)
People who develop complications from listeriosis typically require immediate medical care. For one in five people, a serious infection is fatal.

Related Conditions of Listeria Infection

In otherwise healthy people, listeriosis is mainly associated with mild flu-like symptoms like diarrhea, stomachache, and slight fever, which may lead people to mistake listeriosis for the flu or gastrointestinal (GI) issues.
In high-risk populations — which include newborns, older adults, and people with weakened immune systems — listeriosis can lead to life-threatening conditions, such as sepsis, encephalitis, and meningitis.

Resources We Love

Centers for Disease Control and Prevention

As one of the leading health organizations in the United States, the CDC tracks listeriosis outbreaks, offers answers to the most common questions, and outlines a list of the riskiest foods. You can also keep tabs on CDC projects like whole genome sequencing, which aims to prevent foodborne-illness outbreaks like listeriosis.

KidsHealth From Nemours

KidsHealth.org offers doctor-reviewed advice on hundreds of topics, free lesson plans and programs for educators, and tools like videos and slideshows to help families stay healthy. Check out their informational page on listeria infections, which outlines steps to take if you suspect your child has eaten contaminated food.

Mayo Clinic

Whatever you want to know about listeria infections, the Mayo Clinic has you covered. This site provides info on risk factors, symptoms, diagnosis, and treatment options, as well as tips to help you prepare for an appointment with your doctor.

U.S. Food and Drug Administration

Want to know how to keep listeria out of your food? The FDA outlines which foods are no-no’s and offers key food safety tips to reduce the risk of contamination. You’ll also find advice for maintaining your kitchen and refrigerator as clean, sanitary spaces.

U.S. National Library of Medicine

Have a question about listeria and listeriosis? The U.S. National Library of Medicine will direct you to a number of high-quality government and nongovernment health agencies — like the FDA, CDC, and Mayo Clinic — to help you fill in the blanks. You’ll also find information about clinical trials, journal articles, and high-risk groups.

Additional reporting by Lauren Bedosky.

Editorial Sources and Fact-Checking

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