What Is Listeria?

Listeria monocytogenes (Listeria) is a type of bacteria that causes an infection called listeriosis. The bacterium is found in:

  • soil
  • dust
  • water
  • processed foods
  • raw meat
  • animal feces

Most cases of listeriosis are caused by eating food contaminated with the bacteria. Listeriosis only causes a mild illness for most people. However, it can lead to a much more serious illness in unborn babies or newborns when the mother is infected while pregnant. Infection of the fetus may cause miscarriage or stillbirth. Infection of a newborn can lead to pneumonia and death. For this reason, prevention of listeriosis during pregnancy is extremely important.

Pregnant women should avoid certain types of food, such as hot dogs, deli meats, and soft cheeses in order to lower their risk. Understanding how your food is prepared and following food safety guidelines can also help prevent this infection.

Why Is Listeria More Serious for Pregnant Women?

In healthy adults who are not pregnant, eating food contaminated with Listeria usually doesn’t lead to problems. Listeriosis is rare in non-pregnant healthy adults, but the infection is up to 20 times more common in pregnant women, according to a review in Obstetrics and Gynecology. Most pregnant women have no symptoms or problems from the infection. However, the fetus is highly susceptible to this type of bacterium. The infection can spread into and across the placenta. Infection with Listeria — known as listeriosis — is severe and often fatal for the baby.

What are the Symptoms of Listeria?

Symptoms may start anywhere from two days to two months after exposure to the bacteria. Healthy adults who aren’t pregnant usually show no symptoms at all.

Symptoms in pregnant women may be similar to the symptoms of the flu or cold. They may include:

  • fever
  • headaches
  • muscle aches
  • chills
  • nausea
  • vomiting
  • stiff neck
  • confusion

Be sure to contact your healthcare provider if you are pregnant and experience any of these symptoms. Sometimes a pregnant woman infected with listeriosis won’t feel very sick. However, she can still pass the infection on to her unborn baby without knowing it.

Causes of Listeriosis

Listeriosis is an infection caused by eating foods contaminated with the bacterium Listeria monocytogenes. The bacteria are commonly found in water, soil, and animals. Vegetables can be contaminated from the soil. It can also be found in uncooked meats and unpasteurized dairy products because animals are often carriers for the bacteria, though they don’t get sick from it. Listeria is killed by cooking and pasteurization (the process of heating a liquid to a high temperature to kill germs).

This bacterium is unusual because it grows well at the same temperature as your refrigerator. People typically catch listeriosis by eating the following contaminated foods:

  • ready-to-eat meats, fish, and poultry
  • unpasteurized dairy
  • soft cheese products
  • fruits and vegetables that are contaminated from the soil or from manure used as fertilizer
  • food packaged in unsanitary conditions

Am I at Risk?

Women with certain conditions are at a slightly higher risk of infection. These include the following conditions:

  • diabetes
  • steroid use
  • human immunodeficiency virus infection (HIV)
  • compromised immune system
  • splenectomy
  • use of immunosuppressive medications
  • cancer
  • alcoholism

Many cases of listeriosis occur in healthy pregnant women. Pregnant Hispanic women are also at a higher risk — roughly 24 times more likely than the general population to be infected.

How is Listeria Diagnosed?

A doctor will suspect listeriosis if you are pregnant and have a fever or flu-like symptoms. Listeria is difficult to diagnose. Your doctor will try to confirm a diagnosis by performing a blood culture to test for presence of the bacteria. They may ask you questions about your symptoms and what you’ve eaten recently.

The cultures may take up to two days for growth. Because it is so serious for the baby, your doctor may start treatment for listeriosis even before they get the results.

What Are the Complications of Listeria in Pregnancy?

If you are pregnant and are infected with listeriosis, you are at an increased risk of:

  • miscarriage
  • stillbirth
  • premature delivery
  • deliver of a low birth weight infant
  • death to the fetus

In some cases, the infection can lead to complications in pregnant women, including:

  • bacterial meningitis (inflammation of the membranes surrounding the brain)
  • septicemia (blood infection)

Infection in newborns can cause the following:

Treatment of Listeria in Pregnancy

Listeria is treated with antibiotics. Doctors will usually prescribe penicillin. If you are allergic to penicillin, trimethoprim/sulfamethoxazole may be used instead.

The same antibiotics are given to babies born with listeriosis

What is the Outlook?

A Listeria infection is often severe in babies. It carries a fatality rate of 20 to 30 percent according to a review in Obstetrics and Gynecology. Early treatment with antibiotics helps prevent fetal infection and other severe complications. Not all babies whose mothers are infected will have problems.

Can Listeria in Pregnancy Be Prevented?

The key to preventing listeria infections during pregnancy is to follow guidelines recommended by the Centers for Disease Control and Prevention (CDC). The organization recommends that you should not eat foods with a high risk of Listeria contamination when you are pregnant.

Avoid the following foods:

  • hot dogs, lunch meats, or cold cuts served cold or heated to less than 165˚F. Eating at restaurants that serve deli meat sandwiches is not recommended.
  • refrigerated meat spreads
  • meats cooked “rare”
  • raw produce that hasn’t been washed thoroughly
  • raw (unpasteurized) milk
  • refrigerated smoked seafood
  • unpasteurized soft cheeses, such as feta and Brie cheese. Hard cheeses like cheddar and semisoft cheeses such as mozzarella are OK to consume, as well as pasteurized spreads like cream cheese.

It’s also important to practice food safety and handling guidelines. These include:

  • Wash fruits and vegetables thoroughly in clean water, even if the skin will be peeled.
  • Scrub firm produce like melons and cucumbers with a clean brush.
  • Read ingredient labels.
  • Check expiration dates.
  • Wash your hands often.
  • Keep preparation surfaces in your kitchen clean.
  • Keep your refrigerator at 40˚F or below.
  • Clean your refrigerator often.
  • Cook foods to their proper temperatures. You should purchase food thermometers to be sure foods are cooked or reheated to at least 160˚F.
  • Refrigerate or freeze perishable or prepared food and leftovers within two hours of preparation; otherwise, throw them away.

The United States Department of Agriculture (USDA) and the Food and Drug Administration (FDA) also conduct routine screening and monitoring of the potential food sources of contamination. They will recall any prepared chicken, pork, and seafood products in the United States if there is any concern of contamination.

Ultimately, the Listeria bacterium is so common that exposure can’t always be prevented. Pregnant women should call their doctor if they have any of the common symptoms.