Here’s what to know about listeria in pregnancy.

How do you get a Listeria monocytogenes infection?

Humans get a Listeria infection when they eat food or drink beverages contaminated with the bacteria. Most healthy people can fight off this infection while it’s still in the liver and never experience any serious symptoms. But if a person’s health is compromised — or if the patient is a pregnant woman — infection can take hold.

The infectious dose of Listeria monocytogenes (how much it takes to make you sick) is not well understood. Scientists do not have a clear relationship between the number of Listeria monocytogenes cells in a food and the likelihood of developing listeriosis. It is possible that low-level contamination causes infection in people who are susceptible, including pregnant women, the elderly, and the immunocompromised.

Deli foods are especially susceptible to contamination because of how often slicers, prep counters, and storage drawers are reused without full disassembly cleaning. And restaurant-prepared foods are eaten without a kill step, so any bacteria present survive to make someone sick. The 2024 Boar’s Head deli meat outbreak — which killed at least 10 people and hospitalized more than 60 across 19 states — illustrated how a sanitation failure at one facility can cause harm thousands of miles away. At least one pregnant woman from Minnesota nearly lost her unborn child in that outbreak.

How does Listeria infect you?

When you eat food contaminated with Listeria monocytogenes bacteria, the organism easily survives the low pH of the stomach because it can live in acidic environments. It then moves from your intestines to your liver. The liver in a healthy person can usually destroy this pathogen, but if the immune system is suppressed — or if the person is already sick, especially with a chronic illness — the bacteria can grow rapidly and emerge into the bloodstream.

Pregnant women are more susceptible because their immune systems are altered in response to growing a baby. The same is true of their unborn babies, who lack a developed immune system of their own. If Listeria survives in the liver long enough, it can enter the bloodstream — where it can pass through the blood-brain barrier and infect the brain, the meninges (the membranes around the brain and spinal cord), and the digestive tract. It can also travel to the placenta in pregnant women, with devastating consequences.

The headache and stiff neck experienced by some patients may be symptoms of Listeria meningitis, since the bacteria can infect the meninges. High fever is the body’s way of trying to clear the infection by triggering the growth and movement of immune cells. When the bacterial load is high enough that the bloodstream is infected, the condition becomes Listeria sepsis — a medical emergency.

Pregnant women may only be mildly ill with flu-like symptoms when they are sick with listeriosis. Many women have no symptoms at all, or only diarrhea, stomachache, fever, headaches, or body aches. The full range of listeriosis complications can develop without the mother realizing she’s sick — which is why exposure to a recalled food matters even when you feel fine.

Why are pregnant women at more risk for listeriosis?

Studies show that pregnant women are between 10 and 18 times more likely to contract listeriosis than other healthy adults. The pathogen infects otherwise healthy pregnant women who have few or no other risk factors. Scientists are beginning to understand why: the immune system is suppressed so the embryo isn’t rejected. That makes it easier for the bacteria to establish an infection. While the woman’s immune system may successfully kill some of the pathogen, some may survive, escape into the bloodstream, and travel to the placenta.

In 2015, researchers in Paris discovered a placental breach mechanism for Listeria monocytogenes that may explain why miscarriage, premature labor, and stillbirth are so common among pregnant women with this infection.

The placenta is an immunological barrier between mother and fetus. That means the pathogens that have reached the placenta are not detected by the mother’s immune system. Listeria bacteria may even invade the lining of the uterus directly. The bacteria grow and then “burst” out of the placenta, triggering a miscarriage. This is one theory bolstered by research at Berkeley in 2019. The presence of large numbers of pathogens in the placenta may also eventually trigger the mother’s immune system, which causes a miscarriage as a defense mechanism.

Another theory is that infection in the mother triggers inflammation, which affects the placenta and renders it unable to protect the fetus. The fetal loss rate due to listeriosis in pregnancy is about 20%, and about 3% of newborns die when their mother is infected with this pathogen. Doctors used to think that the fetus is most susceptible in the third trimester, but new research published in 2017 showed that some strains of Listeria may be the cause of many early miscarriages as well.

The risk is even higher for some populations. Hispanic women are about 24 times more likely to contract listeriosis than non-Hispanic women — driven in significant part by exposure to queso fresco-style cheeses, which have repeatedly been involved in outbreaks. The 2021 queso fresco outbreak sickened pregnant women across multiple states, and the 2024 Rizo López cotija cheese outbreak again hit shoppers at major retailers including Costco, Trader Joe’s, HEB, and Albertsons.

What are the symptoms of listeriosis?

The main symptoms of listeriosis in non-pregnant women are high fever, stiff neck, serious headache, muscle aches, nausea, and diarrhea. In pregnant women, symptoms may be very mild and are usually similar to a flu. Often a woman doesn’t even realize she has the infection until something terrible happens to the pregnancy, or the baby is diagnosed with listeriosis at birth or soon after.

Newborns and infants who are sickened with listeriosis can develop a blood infection, meningitis (inflammation of the membranes around the brain and spinal cord), and respiratory pneumonia. Most newborns who contract this infection from their mothers are diagnosed a few days to a couple of weeks after birth. Up to 30% of newborns die despite antibiotic treatment.

The illness a newborn suffers — along with the onset of symptoms and prognosis — varies depending on whether the illness is early-onset or late-onset. Early-onset cases are usually born premature and can suffer the most serious complications; up to one-third of newborns with early-onset listeriosis die. If listeriosis is contracted later in the pregnancy, the infant is more often born full-term. The baby is most likely infected at birth — through contact with the mother’s intestinal flora or the birth canal during delivery. These infants generally have a better prognosis than those with early-onset disease. For a deeper read on what a parent faces, see Pritzker Hageman on babies born with listeriosis.

How are Listeria cases investigated?

Because Listeria monocytogenes infection is a reportable illness, doctors are required to inform public health officials every time a listeriosis infection is diagnosed. The bacteria’s DNA is read (or “sequenced”) using procedures called pulsed-field gel electrophoresis (PFGE) or, more recently, whole genome sequencing (WGS), to create a “fingerprint.” Isolates from patients are stored in a national subtyping laboratory called PulseNet. Because most cases of listeriosis occur individually and not in multistate outbreaks, when someone is sickened officials look through years of data to see if another person was sickened with the same bacteria carrying the same DNA pattern.

If a match is found between two unrelated people, an outbreak is declared. The same process happens if Listeria monocytogenes is found in food: the bacteria is isolated and studied, then officials look through the PulseNet database to see if any human has been sickened with that particular strain. If the food is likely still available for sale, or in consumers’ homes, a recall is usually initiated. At this point people sickened by Listeria will often contact a food safety law firm like Pritzker Hageman to recover medical and other costs and to make sure the companies that broke food safety laws will not do it again.

The illness-onset date in listeriosis outbreaks can extend over years. Patients in a single outbreak may have been sickened months or years apart, because the bacteria can survive so long in food processing facilities. The investigation into Totally Cool Ice Cream showed years of contamination at the facility before a recall was finally issued.

I’m pregnant. How do I know I have listeriosis?

Pregnant women may not know they have listeriosis. If they do show symptoms, the symptoms may feel like a mild flu — nausea, diarrhea, mild fever, muscle aches, backache, fatigue, and vomiting. The most common single symptom of listeriosis in pregnant women is fever. And the incubation time in pregnant women before symptoms appear is often longer than in non-pregnant patients (anywhere from 2 to 10 weeks is common).

The only way to know for certain is to have a test done. A diagnosis is made with a blood test, sometimes paired with cultures of a stool (fecal) sample. These tests can only be performed by a hospital or doctor’s office, with results processed at a medical lab. The detailed clinical workup is on our diagnosis and treatment page.

It’s important that any pregnant woman who has unexplained flu-like symptoms — especially headache or fever — see her doctor as soon as possible. The earlier this infection is diagnosed, the more likely treatment will be effective.

Two deadly listeriosis outbreaks in 2014 were linked to soft cheeses; pregnant women were affected and one newborn was diagnosed with listeriosis. The pattern has continued ever since — the 2024 queso fresco / cotija outbreak, the 2023 Real Kosher ice cream outbreak, and the 2022 Big Olaf Creamery outbreak all sickened pregnant women, and all caused devastating outcomes.

How can women protect themselves against listeriosis?

For listeriosis specifically, a good first step is to avoid deli foods (unless reheated to steaming hot), unpasteurized milk, juice and cheese, products made with raw eggs, and pâté. Follow the FDA’s basic food safety rules for buying, storing, and preparing food.

For general food safety, at the grocery store, buy perishable foods like meats, poultry, eggs, and dairy last so they spend less time out of refrigeration. Keep meats and poultry separated from foods you’ll eat raw — leafy greens and fruits, for example. Always store perishable foods, including cut fruits and vegetables, in the fridge. Store foods that will be eaten raw away from uncooked meats and poultry to avoid cross-contamination. Cook meats, poultry, and fish within three days of purchase, or freeze.

When you’re cooking, always use a food thermometer to check final internal temperatures of beef, chicken, pork, turkey, fish, and egg dishes. Whole cuts of beef and pork (chops and roasts) should reach 145°F. Ground beef, veal, and pork should reach 160°F. All poultry — including ground chicken and turkey — should reach 165°F. Fish and seafood should reach 145°F. Cook shellfish like mussels until their shells open. Cook all egg dishes to 160°F. Hot dogs and luncheon meats (even if already cooked) should be reheated to 165°F before eating.

For a deeper, pregnancy-specific guide that consolidates the CDC and FDA recommendations, see our prevention page.

What is the treatment for a Listeria monocytogenes infection?

Listeriosis in both moms and babies is treated with antibiotics. The antibiotics are typically given intravenously (IV) and may need to be given over ten days or more — the duration depends on how well the antibiotics are clearing the infection. Because pregnant women are at such high risk of fetal loss with this infection, doctors may treat them with a prophylactic (preventative) course of antibiotics if the woman has eaten food that may be contaminated, even before a positive test confirms infection. Some patients need IV fluids for rehydration, and may require hospitalization for additional supportive care.

In healthy non-pregnant people, antibiotics might not be needed at all, and treatment is symptom-based. Treatment in detail is on our diagnosis and treatment page.