Listeriosis and pregnancy: symptoms, high-risk foods, and prevention

By: Dr Elie Servan-Schreiber (Gynécologue-obstétricien) — 11/03/2020

Listeriosis is a bacterial infection caused by Listeria monocytogenes. Rare in the general population (approximately 400 cases per year in France according to Santé Publique France), it is particularly dangerous during pregnancy as it can cause serious complications for the fetus. The good news: simple dietary precautions can prevent it.

Definition and key facts

Listeriosis is a foodborne infectious disease caused by the bacterium Listeria monocytogenes. This microorganism, shaped like a small rod, is a Gram-positive bacillus capable of surviving in a wide range of environmental conditions — including cold, salt, and acidic environments.

Some epidemiological data help to measure the scale of the problem:

• Approximately 400 cases of listeriosis are reported each year in France (Santé Publique France data, 2023).
• The prevalence is estimated at 5 cases per million inhabitants, making it a rare but notifiable disease.
• Pregnant women have a risk approximately 20 times higher than the general population of developing listeriosis, due to the physiological immunosuppression associated with pregnancy.
• Listeriosis has an overall case fatality rate of 20 to 30%, making it one of the most serious foodborne infections (ANSES).

Unlike more common infections such as toxoplasmosis, listeriosis is not systematically screened for at the beginning of pregnancy. Therefore, dietary prevention remains the primary safeguard.

Listeria vs listeriosis: what's the difference?

The two terms are often confused, but they refer to different things:

• Listeria refers to the bacterium itself, Listeria monocytogenes. This germ is very widespread in the environment: it is found in soil, water, vegetation, farms, and food processing plants. Being exposed to the bacterium does not necessarily mean getting sick.
• Listeriosis refers to the disease that develops when the immune system cannot contain the infection. In a healthy person, ingesting Listeria most often has no consequences.

Why is this distinction important during pregnancy? Because pregnancy naturally causes a decrease in immune defenses (to prevent rejection of the fetus). What would be harmless under normal circumstances can become dangerous when pregnant. This vulnerability explains the 20-fold increased risk mentioned above.

Symptoms of listeria during pregnancy

The difficulty with listeriosis is that its symptoms are non-specific and can easily be confused with a common flu-like state. Here are the signs to watch for:

• Fever, sometimes mild (38-38.5°C / 100.4-101.3°F), but which can also exceed 39°C (102.2°F).
• Headaches (cephalalgia) that persist.
• Muscle pain (myalgia) that is diffuse, resembling body aches.
• Digestive problems: nausea, vomiting, diarrhea — often present at the beginning of the infection.
• Significant fatigue and a feeling of general malaise.

A crucial point concerns the incubation period. Unlike most food poisoning cases that manifest within a few hours, listeriosis can take a few days to 2 months to appear (on average 1 to 4 weeks). This long incubation makes identifying the contaminated food particularly difficult.

Any unexplained fever during pregnancy should prompt a prompt medical consultation. Even a mild fever, if not linked to an obvious cold or an identified cause, warrants a blood workup including blood cultures. Since listeriosis is rare, the result will most often be negative — but early diagnosis, when positive, enables rapid treatment that significantly improves the prognosis.

High-risk foods for listeria during pregnancy

This is the most important section of this article. Contamination by Listeria occurs exclusively through food (or more rarely through direct contact with farm animals). The bacterium does not alter the appearance, smell, or taste of foods, making it invisible.

Here is a summary table of foods classified by risk level:

Prohibited foods during pregnancy (high risk)

• Soft cheeses made from raw milk : raw milk camembert, Brie de Meaux, raw milk reblochon, pont-l'évêque, raw milk munster, raw milk maroilles.
• Bloomy or washed rind cheeses made from raw milk: these cheeses provide a moist environment favorable to the growth of Listeria.
• Pâtés, rillettes, and terrines sold at the counter: products sold at the deli counter are more exposed to cross-contamination from cutting utensils.
• Deli meats sold at the counter: sliced ham, beef tongue, head cheese, etc.
• Artisanal smoked fish: artisanal smoked salmon, artisanal smoked trout, homemade taramasalata.
• Raw sprouted seeds: bean sprouts, alfalfa, etc.
• Raw milk that is unpasteurized and unsterilized.
• Raw shellfish and raw crustaceans.
• Raw or undercooked meat: tartare, carpaccio, rare meat.

Foods requiring precautions

• Soft cheeses made from pasteurized milk: they are less risky but it is recommended to systematically remove the rind, as it is on the surface that Listeria develops most.
• Industrially packaged deli meats: vacuum-packed ham, sterilized potted rillettes. Consume quickly after opening and well before the use-by date.
• Industrial vacuum-packed smoked salmon: the risk is lower than for artisanal products, but consumption should remain occasional. Check the use-by date and consume immediately upon opening.
• Surimi and pasteurized seafood products: consume quickly once the package is opened.

Permitted foods (very low or no risk)

• Hard cooked cheeses: Emmental, Comté, Beaufort, Parmesan, Gruyère. Their manufacturing process (high-temperature cooking and long aging) eliminates the bacterium. For more information, see our complete guide on cheeses allowed during pregnancy.
• Processed and spreadable cheeses: industrial products, they are made from pasteurized milk and heated during processing.
• All foods cooked to a core temperature above 70°C (158°F): well-cooked meat, cooked fish, cooked vegetables, stews.
• Canned goods: the sterilization process destroys all bacteria.
• Pasteurized milk and UHT milk.
• Industrial yogurts and fromage blanc.

Unsure about foie gras? Canned (sterilized) foie gras is permitted. However, fresh or semi-cooked foie gras should be avoided as the cooking temperature does not always exceed 70°C (158°F) at the core.

Listeria and cooking

Listeria monocytogenes is destroyed at temperatures above 70°C (158°F). A food item cooked to this core temperature for at least 2 minutes no longer poses a risk of listeriosis. This is the main argument in favor of a diet based on well-cooked products during pregnancy.

However, the bacterium has remarkable resistance properties:

• It resists cold: unlike most pathogenic bacteria, Listeria can multiply at temperatures ranging from 0 to 45°C (32 to 113°F). A refrigerator set to 4°C (39°F) slows its growth but does not stop it. It is one of the rare bacteria capable of slowly proliferating even in the refrigerator.
• It resists salt: salting does not provide sufficient protection. This is why salted products like smoked salmon or certain cured meats remain at risk.
• It resists acidity: it can survive in moderately acidic environments (pH down to 4.4).
• It resists drying: it can persist on dry surfaces for extended periods.

Beware of recontamination risk after cooking. This is an often underestimated point. Even if a food has been perfectly cooked, it can be recontaminated if it comes into contact with a surface, utensil, or another food carrying Listeria. Concrete example: you cook chicken, then cut it on the same board used to prepare raw salmon. The chicken's cooking will have been for nothing.

Consequences of listeriosis during pregnancy

While listeriosis is generally mild in healthy adults, it can have devastating consequences when it occurs during pregnancy. The bacterium can cross the placental barrier and directly reach the fetus.

Risks for the fetus and newborn

• Spontaneous miscarriage: listeriosis can cause a miscarriage, particularly during the second and third trimesters.
• Premature delivery: the infection can trigger premature labor with the associated risks of prematurity.
• Neonatal infection: the newborn may develop septicemia (generalized blood infection) or neonatal meningitis. These neonatal infections are serious and require care in a neonatology unit.
• Stillbirth: in the most severe cases, listeriosis can lead to fetal death. It sometimes happens that listeriosis is diagnosed during the etiological workup of a stillbirth.

Risks for the mother

In pregnant women, listeriosis most often manifests as a moderate febrile syndrome. However, in severe forms, it can progress to:

• Septicemia (bacteremia with signs of severity).
• Meningitis or meningoencephalitis, although these neurological forms are rarer in pregnant women than in non-pregnant immunocompromised individuals.

It is important to emphasize that the maternal prognosis is generally good when treatment is initiated quickly. It is the fetal prognosis that concentrates the severity of this infection.

What to do if you suspect listeriosis?

If you are pregnant and have an unexplained fever, even a mild one, here is what to do:

1. See your doctor or midwife immediately. Any fever above 38°C (100.4°F) during pregnancy, without an obvious cause (cold, already diagnosed urinary tract infection), warrants a same-day consultation.
2. Blood cultures will be ordered. This is a specific blood test that detects the presence of bacteria in the blood. It is the key test for diagnosing listeriosis.
3. Antibiotic treatment will be started without waiting for the results. In case of strong suspicion, the doctor may prescribe amoxicillin (the reference antibiotic against Listeria) even before bacteriological confirmation. Amoxicillin is well tolerated during pregnancy.
4. Fetal monitoring will be put in place: fetal heart monitoring, follow-up ultrasound, depending on the stage of pregnancy.

Early treatment is key. When antibiotic treatment is initiated quickly, before the bacterium reaches the placenta, effectiveness is high and the maternal and fetal prognosis is significantly improved. This is why a fever during pregnancy should never be minimized.

Children born from a pregnancy complicated by listeriosis will typically be hospitalized in a neonatology unit and monitored, particularly for neurological issues (risk of neonatal meningitis).

Prevention: essential hygiene rules

Listeriosis is a disease that can be effectively prevented. The following measures, recommended by ANSES and the French High Council for Public Health, significantly reduce the risk of contamination:

In the refrigerator

• Set the temperature to 4°C (39°F) maximum. Check it regularly with a refrigerator thermometer. Most household refrigerators are set too warm.
• Clean your refrigerator every 2 weeks with bleach water (or a suitable disinfectant), paying special attention to walls, shelves, and vegetable drawers.
• Separate raw foods from cooked foods. Store raw meats and fish in sealed containers at the bottom of the refrigerator to prevent juices from dripping onto other foods.
• Wrap all foods carefully: plastic wrap, airtight containers. Never leave food uncovered in the refrigerator.

Dates and storage

• Strictly respect use-by dates. For high-risk products (vacuum-packed deli meats, smoked salmon), consume them well before the use-by date, ideally within 24 to 48 hours after opening.
• Never refreeze a thawed food item. Thawing promotes bacterial multiplication, and refreezing does not destroy the bacteria that developed in between.
• Consume leftovers within 24 hours and reheat them to above 70°C (158°F) before eating.

In the kitchen

• Wash your hands thoroughly before cooking, after handling raw foods, and after touching animals.
• Wash utensils, cutting boards, and work surfaces between each type of food. A knife that has just cut raw salmon should not be used to slice a ready-to-eat food without being washed in between.
• Wash fruits and vegetables thoroughly, even those that will be peeled. Bacteria present on the surface can be transferred to the flesh during cutting.
• Cook foods to the core: the internal temperature must reach at least 70°C (158°F) for 2 minutes. Use a cooking thermometer if necessary.

These precautions may seem restrictive on a daily basis, but they quickly become second nature. They also protect against other foodborne infections such as toxoplasmosis or salmonellosis.

Our expert's opinion

"Listeriosis is a rare disease, but one whose consequences during pregnancy can be devastating. I understand that the dietary recommendations may seem frustrating — going without certain cheeses or deli meats for nine months is not trivial. But keep in mind that these restrictions are temporary and that they effectively protect your baby. In practice, what I tell my patients is: cook your food well, avoid raw milk products and deli meats sold at the counter, keep a clean and cold refrigerator, and in case of unexplained fever, see a doctor without delay. With these simple precautions, the risk of listeriosis becomes extremely low."

— Dr. Elie Servan-Schreiber, physician