Listeriosis is caused by a pathogen called listeria. This type of bacteria is found everywhere in the natural environment. The main cause of infection is eating contaminated animal-based food products. In most cases, the infection is harmless. However, it can be dangerous for older people, unborn babies and newborns.

At a glance

  • Listeriosis is a foodborne infection caused by a type of bacteria called listeria. These bacteria occur naturally in humans, animals and the environment.
  • Food products can become contaminated by listeria during production and processing. When they are eaten, the bacteria enter the body.
  • In a small number of people, the pathogens spread via the blood and cause life-threatening blood poisoning (sepsis).
  • Most cases of listeriosis are mild. However, it can be severe or even fatal in unborn babies, newborns, older people and those with a weakened immune system.
  • Infection can be prevented by buying fresh produce and eating it before its expiry date.
  • People in at-risk groups should avoid certain food products.

Note: The information in this article cannot and should not replace a medical consultation and must not be used for self-diagnosis or treatment.

Various sausage and cheese products

What is listeriosis?

Listeriosis is a foodborne infection – in other words, a disease that is spread through food. The pathogen responsible is a type of bacteria called Listeria monocytogenes. This type of bacteria is found everywhere in the natural environment.

Listeriosis is a bacterial infection. It is caused by consuming food that has been contaminated by bacteria called listeria.

While most infections are mild, severe cases with life-threatening complications such as sepsis or meningitis can occur. Older people, those with a weakened immune system and unborn and newborn babies are all at an increased risk.

If symptoms occur, the infection is treated with antibiotics that are effective against the listeria bacteria.

To avoid infection, it is advisable to buy fresh food produce and to eat it within a short space of time.

People in at-risk groups can also protect themselves by avoiding certain food products, such as smoked salmon, meat paste or cheese made from unpasteurized milk.

Important: Food manufacturers are required to randomly test their products on a regular basis to ensure that they are free from listeria and guarantee their safety. Compliance with this requirement is monitored by the relevant authorities.

What are the symptoms of listeriosis?

People who don’t have a weakened immune system normally experience a listeria infection without symptoms. Their gastrointestinal tract is only temporarily colonized by the listeria.

In some cases, they may have diarrhea and a slightly elevated temperature for a short period. The time between infection and the onset of symptoms varies from a few hours to 14 days, or even longer in the case of pregnant women.

Often, pregnant women who contract listeriosis only have mild, flu-like symptoms. However, their unborn baby can be severely affected by the infection.

The symptoms are usually more severe in people with a weakened immune system. They usually experience flu-like symptoms like a high temperature and muscle pain, as well as diarrhea and vomiting.

Newborn babies who become infected during birth or in the first few weeks of life show symptoms of blood poisoning (sepsis) and experience breathing difficulties and damage to the skin.

What causes listeriosis?

The most common cause of listeriosis is a type of bacteria called Listeria monocytogenes. In very rare cases, listeriosis is caused by other types of listeria bacteria.

Listeria is found everywhere in the natural environment. It is particularly common in waste water, soil and animal feed. This type of bacteria can therefore contaminate animal-based and plant-based food products.

Listeria is found everywhere in the natural environment. This type of bacteria can contaminate animal-based and plant-based food products.

Listeria can enter food during production or processing. 

The bacteria are particularly common in animal-based products such as:

  • meat and meat products, especially minced meat and raw sausage
  • poultry
  • fish and fish products, especially smoked fish (e.g. salmon or trout)
  • milk and dairy products, in particular cheese, unpasteurized milk and unpasteurized dairy products

However, listeria can also be found in plant-based foods, especially in pre-prepared salads.

Important: Listeria can also multiply in the refrigerator. However, it does this much more effectively at room temperature. For this reason, people in at-risk groups are advised to buy fresh food and consume it within a short period of time.

How do people become infected with listeriosis?

People usually become infected with listeriosis when they eat food that is contaminated with listeria. If a woman becomes infected while pregnant, the listeria is transmitted to the unborn baby through the placenta and umbilical cord. The baby may also become infected during the birth or shortly afterwards through close contact with the mother.

Who is at risk of a particularly severe listeriosis infection?

Anyone with a weakened immune system is at an increased risk of becoming seriously ill with listeriosis. This includes the following groups:

  • unborn and newborn babies, as they have an immature immune system
  • older people, as the immune system weakens with age
  • people who have diseases that weaken the immune system, such as cancer or an HIV infection
  • people taking medication that suppresses the immune system – for example, following a transplant
  • people who take proton pump inhibitors on a regular basis – these are drugs that inhibit the production of stomach acid

How common is listeriosis?

The number of listeriosis infections varies from year to year. In Germany, there are between 200 and almost 800 infections per year on average.

One in ten people infected are pregnant women or newborn babies. The remaining cases largely affect people over the age of 50 and those with certain pre-existing health conditions. Men are more commonly affected than women.

What are the possible consequences of listeriosis?

Around 30 percent of people who become infected with listeriosis develop blood poisoning (sepsis) or purulent inflammation of the meninges, i.e., the fluid and membranes surrounding the brain and spinal cord (meningitis). This often affects newborn babies and people with immune deficiencies.

If the bacteria spread around the entire body, other centers of purulent inflammation may occur in other sites, such as the joints or heart valves.

An infection with listeriosis during pregnancy can lead to premature delivery or stillbirth.

While listeriosis is not a common foodborne infection, it is the one with the highest mortality rate. Around 7 percent of those who develop a severe infection die as a result. 

How can listeriosis be prevented?

Contamination with listeria can occur during the processing, packaging or preparation of animal-based and plant-based foods.

To prevent infections and large outbreaks, food products in Germany and other EU Member States are randomly tested for listeria on a regular basis.

Listeria is particularly common in animal-based foods that are not heated before consumption. The bacteria can multiply on food despite vacuum packaging and storage in a refrigerator. For this reason, vacuum-packed food products should be eaten as soon as possible after purchase. As listeria doesn’t cause food to spoil, it cannot be detected by sight or smell.

People in at-risk groups, such as pregnant women, older people and those with a weakened immune system, should also avoid the following foods:

  • raw meat products (e.g., minced meat) and raw sausage (e.g., salami)
  • raw, smoked or marinated fish or fish products
  • hard and soft cheeses made from unpasteurized milk
  • pre-prepared, pre-packaged salad leaves
Pregnant women and people with pre-existing conditions should avoid eating the following foods: raw meat and raw sausage, raw and processed fish, cheese made from unpasteurized milk, pre-packaged leafy salads.

More information about foodborne infections and advice on handling food is provided in the article Food hygiene – don’t give germs a chance.

For more detailed information about avoiding an infection with listeria, see the consumer advice provided by the Federal Institute for Risk Assessment.

How is listeriosis diagnosed?

If doctors suspect listeriosis based on the patient’s symptoms and medical history, they normally take a blood sample or a sample of cerebrospinal fluid – the fluid surrounding the spinal cord in the spine.

They may also take additional samples – for example, of mucus, stool or vaginal discharge – depending on the patient’s symptoms.

These samples are then tested for listeria in the lab. If the pathogens are detected, doctors are required to inform the local health authority.

The authorities will then conduct an investigation to determine where the infection originated. 

How is listeriosis treated?

Doctors usually treat listeriosis with a combination of two different antibiotics. Depending on the active ingredient, these kill the listeria or inhibit their growth.

Treatment isn’t straightforward because the bacteria enter the cells of the body and multiply there. To avoid relapses, doctors prescribe the antibiotics for at least 3 weeks. Treatment may extend to 6 weeks if the brain or heart are affected by the infection.

Reviewed by the German Society for Hygiene and Microbiology (Deutsche Gesellschaft für Hygiene und Mikrobologie e.V. – DGHM).

As at:
Did you find this article helpful?