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.

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.

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.

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 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.

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.

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

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.

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. 

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

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.

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. 

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.