Listeriosis is a bacterial infection caused by Listeria monocytogenes. Listeria is transmitted via food. The bacterium can multiply between 1 and 45°C, in the presence or absence of oxygen. It can survive in the environment and multiply at refrigerator temperature (4°C). The bacteria can withstand freezing, but is sensitive to cooking.

There is no human-to-human transmission, but pregnant women can pass the infection on to the fetus in utero during pregnancy or in rare cases during childbirth.

The incubation period for the appearance of the first symptoms is 1 to 2 weeks on average. In pregnancy-related infections, the incubation period is longer, with an average of 3 to 4 weeks. In general, the incubation period can vary between a few days and up to 3 months.

In healthy individuals, listeriosis usually causes isolated inflammation of the intestine (gastroenteritis) with fever and the infection is usually mild. However, it can progress to an invasive form (blood poisoning/sepsis or central nervous system infection/meningitis) and is then associated with a high mortality rate (20%).

The invasive forms mainly occur in people with an impaired immune system (e.g. in individuals using immunosuppressants, i.e. drugs that suppress the immune system, or corticosteroids and in the case of chemotherapy, cancer, diabetes or alcoholism) as well as in elderly people over 60 years of age. In pregnant women, the infection can lead to miscarriage, premature birth or infection of the newborn child. 

Listeriosis is caused by the consumption of food that is contaminated with Listeria monocytogenes. If an infection occurs during pregnancy, the bacteria can spread to the baby through the placenta.        

In 2016, the National Health Council published a list of foodstuffs that people at risk should avoid:

• fresh cheese (e.g. mascarpone) and soft cheese (e.g. Brie) based on raw or pasteurized milk
• cooked charcuterie (e.g. pâté, chicken fillet, cooked ham, ham sausage, veal sausage, etc. both pre-packed and pre-sliced) 
• cold smoked fish 
• chopped and prepackaged leafy vegetables (lettuce, herbs, spinach) 
• sprouts
• sliced ​​and prepackaged melon or fruit salad with melon 
• raw meat and raw fish
• mayonnaise-based spreads for sandwiches (chicken and curry, meat spread, salmon spread, etc.) 
• pre-packaged sandwiches or salad meals (with cooked ham, smoked salmon, soft cheese, etc.).

It is also important:

• to clean the refrigerator regularly
• to not eat expired foods
• to make sure that the temperature of the refrigerator is low enough (<4 °C)
• to wash foods properly 
• to observe good hygiene when preparing food. 

The doctor will take a sample (from a tissue, blood or spinal fluid that is normally sterile) and send it to a laboratory for identification. If Listeria is isolated in the culture, the diagnosis of listeriosis will be confirmed. The laboratory send the human bacterial strain to the National Reference Center for Listeria at the Bacterial Diseases department of Sciensano.

If no bacteria can be isolated by culture, a molecular test (PCR) can be performed on the human sample for molecular detection of Listeria.

The risk groups are mainly people with impaired immunity:

• pregnant women
• immuno-depressed patients (like people with cancer or diabetes, or people who receive immuno-suppressive treatment)
• elderly people.

Although the risk of contracting listeriosis is not very high, it can have dangerous consequences for pregnant women. In pregnant women, an infection can lead to miscarriage, stillbirth, premature birth or infection of the newborn child. It is therefore important to avoid certain foods during pregnancy and practice hygiene measures before preparing and storing food.