In Germany, illnesses caused by Listeria are comparatively rare – at least according to the statistics of reportable infectious diseases: around 400 to 600 cases are reported in this country every year. However, the majority of infections remain undetected.
The problem: For some people, a listeria infection means an increased risk of becoming particularly seriously ill and, in extreme cases, even dying. So that you can protect yourself if necessary, we have compiled the most important information about pathogens in the form of questions and answers for you below.
What is listeria?
Listeria is a bacteria that occurs everywhere in nature – for example in soil, water, plants and the digestive tract of many animals (such as cattle, sheep, goats, pigs, chickens and rodents). Infected animals can excrete the germs in their feces and thus spread them further.
There are different types of listeria. Most are harmless to humans. But some – mainly representatives of the species Listeria monocytogenes – can cause listeriosis in humans. This disease can range from mild to fatal. What is crucial for the course of the disease (in addition to the number of pathogens ingested) is how good the immune system of the infected person is.
Some people are also immune to listeriosis. This means that they remain completely healthy even if they are proven to be carrying Listeria monocytogenes. This is due, among other things, to the widespread distribution of Listeria: This means that repeated contact with the bacteria is practically unavoidable – and the human immune system can learn to specifically ward off these.
What causes Listeria infection?
Listeriosis is usually the result of food poisoning. This means that those affected become infected by eating food contaminated with listeria. Infection often occurs through unheated animal products. This primarily includes milk and milk products such as:
- Raw milk and raw milk products, for example raw milk cheese (except long-matured hard cheeses such as Emmental, Gruyère or Parmesan) or homemade ice cream
- semi-hard blue cheese (such as Gorgonzola)
- Soft cheese with surface smear (such as Esrom, Handkäse, Harzer, Limburger, Mainzer, Munster or Tilsiter)
- Pickled cheese (such as sheep’s cheese or mozzarella) or cream cheese offered in open containers
- ready grated cheese
In addition, many other animal foods can be contaminated with listeria. The risk of infection exists, for example, when eating raw meat, raw sausage (such as chorizo or salami), delicatessen salads with animal ingredients (such as tuna or chicken salad) and raw or smoked fish.
The food doesn’t even have to come from animals that are infected with listeria. The bacteria can also subsequently get onto the food – during its extraction, processing, storage or preparation (e.g. through contact with contaminated hands, equipment, surfaces or other foods).
Plant-based foods can also be contaminated with listeria. Typical sources of infection are, for example, pre-cut and (vacuum-)packed leaf lettuce, ready-made salads (such as coleslaw or mixed salad), raw sprouts and seedlings as well as pre-cut melons.
Listeria infection during pregnancy
Listeria is indeed contagious. However, transmission from person to person is unlikely – except during pregnancy: If a pregnant person becomes infected with listeria, the bacteria can easily be passed on to their unborn child. This usually happens during pregnancy (through the placenta), sometimes only during birth in the birth canal.
Rare routes of infection
In principle, (skin) contact with infected people and animals, contaminated soil or other contaminated material can also lead directly to a listeria infection. In such a contact or smear infection, the germs can, for example
- penetrate the skin or mucous membrane directly through the smallest of injuries or
- reach the mucous membranes of the mouth, nose or eyes through unwashed hands.
However, it is rare for direct contact with Listeria to lead to infection. For example, the following may be affected:
- Newborns whose mothers or other close contacts are infected with listeria (postnatal infection)
- other immunocompromised people – such as those who are seriously chronically ill – who come into contact with listeria in hospital (nosocomial infection)
- People who have intensive contact with infected animals – for example due to veterinary or agricultural work.
What symptoms can listeria cause?
In healthy people, an infection with listeria usually has no symptoms. However, if the bacteria enter the intestine in sufficient quantities through food, penetrate its mucous membrane and multiply there, some of those affected appear to be in otherwise excellent health
- a mild febrile reaction that could have any number of causes, or
- Symptoms of a listeria infection limited to the intestines (non-invasive listeriosis).
A listeria infection limited to the intestines typically leads to gastrointestinal inflammation (gastroenteritis). Such non-invasive listeriosis often causes flu-like symptoms such as fever and body aches as well as gastrointestinal complaints such as diarrhea and vomiting.
The symptoms of non-invasive listeriosis are usually mild. However, after ingesting a particularly large amount of listeria, the symptoms of the gastrointestinal infection can be more severe. In any case, the disease usually goes away on its own after one to three days.
Listeriosis of the skin
The rare skin listeriosis can occur after direct skin contact with listeria. The symptoms are then localized to the contact point. Typically, nodules (papules) or pus-filled blisters (pustules) form there. The skin changes are not associated with pain or itching, but can transform into small ulcers. But the skin infection is usually uncomplicated and – like the gastrointestinal infection – heals on its own.
In rare cases, a listeria infection spreads beyond the intestines. This happens when the bacteria from the intestinal mucosa enter the blood or lymphatic system and spread to other organs. Such invasive listeriosis causes severe symptoms, some of which can be life-threatening.
The most common symptoms of an invasive Listeria infection are sepsis (“blood poisoning”) , meningitis and/or encephalitis. In principle, the bacteria can also affect any other organ – such as the eyes, joints, heart and kidneys – and lead to localized purulent inflammation.
What symptoms does Listeria cause during pregnancy?
Anyone who is pregnant is significantly more susceptible to a listeria infection than other people. On the whole, listeria causes similar symptoms during pregnancy as in healthy non-pregnant people.
Accordingly, gastrointestinal complaints such as diarrhea during pregnancy can indicate listeria in the intestines. However, sometimes the infection goes completely unnoticed.
Even with an invasive Listeria infection during pregnancy, the symptoms that the pregnant person develops usually do not suggest a serious illness. Often only flu-like symptoms such as fever, fatigue and body aches occur.
When does listeria become dangerous during pregnancy?
Pregnant women can also become seriously ill with invasive listeriosis. However, this rarely happens. The risk of a Listeria infection being passed on to the unborn child during pregnancy is significantly higher. Depending on when this happens, the consequences are different – but always dangerous:
- Infection early in pregnancy can result in miscarriage or stillbirth.
- Infection later in pregnancy can cause premature birth and severe listeriosis in the newborn.
- If the child becomes infected with listeria during birth, it can also become seriously ill.
In the event of an early infection, the general condition of the newborn is impaired. Sepsis, shortness of breath and skin changes often occur. A late infection, however, only becomes apparent from the second week of life onwards: Listeria often causes meningitis and skin changes. Overall, listeriosis puts newborns at high risk of permanent damage or even death.
Who else can a listeria infection be dangerous for?
In addition to unborn children and newborns, people who have a weakened immune system are particularly susceptible to listeria infections. These include, for example, people who…
- are over 65 years old.
- have certain underlying diseases (such as cancer, diabetes, liver or kidney disease, alcoholism , HIV or AIDS).
- receiving treatment that suppresses the immune system (such as “cortisone” treatment or chemotherapy ).
How long is the incubation period for Listeria?
The time between infection and the appearance of the first symptoms – technically called the incubation period – varies greatly with listeria. Anyone who ingests the bacteria through food can experience signs of an intestinal infection after just a few hours to days. Listeria-related food poisoning usually becomes apparent within 24 hours of infection.