Listeria are bacteria. They are the causative agent of listeriosis, a rare, mainly food-borne disease.
Listeria are widely spread in the environment, e.g. in waste water, in soil and on plants. Food of animal origin such as raw milk and raw milk products and raw meat, but also meat and fish products such as sliced, packaged sausage and smoked fish may contain Listeria. Products made from pasteurised milk, such as soft or cream cheese, can get contaminated during production.
L. monocytogenes are found in the environment, soil and water. Although animals can carry the pathogen without becoming ill, miscarriages caused by listeria occur in ruminants. Food processing plants can be a reservoir for these pathogens, therefore (further) processed food can get contaminated. Due to their ability to grow even at low temperatures, Listeria even multiply in the refrigerator; therefore, contaminated food may contain high bacterial counts after storage in the refrigerator.
Mode of transmission
The pathogen is mainly absorbed by the consumption of contaminated animal and plant products. In pregnant women, the pathogens can also be transmitted to the unborn child without any symptoms of maternal illness. Very rarely, further spread occurs through human-to-human transmission (hospital infections of newborns) and through direct contact with infected animals (skin infections).
In case of an infection via contaminated food, the first symptoms can appear within 1-70 days. Septicemic courses: 1-12 days (median 2 days); neurological courses: 1-14 days (median 9 days); pregnancy-associated cases: 17-70 days (median 27.5 days).
In healthy adults, an infection usually progresses without symptoms or only with diarrhoea. In general, the human immune system provides sufficient protection against severe disease progression and many infections pass unnoticed and without consequences. Serious cases mainly develop in immunocompromised people (e.g. people suffering from cancer, patients under high-dose cortisone therapy, etc.). If listeriosis is diagnosed, there is almost always an invasive course of the disease, which means that the bacteria spread beyond the digestive tract. Invasive listeriosis manifests itself by severe headaches, severe fever, nausea and vomiting. As a result, it can lead to meningitis or sepsis, which is fatal in about a quarter of patients. The pathogen can also cause inflammatory processes in other parts of the body (e.g. inflammation of the vertebral bodies), but these symptoms are rarely observed. In pregnant women, there is a risk of infection of the unborn child with the risk of premature or stillbirth. Sepsis and meningitis may develop in the infected newborn.
If consumers are concerned that they may have eaten food contaminated with listeria because of product recalls or warnings, the doctor can send a stool sample to a microbiological laboratory for listeria exclusion. A negative laboratory result should eliminate any concerns. Only if listeria are detected in the stool prophylactic administration of an antibiotic can be considered. However, without cultural pathogen detection, prophylactic antibiotic administration is considered contraindicated, as the risk of a severe antibiotic side effect is significantly higher than the very low risk of developing invasive listeriosis.
In case of invasive listeriosis, administration of antibiotics is necessary. Nevertheless, up to 30% of invasive listeriosis cases are fatal despite targeted therapy.
General basic rules to minimize the risk of foodborne infections are:
- Rinse fruits, berries, vegetables and pre-cut packed lettuce thoroughly with tap water before consumption or further processing
- Cook meat and fish dishes thoroughly
- Boil raw milk before consumption
- Do not eat raw minced meat
- Always store potentially hazardous foods such as soft cheese, sliced sausages or smoked fish separately from other food products
- Immunocompromised people, pregnant women and the elderly should refrain from eating potentially risky foods. Do not eat these products after the expiration date has passed