Listeria is widely distributed in the environment, e.g. in sewage, 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 smear or soft cheeses, can become contaminated during manufacture.
L. monocytogenes can often be found in the environment, soil and water. Animals can carry the pathogen without becoming ill, but miscarriages caused by Listeria occur in ruminants. Food processing plants can be a reservoir for these pathogens, resulting in contamination of (further) processed food. 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.
Listeria are mainly ingested through the consumption of contaminated animal and plant foods. In pregnant women, the pathogens can also be transmitted to the unborn child without any symptoms of maternal disease. Very rarely, further spread also occurs through person-to-person transmission (hospital infections of newborns) as well as through direct contact with infected animals (skin infections).
In healthy adults, an infection usually runs its course without signs of illness or only with diarrhea. In general, the human immune system provides sufficient protection against severe courses of disease, and many infections pass virtually unnoticed and without consequences. Severe illnesses mainly affect immunocompromised people (e.g. people suffering from cancer, patients under high-dose cortisone therapy, etc.). If listeriosis is diagnosed, it is almost always invasive, meaning that the bacteria spread beyond the digestive tract. Invasive listeriosis is manifested by severe headache, high fever, nausea and vomiting. This can lead to meningitis or sepsis (blood poisoning), which is fatal in around a quarter of patients. The pathogens can also cause inflammatory processes in other parts of the body (e.g. pus accumulation in the spine or in joints), but these consequences are rarely observed. In pregnant women, there is a risk of infection of the unborn child with the risk of premature birth or stillbirth. Sepsis and meningitis may develop in the infected newborn.
General basic rules to minimize the risk of foodborne infections: Rinse fruits, berries, vegetables and pre-cut packaged leafy salads thoroughly with tap water before eating or processing, cook meat and fish dishes thoroughly, boil raw milk before eating, do not eat minced meat raw, always store possible risk foods such as soft cheese, smear cheese, sliced sausages or smoked fish separately from other foods. Immunocompromised people, pregnant women and the elderly should refrain from eating possible risk foods and should never eat them after the expiry date.
In 2020, 41 laboratory-confirmed cases of invasive listeriosis were reported. The 28-day lethality (= total lethality within 28 days of diagnosis) for invasive listeriosis was 30% (12 of 41 cases). No test material was sent to the national reference centre for listeriosis from one patient, therefore only 40 cases appear in the 2020 annual report.
In 2020, 3,335 food samples were tested for Listeria, of which 3,170 were tested by qualitative method (in 25 g); L. monocytogenes were detected in 58 samples:
- in two of 97 (2.1%) tested samples of raw, ready-to-eat pork products and in three of 32 (9.4%) unspecified, ready-to-eat pork products
- in two of 12 (16.7%) samples of mixed, non-ready-to-eat meat products
- in three of 140 (2.1%) samples of cooked, ready-to-eat meat products and in 12 of 119 (10.0%) fermented sausages tested
- in two out of 36 (5.6%) samples of ready-to-eat fish products tested and in four out of 79 (5.1%) smoked fish products tested
- in one out of 49 tested samples of sliced fruits
- in 13 out of 608 (2.1 %) other ready-to-eat meals tested
One sample (ready-to-eat pork product) contained more than 100 colony-forming units of L. monocytogenes per gram of food tested (CFU/g), and four samples contained L. monocytogenes between 10-100 CFU/g: one sample of ready-to-eat pork, one sample of cooked meat products, one sample of the sausages tested, and one sample of Gravad salmon.
L. monocytogenes was not detected in 612 samples of fresh, hard, and soft cheeses made from pasteurized milk and raw milk, in 115 samples of pastry products, in 22 egg products, in 20 samples of spices, in 41 samples of smoked fish, and in 70 samples of ready-to-eat salads.
In most cases, L. monocytogenes is not introduced into food via the animal but via the inanimate environment during processing. Monitoring of livestock for Listeria is therefore not considered appropriate. In the case of raw milk, contamination with faeces is considered to be the most frequent source of introduction; in isolated cases, direct introduction of the bacteria via mastitis has been documented.
Cultural pathogen detection from blood, cerebrospinal fluid, pus, punctates, or (in newborns) swabs from the umbilicus, ear, or meconium should be sought. Listeria can be detected by standardized qualitative, quantitative and molecular biological methods. PCR from CSF can be used if cultural pathogen detection is unsuccessful after antibiotic pretreatment. Serological tests are difficult to interpret because cross-reactions in healthy individuals and lack of antibody detection despite infection are common. Nearly 90% of ill people are associated with the three serovars 4b, 1/2a and 1/2b.
Our laboratory diagnostic services:
Test material: culture isolate
- Identification by biochemical methods
- Identification by sequencing
- Determination of minimal inhibitory concentration by E-test
- PCR for Listeria monocytogenes
- Clinical examination material: direct cultivation on solid and liquid selective culture media
As a rule, the routine tests mentioned are sufficient for a reliable answer to all clinical-diagnostic and epidemiological questions. PCR from CSF can be used if cultural pathogen detection is unsuccessful after antibiotic pretreatment. Serological tests are difficult to interpret because cross-reactions in healthy individuals and lack of antibody detection despite infection are frequent, especially with the Widal test. We therefore recommend serological tests only in individual cases (e.g. suspicion of rhombencephalitis) if direct pathogen detection is not possible.
Obligation to report: According to § 1 Epidemic Act 1950, Listeria must be reported as a pathogen of bacterial food poisoning or as a pathogen of invasive bacterial diseases (sepsis, meningoencephalitis). For the reporting of pregnancy-associated cases of listeriosis, the following has also applied since June 2013: any miscarriage or stillbirth due to pregnancy-associated listeriosis in the mother must be reported. The listeriosis illness of the mother is to be considered a separate, reportable case.
Germ reservoir food
Provided there is no surface contamination or subsequent contamination after opening the packaging, some foods are largely free of Listeria: In untreated foods, e.g. carrots, tomatoes and acidic fruits such as apples and pears, the risk is extremely low, especially if any surface contamination has been removed by washing or peeling.
Contamination of food with Listeria can occur at various stages of production and processing. In particular, food of animal origin such as raw milk and raw meat can be contaminated during production, e.g. during milking or slaughter. In the case of cheese made from unpasteurised raw milk, contamination of the raw milk cannot be excluded as a cause for the presence of Listeria in the final product. In the case of cheese made from heat-treated milk, the Listeria are killed during pasteurisation. However, if hygiene is poor in the processing procedure, there is a possibility of renewed contamination of the product after heat treatment. In most cases, the contamination of cheese relevant for infection transmission only occurs during ripening via colonisation of the rind. In cheeses with a soft, greasy rind, Listeria can then multiply massively in the course of ripening. They are often not distributed evenly over the entire surface, but rather in micro-colonies at specific points.
The ability of Listeria to survive and multiply in food depends on the technological treatment or the production process. Cooking, frying, sterilizing and pasteurizing kills the bacteria. In foods that contain little water, a lot of salt or preservatives, or that are very acidic (e.g. sauerkraut, mixed pickles and yoghurt), reproduction is only possible with a delay or not at all. In comparison to competing germs, Listeria have good growth possibilities in the case of reduced oxygen supply (e.g. in vacuum packaging of cooked sausages, salmon and smoked fish) and long storage times of the food under refrigeration.
National Reference Laboratory for Listeria: Analysis of isolates from food and environmental samples.
The isolates received are typed by whole-genome sequencing (Van Walle et al. 2015). To clarify epidemiological questions such as the confirmation of a foodborne listeriosis outbreak, the sequences are evaluated using core genome MLST (cgMLST) analysis (Rupptisch et al. 2015). This in-house method was developed in 2015 in collaboration with Münster University Hospital and Ridom Bioinformatics and has since been used internationally (Van Walle et al. 2018). The cgMLST scheme is integrated in the Ridom SeqSphere+ software. For typing, 1701 defined target sequences are analyzed and aligned via a nomenclature server(https://www.cgmlst.org/ncs/schema/690488).
Van Walle I, Torgny Björkman J, Cormican M, Dallman T, Mossong J, Moura M, Pietzka A, Ruppitsch W, Takkinen J, European Listeria WGS typing group. Retrospective validation of whole genome sequencing enhanced surveillance of listeriosis in Europe, 2010 to 2015. Euro Surveill. 2018;23(33):pii=1700798. https://doi.org/10.2807/1560-7917.ES.2018.23.33.1700798
Van Walle I, Pietzka A, Moller Nielsen E, Takkinen J, Damjanova I, Michelacci V, Mossong J, Eelco F, Van Pelt W, Wolkowitz T, Borges, V, Jernberg C, Fisher I, Peters T, Agren J, Rizzi V, Da Silva Felicio MT, Struelns M, Palm D. European Centre for Disease Prevention and Control. Expert Opinion on the introduction of next-generation typing methods for food- and waterborne diseases in the EU and EEA. Stockholm: ECDC; 2015. technical report - October 2015. ISBN 978-92-9193-723-3; doi 10.2900/453641; catalogue number TQ-02-15-849-EN-N.Ruppitsch W, Pietzka A, Prior K, Bletz B, Lasa Fernandez H, Allerberger F, Harmsen D, Mellmann A. Defining and evaluating a core genome MLST scheme for whole genome sequence-based typing of Listeria monocytogenes. J Clin Microbiol. 2015;53(9):2869-76. doi:10.1128/JCM.01193-15.
Last updated: 23.02.2022