Human infections are primarily caused orally via contaminated food. Raw foods are the main source in this case. There are only very few situations in which major outbreaks can be caused. Thus, only one person might get infected after eating, as the bacteria were not spread equally, so that one piece of the cheese is contaminated while another one is not. Moreover, the varying susceptibility of the individuals affected also plays a major role for the severity of the symptoms. This explains why cases of listeriosis often occur only sporadically.

Pathogenic listeria may also be transmitted after contact with infected individuals or animals, in rare cases; thus, sporadic infections were registered in vets and farmers after they came in contact with infected materials.

Similarly, listeria can also occur as a nosocomial (hospital-acquired) infection. There are several reports of infections having occurred in the delivery room or neonatal units. In these cases, it was claimed that the cause of these infections could be traced back to the hands or scrubs of staff members, scales, stethoscopes, thermometers, baby oil and other items.

We come into contact with listeria on a regular basis, as they are spread ubiquitously. In most cases, the infection will be unapparent in healthy individuals. The temporary presence of listeria can be detected in the stools of 1-5 % of all persons. When exposed on a larger scale, enteritis, which is self-limited thanks to an intact immune system, could be observed. Most adults have already gone through an immune reaction through earlier contact with listeria in food. However, this cell-mediated immunity is limited during pregnancy and even more in individuals with immune deficiencies.
The first symptoms show between three and 70 days after food-borne infection.

Pregnant women have a risk of suffering a listeriosis infection that is 15 times higher than average. There is a potential risk that the listeria are transmitted from the mother to the child during pregnancy. The listeria can already be transmitted to the foetus in utero, causing abortions, premature deliveries or stillbirths. Neonatal listeriosis may occur in the form of "early onset" listeriosis within the first week of life or as "late onset" listeriosis from the second week onwards.

Another group of people predisposed for systemic listeria infections are individuals with compromised immune systems: older individuals, patients with chronic diseases, patients with immunosuppression  (patients with transplants, AIDS or corticoid therapy). In these cases, the infection often manifests as meningoencephalitis (special form: rhombencephalitis/brainstem encephalitis).

Thirty-six cases of invasive listeriosis infections were reported in 2013.

The favoured treatment is the synergetic combination of Ampicillin + Gentamicin (only Ampicillin for pregnant women). High doses of Ampicillin should be administered in cases of meningitis and rhombencephalitis (minimum 6 g/day for adults), in particular. Immunosuppression issues should be eliminated as widely as possible, if necessary.

Cotrimoxazole (trimethoprim-sulfamethoxazole) is recommended for contraindications for one of the two drugs (not for pregnant women!). Other, not undisputed treatment options are: Vancomycin, Rifampicin, Meropenem.

Treatment duration: 2-3 weeks, in the case of rhombencephalitis up to 6 weeks.
Cephalosporins are ineffective.

Antibiotic resistance plays an insignificant role in treatment with Ampicillin, Gentamicin and Cotrimoxazole at present, thus, ensuring that a treatment decision can be made before the antibiogram is available. However, inadequate response to the therapy is still rather common and can be traced back to the intracellular behaviour of the pathogen, the immunesuppression in many patients and difficult, often late, diagnosis. About 20 % of invasive listeria infections are terminal, despite targeted treatment.

Listeria monocytogenes was found in 25 g of the following food samples examined in 2013: in four of 1,058 samples of milk, dairy products and cheese; in seven of 222 samples (3.2 %) of fish and fish products. A total of 21 of 855 samples taken from meats of different animal species (both raw and cooked, ready-to-eat and not ready-to-eat) contained Listeria monocytogenes, although the share of listeria-positive samples in the various meat samples (beef, pork, mixed meats and meat products) was distributed equally, with the exception of pâté products (3 out of 24 samples). L. monocytogenes was only detected in 1 % of other food samples (7 out of 497) taken from baked goods and pastries, ready-to-eat products, vegetables, salads and sauces.

More than 100 colony forming units of L. monocytogenes per gram (cfu/g) were found in one sample of goat cheese, one sample each of ready-to-eat food and meat contained 10-100 cfu/g of L. monocytogenes. L. monocytogenes was found in quantities of fewer than 10 cfu/g in all other food samples that tested positive for listeria (detectable in 25 g).

Food as a Reservoir for Pathogens

Some foods are mainly listeria-free as long as there has been no prior surface contamination or contamination after the opening of the package. Untreated food, such as carrots, tomatoes and acidic fruit (e.g. apples, pears) have very low contamination risks, in particular after washing or peeling when a possible surface contamination could be removed.

Food can be contaminated with listeria during a number of production and processing stages. Animal based foods, such as unpasteurised milk and raw meat, may already be contaminated during production -- e.g. milking or slaughtering. In the case of cheese made from unpasteurised milk, the contamination of the original milk cannot be excluded as the cause of listeria in the finished product. However, there are additional sources of contamination after the heat treatment of products, if hygiene standards during the manufacturing process are neglected. In most cases, the type of contamination relevant to infections occurs during the maturing process when listeria can settle on the rind. The listeria can proliferate considerably in cheeses with soft, spreadable rinds during maturing. Often, they do not spread evenly over the entire surface, but in patches of microcolonies.

The ability of listeria to survive and multiply in food depends on technological treatments and manufacturing process. Boiling, roasting, sterilising and pasteurising kill off the bacteria. Foods containing little water, lots of salt or preservatives, or foods that are very acidic (e.g. sauerkraut, mixed pickles and yoghurt) delay or even completely inhibit  bacterial proliferation. Unlike other, competing bacteria, listeria has the ability to grow in anaerobic environments (e.g. in vacuum packages of sausages, salmon and smoked fish) and in foods that are refrigerated for longer periods.