The infectious agent responsible for listeriosis is the bacterium Listeria monocytogenes. Due to its ubiquitous nature – it is present in water, soil, plants and the intestine of many animals – and its physicochemical properties, this bacterium has the capacity to colonize food-processing environments. It causes foodborne infections, especially from contaminated dairy or meat products, and also outbreaks, in cases where there has been wide distribution of contaminated food.
Symptoms and treatment
The MONALISA prospective national study conducted at the National Center for Listeria, in cooperation with Santé publique France, enabled the clinical signs and prognostic factors of infection to be more clearly delineated.
- In adults the disease causes an infection of the blood (septicemia) or even of the central nervous system, leading to meningo-encephalitis (an infection of both the envelopes of the brain, called meninges, and the brain itself).
In pregnant women the infection is benign for the mother. It can even go unnoticed, or may manifest as contractions, or as fever. However, it can lead to fetal death, and the premature delivery of an infected newborn. Neonatal infection is severe, because of the prematurity, and the infection itself, which can manifest as septicaemia, infections of the lungs, nervous system, and sometimes the skin. Antibiotic treatment is available, and is more effective when administered early. However, even with targeted early treatment the outcome may be fatal.
The incubation period of listeriosis lasts from a few days to 6 weeks; it lasts longer in its maternal form (1 month) than in the septicemic or neurologic form (a few days).
The bacterium Listeria monocytogenes was first described in the 1920s, but has been considered a serious public health problem only since 1981, when an outbreak in Canada brought to light the foodborne origin of this infection in humans. It has been a notifiable disease in France since 1998.
Listeriosis occurs as sporadic cases, clusters of cases and sometimes outbreaks. About 350 cases are reported in mainland France each year. Until 1992 there were about 750 cases, including 279 epidemic cases linked to the consumption of jellied pork tongue. No massive epidemic has been reported and the number of sporadic cases have considerably fallen in the last 15 years, particularly for the neonatal forms of the disease, a resurgence in cases which consist mainly in septicaemia in the elderly has been observed in the last few years. In 2018, the most serious epidemic reported to date occurred in South Africa, with more than 1,000 infected patients, of which more than 40% were newborns. The source of contamination identified was a type of industrial sausage often consumed in this country.
The most common contamination mode in humans is the ingestion of contaminated food . Although killed by heat, the bacterium is still able to multiply at 4°C (refrigerator temperature). Longer cold chains (industrial cold storage plants and domestic refrigerators) therefore favor contamination of food products by Listeria monocytogenes. Unlike most other foodborne pathogens, Listeria monocytogenes does not alter the taste of food, unlike most food pathogens, which explains why this bacterium may be repeatedly ingested or ingested in large quantities without being noticed.
In France, the foods most often contaminated by Listeria monocytogenes are dairy products (especially soft and raw milk cheeses), delicatessen meats (tongue, brawn, rillettes), processed fish products, chilled sprouted seeds and non-annealed catering preparations.
For at-risk groups (pregnant women, the elderly and immunodeficient individuals, either as a result of immunosuppressive treatments or a particular pathology such as cancer, cirrhosis, diabetes), prevention consists of avoiding dairy products, delicatessen meat products, rillettes, pâté, foie gras, soft cheeses, smoked fish, raw shellfish, crab sticks, tarama, and raw seed sprouts. The recommendation is to cook all animal products thoroughly, remove every cheese rinds, carefully wash vegetables and herbs and reheat all pre-prepared food to boiling temperature.
Raw food should be stored separately from cooked or ready-to-eat food, to avoid cross-contamination (contamination of one food by another). Packaged food is preferable to loose products or those sliced to order, which should always be consumed promptly after purchase. The usual food hygiene rules, which concern not only Listeria monocytogenes, should be scrupulously observed:
- Leftovers and pre-prepared food should be heated thoroughly and before being consumed;
- Refrigerators should be cleaned regularly, and disinfected with a bleach solution;
- Refrigerator temperatures should be monitored to ensure that they are low enough (4°C);
- Use-by dates should be respected;
- Hands should be washed and kitchen utensils cleaned following the preparation of raw food.