What is it?
Listeriosis is an infectious disease caused by bacteria. It is an infection caused by the bacterium: Listeria monocytogenes. This disease can be serious. Indeed, the infected individual may suffer from septicaemia (a generalised infection of the body through the passage of the bacteria through the blood vessels) or an infection of the brain (1).

An infection by Listeria monocytogenes in a pregnant woman is even more serious in the sense that it can lead to abortion, neonatal infection or premature delivery.

The prevalence in France (number of cases of the disease in a given population at a given time) is low: 5/ 1 000 000. However, there is a 20-30% risk of death for the infected person. The risk of infection should therefore not be neglected. (1)
Listeria monocytogenes infections are generally sporadic cases, which means that the disease only affects certain people at random and without any contagion.

In addition to the fact that listeriosis can be even more serious in pregnant women, certain populations of people may also have a privileged terrain for the development of the bacteria. These are: (2)
- the elderly ;
- people with underlying diseases (cancer, diabetes, immunodeficiency, autoimmune disease, etc.);
- people who have undergone organ transplants.

The most common symptoms associated with the disease are: digestive disorders, diarrhoea, etc. (2)

The bacteria involved: Listeria monocytogenes is a ubiquitous bacterium (present everywhere in the environment and very resistant). The bacterium is said to be "psychrophilic", i.e. it is capable of growing at very low temperatures (refrigerators) but is destroyed during cooking. In this sense, this bacterium can be present in silage and thus contaminate ruminants. In addition, the bacterium is mainly present in the environment through the faeces of healthy and sick animals.
Animals contaminated with L. monocytogenes are therefore the predominant source of contamination for humans by this bacterium.

The resistance of the bacterium in the environment allows it to contaminate a number of foods. Thus, exposure to L. monocytogenes is frequent but does not necessarily cause serious symptoms (generally digestive disorders and diarrhoea). (2)

The symptoms
The general symptoms associated with listeriosis are primarily intestinal and gastric: digestive disorders, diarrhoea, abdominal pain, etc. Fever and body aches may also occur as a result of these primary symptoms. (3)

In the event that the diagnosis of L. monocytoges infection is made, the form of the disease is called invasive. This form can be manifested by : (3)
- neurological damage: headaches, nausea, fever, vomiting, behavioural disorders that can lead to the development of meningitis or meningoencephalitis;
- Bacteremia: the passage of the bacteria into the bloodstream causing fever or chills;
- spontaneous abortions in the first half of pregnancy, premature delivery in the case of infection between the 6th and 9th week of pregnancy, and even foetal death.
- Febrile gastroenteritis or bacteremia in patients with prostheses may also be associated with the disease but remain rare.

There is no specificity of symptoms of the disease but symptoms vary from patient to patient.
In a healthy individual, an infection with L. monocytogenes may cause no symptoms or only a transient febrile gastroenteritis.
It is therefore in "fragile" people (elderly, immunocompromised, pregnant women, etc.) that the disease can be severe. It is expressed by the development of meningitis, bacteremia, isolated fevers, spontaneous abortions, etc. (3)

It is remarkable that in pregnant women, the infection is generally of no consequence to the mother or is only expressed through febrile periods. On the other hand, the newborn may be affected through skin manifestations, respiratory abnormalities or neurological signs. (1)

The origins of the disease
The main source of L. monocytogenes contamination is food. Transmission of the bacterium occurs mainly through the ingestion of contaminated food.
Transmission by direct contact with the animal or nosocomial transmission (infection during a stay in hospital) is possible but remains rare (3).

As the placenta is not a barrier for the foetus, transmission from the mother to the child is essentially via this route or via the genital tract during childbirth.

Food contamination by this bacterium can occur at all stages of the food chain: breeding, production, conservation, preparation, storage, etc. Moreover, it concerns both small-scale and industrial production chains.

Contamination by L. monocytogenes is increased by the ingestion of raw or poorly cooked foods, mainly meat, smoked fish, vegetables, raw milk or raw milk cheeses (3).

The psychrophilic nature of this bacterium gives it the power to grow at 4°C (refrigerator temperature). Extending the cold chain can therefore favour the development of the bacteria and be an additional risk factor for contamination. (2)

Risk factors

The main risk factors for L. monocytogenes contamination are

- Extending the cold chain;

- consumption of raw or undercooked food

- poor hygiene at different stages of the food chain (production, storage, preparation, etc.).

Prevention and treatment

As Listeria monocytogenes is a ubiquitous bacterium, its presence in the environment and its increased resistance to it leads to a high rate of exposure for individuals.

Many people exposed to the bacteria have no symptoms or even symptoms similar to gastroenteritis. In this case, diagnosis is difficult.

However, if the symptoms are persistent, the isolation of the bacterium allows the diagnosis to be made or not. This bacterial detection is based on a blood culture (culture of blood cells), from cerebrospinal fluid following a lumbar puncture, following a vaginal swab, from placenta or from gastric fluid (3).

The diagnosis of the presence of the bacteria in the body is often followed by hospitalisation.

The general treatment of listeriosis is mainly based on antibiotic therapy. (3)

The vital prognosis for this pathology is 20 to 30% for fragile subjects and is often better in young people with no previous history (2).

Prevention is a very important concept in this context in order to avoid contamination by L.monocytogenes.

This prevention is achieved through good agricultural, industrial and domestic hygiene practices, such as: (2)

- storing food in a refrigerator set at +4°C;

- cleaning food properly

- respecting the rules of domestic hygiene (cleaning utensils and work surfaces, etc.);

- Wash hands thoroughly after handling raw food;

- Keep leftovers in the refrigerator for less than 3 days;

- Respecting use-by dates;

- etc.

People at risk are advised to avoid eating jellied meat products (rillettes, pâté, foie gras, etc.), raw milk products, smoked fish, raw shellfish, etc. (2)

Rédaction : Delphine Waquier
Rédactrice scientifique
Juillet 2016
Source: https://www.passeportsante.net/fr/Maux/Problemes/Fiche.aspx?doc=listeriose
(1). INSTITUT PASTEUR. 2013. Listériose. [En ligne]. http://www.pasteur.fr/fr/institut-pasteur/presse/fiches-info/listeriose. (Consulté le 08.06.2016).
(2). ANSES. 2013. Listériose. [En ligne]. https://www.anses.fr/fr/content/list%C3%A9riose. (Consulté le 08.06.2016).
(3). INVS. 2015. Listériose. [En ligne]. http://www.invs.sante.fr/Dossiers-thematiques/Maladies-infectieuses/Maladies-a-declaration-obligatoire/Listeriose/Aide-memoire. (Consulté le 08.06.2016).