Listeriosis is a serious bacterial infection caused by Listeria monocytogenes. This pathology, often linked to the consumption of contaminated food, can have severe consequences, particularly in vulnerable people such as pregnant women, newborn babies, the elderly and immunocompromised individuals.
What bacteria are responsible?
Listeria monocytogenes and Listeria ivanovii are bacteria present in the environment. Listeriosis is a bacterial disease affecting several animal species, caused mainly by L. monocytogenes and transmitted mainly through food. The disease can manifest itself in septicaemic, nervous or genital forms in different species.
Listeria monocytogenes, a Gram-positive bacillus, was named after Joseph Lister. It is the only human pathogen in the Listeria genus. This bacterium is small, non-spore-forming, facultative aero-anaerobic, ubiquitous and mobile at 20°C. Approximately 1-10% of humans are healthy carriers in their intestine.
Discovered in 1926, L. monocytogenes has been linked to food contamination since 1953. It survives for several months in external environments such as soil and plant debris, but is destroyed by pasteurisation and pH levels below 4.
L. monocytogenes is resistant to most common disinfectants. However, their mobility and viability are reduced at low temperatures. This bacterium is found in at least 37 species of mammals and 17 species of birds. It can develop at temperatures close to 0°C, which poses a problem for refrigerated foods.
The bacterium can survive in frozen food and start growing again when it is defrosted. It resists lysosomal degradation and uses the actin cytoskeleton to propagate between cells. Two surface proteins, InIA and InIB, enable L. monocytogenes to cross biological barriers such as the placenta and the blood-brain barrier.
Listeria ivanovii is pathogenic to animals and rarely to humans. Reservoirs of L. monocytogenes include poorly preserved silage and the digestive tract of sick animals or asymptomatic carriers. It is resistant to salty environments and thrives at refrigeration temperatures, but is destroyed by appropriate cooking.
What does infection in animals look like?
Listeria mainly infects domestic ruminants, but also other mammals, birds and fish, often as asymptomatic carriers. Listeriosis is a bacterial infection with a worldwide distribution, observed in France in small ruminants. It is transmitted by ingestion of contaminated plants andsilage. Listeria can survive for several months in theenvironment, contaminating the external environment for a long time.
Symptoms include abortions, nervous disorders, imbalance, gait disorders and generalised infections. Local forms may include diarrhoea, mastitis, conjunctivitis and respiratory problems. Many mammals such as ruminants, rodents, lagomorphs, equines, suids and carnivores are susceptible to listeriosis. Birds such as chickens, turkeys, ducks, canaries and geese can also be infected.
The reservoir of Listeria includes susceptible animals,humans, animal products (milk, eggs, meat) and theenvironment (soil, water, plants). The faeces of carriers often contaminate birds and other animals indirectly through the contaminated environment. In birds, intercurrent conditions such as salmonellosis and coccidiosis are often associated with the disease, which affects young birds more severely.Symptoms include anorexia, cyanosis of the mucous membranes, diarrhoea and sometimes nervous disorders . Mortality can be as high as 40%.
In ruminants, listeriosis takes three forms: septicaemic (rapid death of young animals), genital (abortions and metritis) and nervous (ocular and general signs). Nerve lesions are located in the brain stem.
In rodents and lagomorphs, the acute meningoencephalitis form affects the young, causing rapid death. In other animals(equidae, suidae, canidae, felidae), listeriosis is rare. It manifests itself in septicaemic, meningoencephalitic or abortifacient forms. Clinical signs are rare in birds, rabbits, hares, pigs, dogs and cats.
How is the disease transmitted?
Listeriosis is mainly transmitted by ingestion ofcontaminated food (cheese, dairy products, cold meats, etc.). In exceptional cases, it is transmitted by skin contact with abortion or farrowing products. In France, there are between 250 and 300 cases a year. Listeriosis poses no particular risk to people in good health, except for pregnant women and people with weakened immune systems.
Professional activities at risk include vets, breeders and laboratory staff who come into contact with contaminated abortion products. Compliance with good professional practice is essential to prevent transmission.
Sources of Listeria include the external environment (soil, water, plants), animals (arthropods carrying Listeria) andfood (animal products such as meat, milk, eggs and cheese). Humans can be healthy or chronic carriers.
The modes of transmission are varied. Direct transmission occurs mainly via the blood-borne route (congenital listeriosis) and can occur from mother to foetus via the digestive or respiratory tracts. Transmission by contact, although rare, is possible.
Indirect transmission occurs via inanimate vectors such as animal products or contaminated surfaces. Contamination via the digestive tract is the cause of sporadic or epidemic cases. Respiratory transmission is rare, but possible in confined environments such as sheep pens.
Listeria monocytogenes can survive for long periods in the environment and in food. Preventive measures include rigorous hygiene, particularly in the agri-food industry, with the application of the HACCP protocol.
During pregnancy, transmission to the foetus can occur via the placenta or by contact with the genital tract during delivery. Strict hygiene is essential to prevent listeriosis.
How does listeriosis affect humans?
Listeriosis is often asymptomatic. Sometimes it causes fever and fatigue. Very rarely, it causes skin lesions. In pregnant women, there is a risk that the foetus may be affected, leading to abortion, premature birth or serious infection of the newborn. For immunocompromised people, listeriosis can be serious, affecting the nervous system.
In pregnant women, the disease develops insidiously, with a febrile flu-like syndrome. The consequences for the foetus are serious if the disease is contracted early in pregnancy: abortion, premature delivery, neonatal septicaemia or meningitis.
In adults and children, the infection manifests itself as fever, headache and widespread pain. A meningoencephalitis form, associated with nervous disorders, often affects people over 50 and immunocompromised individuals. Mortality is lower, mainly due to intercurrent pathologies such as AIDS. Cases of endocarditis, arthritis and peritonitis are also possible. Listeria monocytogenes is an opportunistic bacterium, preferentially affecting immunocompromised subjects: the elderly, pregnant women, newborns and immunocompromised patients.
In the elderly or immunocompromised, listeriosis can lead to vomiting, diarrhoea, constipation, headaches, fever, meningitis or septicaemia. Hospitalisation is necessary in over 90% of cases, with a mortality rate of 25-30%.
In the case of invasive listeriosis, symptoms vary depending on the area affected: headaches and stiff neck in the case of meningitis, mental confusion and loss of balance. The fatality rate is 20-30%. Thehospitalisation rate is over 97%. If the disease is diagnosed in time, antibiotic treatment is possible, adapted to the specific needs of each patient.
How is the disease diagnosed?
Theincubation period for listeriosis can last from 2 to 70 days after ingestion of the contaminated food, complicating the retrospective search for the foods involved in a clinical episode.
Isolated or grouped infections can form a listeriosis epidemic. In France, the authorities have set up a surveillance system to identify the foods responsible. This system has shown that industrial cold meats (rillettes and jellied pork tongue) and certain cheeses are responsible for recent epidemics.
On the animal
The clinical diagnosis of listeriosis is based on several factors:
- Epidemiological elements: It is a sporadic to enzootic disease in ruminants, which can become epizootic in poultry and rodents. In ruminants, it is often associated with the consumption ofpoorly preserved silage.
- Non-pathognomonic clinical signs: In the event of repeated abortions in ruminants, and after ruling out brucella and chlamydial infections, listeriosis should be considered.
- Necropsy: This reveals foci of necrosis in septicaemic and abortifacient forms, and microabscesses in nervous forms.
Several samples need to be taken. From alive animal:
- Blood sampling
- Collection of placenta and abortus
- Faeces (septicaemic form)
- Puncture of cerebrospinal fluid (nervous form)
Ondead animals:
- Sampling of the brain and spinal cord (nervous form)
- Collection of blood, spleen, liver and heart (septicaemic form)
Samples of soil, manure andsilage are taken from the outside environment. Bacteriological diagnosis is made by direct examination if the sample is rich or by culture and enrichment of the culture medium. After 1 to 2 days of culture, Listeria can be identified and serotyped. Serological diagnosis is not satisfactory. Experimental inoculations can be carried out conjunctivally in rabbits, intraperitoneally in mice, or on embryonated chicken eggs.
In humans
Listeria monocytogenes infection is identified in blood, cerebrospinal fluid or other samples by bacteriology laboratories. The laboratory provides useful indications for therapy. Serology has limited use.
Precise molecular typing methods are used, targeting proteins andDNA. Protein typing is carried out using isoenzyme electrophoresis (Multilocus Enzyme Analysis), which measures variations in the electrophoretic mobility of enzymes. This method identifies two groups of L. monocytogenes: 1/2 b and 4 b, as well as 1/2a and 1/2c.
DNA typing uses conventional electrophoresis after the action of restriction enzymes (EcoRI). Professionals diagnose listerial infections by culturing blood or cerebrospinal fluid. If L. monocytogenes is suspected, it is essential to inform the laboratory, as this microorganism can be confused with diphtheroid germs.
In all cases of listeriosis, the peak inIgG agglutinins occurs 2 to 4 weeks after the onset of infection. Doctors take a blood sample or perform a lumbar puncture to obtain cerebrospinal fluid. The samples are cultured to confirm the presence of the bacteria.
Diagnosis is based on the detection of the bacteria in a sample of blood (blood culture), cerebrospinal fluid or another sterile site. The placenta may show macroscopic foci of infection, and its culture can be used to isolate the bacteria. For infants, meconium analysis can also isolate L. monocytogenes.
Ifcontaminated food is suspected, bacteriological analyses must be carried out to detect the contamination. Diagnosis is based on isolation of the bacteria from normally sterile sites, such as blood, cerebrospinal fluid, vaginal swabs, placenta or gastric fluid in newborns, or even joint or pleural fluid. Serological tests are not reliable and should not be used.
How is the disease treated?
In poultry farming, farmers generally use cyclin antibiotics. In the case of more severe forms, vets prescribe beta-lactam-aminoside combinations in higher doses than usual, mainly because of the presence of the intracellular germ.
In sheep, standard treatments include tetracycline,ampicillin and gentamicin, combined with a corticosteroid and vitamin B to help protect nerve cells. Recovery is possible but may be incomplete due to damage to the nervous system, which can lead to complications such as permanent blindness or myelitis.
Antibiotic therapy is not routinely given to patients with few symptoms who are not immunocompromised. Doctors prefer amoxicillin or ampicillin in pregnant women, sometimes in combination with an aminoglycoside or co-trimoxazole. In non-pregnant patients at risk of neuro-meningeal damage, doctors prefer dual therapy. There is no effective vaccination against listeriosis.
Listeria susceptibility to antibiotics has changed little over the last few decades.Ampicillin andamoxicillin remain effective, especially when combined with aminoglycosides, with a strong bactericidal action. Tetracyclines are also active, and quinolones have demonstrated their efficacy, as has the combination of sulphonamide and trimethoprim.
The treatment of listeriosis is mainly based on the use of antibiotics, in particular penicillin, streptomycin and sulphonamides. However, results are variable and depend largely on the state of the patient’s immune system. Early treatment is essential for optimal efficacy.
To treat listerial meningitis, doctors prefer to administer ampicillin intravenously, often in combination with gentamicin for greater synergy. Cephalosporins are not recommended in this case. For endocarditis and primary listerial bacteremia, treatment combinesampicillin and gentamicin for a prolonged period after resolution of symptoms. Oculoglandular and dermatological infections respond well toerythromycin or the trimethoprim/sulfamethoxazole combination, administered as appropriate.
How can transmission be prevented?
Animal health is not generally affected by listeriosis. There is currently no occupational disease table for listeriosis. Listeria monocytogenes and L. ivanovii are classified as hazard group 2 under the French Labour Code.
As far as animals are concerned, prophylaxis is mainly of a sanitary nature, with vaccination being of little interest and tetracycline-based chemotherapy being preferred during enzootic outbreaks.
It is important to :
- detect and isolate sick animals
- treat or eliminate affected animals,
- destroy cadavers, runts and placentas,
- clean and disinfect premises
- bacteriological and chemical control of silage,
- and to limit stress factors and other intercurrent conditions that could favour the appearance and development of listeriosis.
For humans, rigorous hygiene is recommended after handling raw meat, with pasteurisation and cooking being effective ways of destroying the bacteria. It is advisable to wash food thoroughly and remove the rind from cheeses (particularly those made from raw milk).
However, research shows that the rind of raw-milk cheese can harbour a diverse native microbiota that regulates the proliferation of Listeria monocytogenes, thus offering food safety. Pregnant women should avoid handling animals at risk and follow specific dietary recommendations to prevent listeriosis.
Mandatory reporting of the disease is in force in France, Belgium and Canada. Listeriosis can also be a form of collective food poisoning, occurring after the consumption of contaminated products. The people most at risk are the elderly, pregnant women, newborn babies, people with weakened immune systems and those suffering from certain illnesses.
Some epidemiological data…
Listeriosis is a disease that is widespread throughout the world, but appears to be more common in industrialised countries, perhaps because of better detection or differences in eating habits. The most numerous cases are reported in Europe, the United States and Canada.
In Europe, there were just over 1,500 cases in 2007, with a mortality rate of around 20%. In 2011, 1,470 cases were reported, with a mortality rate of 12.7%. The toll subsequently rose to 1,763 cases in 2013, resulting in 64 deaths in France. Listeriosis is less common than other food-borne infections such as campylobacteriosis, salmonellosis and yersiniosis. Every year, the authorities report between 400 and 500 cases of listeriosis in mainland France. Although rare and small today, outbreaks have been linked to dairy products, meat products, processed fish and fresh and frozen vegetables.
The people most at risk are the elderly, the immunocompromised, pregnant women and infants. In recent years, there has been an increase in cases of septicaemia, while the number of maternal-neonatal cases has remained stable. Following epidemics in the 1990s, the agri-food industry introduced strict surveillance, which led to a drop in the incidence of the disease. However, there has been a resurgence since 2006, although incidence has stabilised since 2008. In France, there are between 350 and 400 cases each year, making it the second leading cause of food-related death.
Unlike other food-borne infections, the majority of listeriosis cases are isolated and often cannot be attributed to a common food source. The disease generally develops as sporadic cases, clusters of cases or small epidemics, facilitated by the wide distribution of food products.
In France, around 300 to 400 cases of listeriosis are diagnosed each year, corresponding to an annual incidence of 5 to 6 cases per million inhabitants, according to Santé Publique France.
Action by the health authorities
Listeriosis surveillance is a complex process involving several organisations and government agencies. In France, the Direction Générale de l’Alimentation (DGA ) and the Direction Générale de la Concurrence, de la Consommation et de la Répression des Fraudes (DGCCRF ) are responsible for monitoring food production, processing and distribution. Every year, the DGA takes over 60,000 samples. The DGCCRF ensures that professionals comply with French and European standards. To do this, they implement self-checking procedures.
If contamination in excess of established thresholds or even the mere presence of the bacteria is detected, the food products concerned are withdrawn from the production chain or from sale. Clinical and microbiological data are then transmitted to theInstitut de veille sanitaire (InVS), which oversees the clinical and epidemiological aspects of the disease.
Listeria strains are sent to the National Reference Centre (CNR ) for Listeria, located at theInstitut Pasteur. This centre carries out molecular typing to detect any cluster cases or a common food source. If grouped cases are detected, an interministerial listeria unit may be activated to take additional measures, such as product recalls or additional checks.
At European level, theEuropean Food Safety Agency (EFSA) and theEuropean Centre for Disease Prevention and Control (ECDC ) coordinate surveillance activities.
TheAgence nationale de sécurité sanitaire de l’alimentation, de l’environnement et du travail (Anses) also plays a crucial role. As a national reference laboratory, it defines standardised analysis methods for all official control laboratories. It also contributes to monitoring the bacterium. It also carries out risk assessments and makes recommendations to professionals and public authorities.
