On April 5, 2022, a national recall campaign was launched for Graindorge brand cheeses (Lactalis group) in response to the discovery of Listeria during a routine check in a retail store. This reminder had a national audience; it is also very common for RappelConso to report products being withdrawn from sale due to the presence of these bacterial strains. Indeed, in France they are the second cause of death from foodborne infections: they are therefore under special surveillance. What do we know about this human pathogen?
From a naturally occurring bacterium to a public health problem
Listeria monocytogenes are bacteria that are widely distributed in nature: they have been isolated from soil, water, plants and some animals, in particular insects, birds, wild or domestic mammals. In the natural environment, they are often harmless and feed on decaying organic matter (saprophytic bacteria).
These bacteria, first discovered in laboratory rabbits in 1924 were not included in the list of probable sources of human foodborne infections until 1979, when 23 patients became ill in a hospital in Boston (USA). It was in 1981 during an epidemic in Canada due to the consumption of contaminated cabbage that the final evidence of transmission of the infection through food was established.
Since then, there have been several other epidemics of listeriosis, such as the one that occurred in 1998-1999 in the United States, when contaminated hot dogs killed 21 people. In France, the most serious infection occurred in 1992, with 63 deaths and 22 women having abortions after eating pork tongue in jelly. To date, the most serious reported outbreak occurred in South Africa in 2018, when more than 1,060 patients were infected and 204 people died, 42% of whom were newborns.
Researchers believe that several simultaneous factors contributed to these epidemics, including the industrialization of food production, the ubiquity of refrigeration equipment, and an increase in the population at risk. Thus, listeriosis became the subject of observation; although rare, it is serious and since 1998 is subject to mandatory reporting.
Listeria monocytogenes is able to quickly respond to changing environmental conditions. It can, by colonizing the intestines, turn from a saprophytic bacterium into a human pathogen. We have been able to show that in this case, regions of its genome (all the genes of a living being) will express themselves, while others will fall silent. The appearance of the disease also depends on various factors: the immune status and acidity of the stomach (stomach) of the infected person, as well as the virulence of bacteria (the ability to develop the disease). High concentrations of bacteria also contribute to the emergence of the disease. In most cases, the human immune system is able to suppress the infection, and many people are asymptotic carriers of the bacteria. However, sometimes these bacterial strains cross the intestinal barrier and multiply in organs such as the kidneys, then diffuse into the bloodstream and reach the placenta or brain, where they destroy cells.
In 99% of cases, infection occurs through the use of contaminated food.
Because these bacterial strains are widespread in the environment, they easily contaminate meat, seafood, cold cuts, dairy products, and may be present on vegetables or fruits that have come into contact with infected feces.
Another problem: these bacteria are able to withstand and grow at low temperatures, as well as in saline (brines) or acidic (fermented products) environments. As a result, it is possible that they contaminate all stages of the food chain by colonizing processed food production sites and continue to evolve during food preparation, distribution and storage. Obviously, after contamination of food, bacteria continue to multiply in a family refrigerator that is not cold enough (above 4°C).
Thus, food products that have a long shelf life and have not been heat treated or reheated are most likely to become contaminated. In France, for example, epidemic-causing foods were sausages (pork’s tongue in jelly, riettes, mortadella, etc.), cheeses (brie, pont-levecq, epoisses, etc.) and ready meals. But elsewhere in the world, other epidemics have broken out with smoked fish, vegetables (melons, sprouted seeds, ready-to-eat salads), ice cream. More surprisingly, epidemics have been reported after eating frozen foods (corn and vegetables).
There is another way of infection: transmission from a pregnant mother to a fetus. after passage of the bacterium through the placenta or from mother to newborn by passage at the level of the genital organs during childbirth. In very rare cases, listeriosis can be transmitted through contact with an infected animal (cases seen in veterinarians or farmers) or in a hospital (nosocomial transmission).
Cheese aging (Livarot) at the Graindorge dairy (Lactalis group) © JIMMY BEUNARDEAU / HANS LUCAS / AFP
What are the symptoms of listeriosis?
While Listeria monocytogenes can contaminate many unsterilized foods, serious infections are rare because ingestion of small amounts of these bacteria often goes unnoticed; in large quantities, they sometimes cause gastroenteritis (nausea, vomiting, diarrhea, fever). These signs appear between 6 hours and 4 days after infection.
Severe forms develop in weakened people with weakened immune defenses (cancer patients, transplant recipients, hemodialysis patients, HIV-infected people), in people with kidney or liver failure, in people with inflammatory diseases. The risk group also includes the elderly or people with concomitant diseases (diabetes, cirrhosis of the liver).
In all these cases, the onset of symptoms is highly variable, ranging from 24 hours to 14 days. The disease then manifests itself with a more or less high fever, accompanied by headaches and, sometimes, digestive disorders (nausea, diarrhea, vomiting). Then there are complications with infections of the bloodstream, blood (bacteremia, sepsis) or various infections of the brain (meningitis, abscess).
In the case when the infection affects a pregnant woman, the incubation period is very long (from 17 to 67 days), although in half of the women symptoms appear around the 28th day after infection. The infection may go unnoticed or lead to contractions or flu-like symptoms (fever, chills, back pain). However, the risks and consequences can be serious for the unborn child: spontaneous abortion, fetal death, premature birth or neonatal infection.
What are the treatments?
Treatment for listeriosis infection is based on antibiotics. The sooner they are introduced, the greater the chance of survival. There is no vaccine against listeriosis.
What are the risks of contracting listeriosis in France? What is the mortality rate?
In France, for ten years, the number of cases has been less than 400 per year. Since 2015, less than 10% concern pregnant women. However, the study (MONALISA) showed that the life prognosis in this disease is still poor, with mortality after 3 months in 46% of cases of bloodstream infection and in 30% of cases of infection of the central nervous system. (brain).
Number of listeriosis cases in France between 2009 and 2019, classified by type of infection and number of deaths Foodborne Biohazard Description Sheet: Listeria monocytogenes – Anses – April 2020.
What prophylaxis can be put?
The first prevention is the strict application of hygiene and control measures at all stages of the food chain, from production to food preparation, both in industrial premises and in the home.
For individuals, the Pasteur Institute website provides a series of tips on how to protect yourself from infection and lists all foods to avoid when you are at risk. In case of doubt, it is always preferable not to consume the product, and it must be remembered that these bacteria are sensitive to heat: cooking at 60° for 30 minutes destroys them.
Whereas during the 1992 epidemic the public authorities refused to indicate the brand of the product at the point of origin of the infection, today transparency has become essential: the French authorities have created RappelConso website a list of batches of contaminated foodstuffs, and whenever clustered contaminations appear, warnings are published through the press stating the name, brand and origin of the contaminated foodstuffs. From that point on, immediate withdrawal measures are usually taken.
National Monitoring was established in 1982 with the creation of the CNR (National Reference Centre); this measure was reinforced by a declaration commitment in 1998. In case of bulk contamination, the national public health agency (SPF) conducts an investigation in cooperation with the CNR.
However, repeated seizures of contaminated products from food companies show that there are still holes in the network. According to NGOs such as Foodwatch, better control over these production chains is needed..