A foodborne infection, listeriosis is a potentially serious disease during pregnancy and in the most fragile people.
According to figures from Santé Publique, France, there are, on average, between 300 and 400 cases of listeriosis each year in France. It is the second most common cause of food-borne death in France, after salmonella infection. Listeriosis has been a notifiable disease since 1998, which means that when a case is detected, it must be immediately reported to the health authorities. Although cases of listeriosis are still rare, the disease can be serious in frail people, who account for 80% of cases of infection with the bacteria. Pregnant women and the elderly or immunocompromised are most at risk of a severe form.
1. What is listeriosis? Definition
Listeriosis is a foodborne infection caused by the bacterium Listeria monocytogenes. It is a bacterial zoonosis since the bacteria affects both humans and animals. In 99% of cases, the bacterium is transmitted by food, although, as we shall see, other modes of contamination are possible, although they are much rarer. The bacterium responsible for listeriosis is present in water, plants, and the intestines of many animals and can also be found on surfaces (floors, kitchen worktops, refrigerators, freezers, etc.). This explains why it can colonise food manufacturing sites, but also our kitchens.
Listeria monocytogenes has a characteristic that distinguishes it from many other bacteria, it is psychotropic. This means that it is able to survive and grow at very low temperatures, down to minus 5 degrees.
2. The incubation period of listeriosis
The incubation period of listeriosis varies greatly from person to person, ranging from 2 days to several weeks. But on average it lasts from 10 to 28 days.
3. Listeriosis: who is most at risk?
We are not all equal when it comes to Listeriosis and although in some people the disease can go unnoticed, it can have deleterious consequences in certain vulnerable individuals who will then develop a serious form.
According to the Institut Pasteur, the people most at risk are:
- Pregnant women
- Newborns
- People over 80 years old
- People on dialysis, insulin-dependent diabetics
- People who are immunocompromised due to a treatment or a disease (HIV, etc.)
- People with cirrhosis
- Transplant patients
- People suffering from cancer
4. How is listeriosis transmitted? The different modes of contamination
In the vast majority of cases, listeriosis is transmitted through food, i.e. The ingestion of food contaminated with Listeria monocytogenes. Listeria monocytogenes do not give any particular taste to food, so it is impossible to know that you have just eaten contaminated food. The bacteria are naturally present in the digestive tract of many animals (cattle, chickens, sheep, etc.) and the environment can then be contaminated by their faeces. If present in a food factory, the bacteria can contaminate food when it is handled and then persist if cleaning is not done properly or if disinfection is inadequate.
Listeria monocytogenes are very rarely found in cooked products because it is very sensitive to heat (30 minutes at 60°C will destroy it) unless the food is contaminated after cooking. It is mainly found in dairy products, vegetables, seafood and certain delicatessen products.
In addition to the transmission through food, the other mode of transmission observed is that of contamination from the pregnant woman to the foetus during pregnancy, but this is extremely rare, or from the newborn to the baby during delivery. The Pasteur Institute adds that direct transmission has also "been observed in veterinarians and farmers after the delivery of an infected animal or during abortions linked to animal listeriosis".
5. The symptoms of listeriosis
Infected people are often asymptomatic, especially pregnant women. The symptoms of listeriosis are not very specific: fever with sometimes headaches, chills, and digestive disorders such as diarrhoea, nausea and vomiting.
There are two clinical pictures: the invasive form and the non-invasive form. The latter is rare and usually takes the form of febrile gastroenteritis. In the invasive form, the bacteria will spread outside the digestive tract, causing neurological complications such as meningitis, encephalitis and sometimes brain abscess, septicaemia or bacteraemia. Depending on the study, the case fatality rate is 20-30%.
The likelihood of an individual developing a severe form of the disease depends on both the immune status of the individual and the virulence of the bacterial strain.
6. Listeriosis in pregnant women
Listeriosis is one of the most feared diseases during pregnancy, which explains the many dietary restrictions that pregnant women must observe. As we have seen, listeriosis is usually asymptomatic in expectant mothers, apart from a slight fever. The danger lies mainly in the transmission to the foetus. If the pregnant woman is infected in the first six months, there is a risk of miscarriage. Between the 6th and 9th months, listeriosis can cause premature delivery and in some cases in-utero foetal death.
Sometimes the infection is also diagnosed in the baby after birth. The symptoms are often those of sepsis with respiratory distress, sometimes meningitis. The infection is very dangerous in the newborn with a high case fatality rate.
7. Listeriosis: how to make the diagnosis?
When a case of listeriosis is suspected because of the presence of symptoms, the diagnosis can only be confirmed by isolating the bacteria by means of a blood sample, a sample of cerebrospinal fluid taken during a lumbar puncture, a vaginal swab or an analysis of the placenta. In neonates, the germ can be isolated from blood or cerebrospinal fluid, and sometimes also from the gastric fluid by aspiration.
8. Foods affected by listeriosis
Among the foods most frequently contaminated with Listeria monocytogenes are
raw milk cheeses, cheese rind, cheese sold grated
raw milk
smoked fish (salmon, trout, etc.)
raw fish: sushi, sashimi
fish eggs
raw shellfish: oysters, clams, etc.
charcuterie products: rillettes, pâtés, foie gras produced in jelly, etc.
cut meats: ham, etc.
raw or undercooked meat (carpaccio etc.)
raw sprouted seeds
9. Treatment of listeriosis
The treatment of listeriosis is based on antibiotic treatment, usually, high-dose amoxicillin combined with another antibiotic, which must be administered very quickly.
10. Preventing listeriosis: hygiene measures and foods to avoid
The risk of contracting listeriosis can be prevented, or at least reduced, by taking certain precautions. These are especially important for susceptible people, and in particular pregnant women, because of the risk that the bacteria poses to the foetus.
The first thing to do is obviously to avoid the risky foods mentioned above. During pregnancy, it is imperative to be really vigilant. For example, cheese lovers should opt for cheese made from pasteurised milk. Meat should be eaten well-cooked.
Other hygiene measures should be applied in the storage of food and in the preparation of meals:
- Wash your hands thoroughly before handling food and wash them again, for example, if you have to handle meat after cleaning vegetables.
- Do not keep leftovers in the refrigerator for more than 3 days.
- change utensils for handling cooked and raw food.
- clean vegetables and herbs well.
- Cook meat and fish thoroughly.
- Clean and disinfect the refrigerator regularly.
- remove the rind from the cheese
- and eat the cut products quickly.
- Respect the use-by date of food, do not keep products beyond this date.
- Store at-risk foods in the coldest part of the refrigerator, where the overall temperature should be below 4 degrees.
- Reheat food that is to be eaten hot.
- Respect the cold chain, do not refreeze a thawed product.
- regularly clean and disinfect the kitchen worktop.
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