Salmonella infection (salmonellosis)
-
198,000
cases of salmonellosis every year in France, 183,000 of them foodborne infections, according to Santé publique France
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Between 8,000 and 13,000
strains of Salmonella spp analyzed each year by the CNR
What are the causes?
Salmonellosis is the name given to infections caused by bacteria of the genus Salmonella, generally ingested in contaminated food. More rarely, Salmonella infection can also occur after contact with warm- or cold-blooded animals carrying the bacteria. Salmonella bacteria can also survive for periods of time in soil or water.
How do the bacteria spread?
Salmonella bacteria are mainly spread through the consumption of contaminated food or water.
Since Salmonella can be carried by a number of different animals, a wide variety of food products, either eaten raw or undercooked or contaminated after cooking, may lead to human infection. Meat-based products (poultry and beef) including cured meat, egg-based products, and raw (unpasteurized) milk-based products including some cheeses may potentially pose a risk.
Finally, contaminated plant-based foods that are not properly washed or cooked prior to consumption may also cause Salmonella infection.
Human-to-human transmission is also possible via the fecal-oral route.
More generally, inappropriate food handling and storage conditions, as well as poor personal hygiene practices, can encourage the spread of Salmonella bacteria. Salmonellosis can also be spread from one person to another if hygiene measures are not strictly applied, especially in settings such as kitchens or restaurants.
More rarely, individuals can be infected by directly handling an animal that is either sick or a healthy carrier, especially reptiles or birds. It is worth noting that the vast majority of reptiles are healthy carriers of Salmonella. Pet reptiles can cause severe infections in infants.
What are the symptoms?
Most infections caused by Salmonella bacteria present as gastroenteritis. Symptom onset generally takes 1 to 2 days and depends on the dose of bacteria ingested, the health of the host and the characteristics of the Salmonella strain. The main symptoms of infection are diarrhea, fever, vomiting and abdominal pain.
Infections caused by some types of Salmonella may also present as typhoid or paratyphoid fever.
In elderly people, infants, pregnant women or immunosuppressed individuals, the bacteria may spread to the bloodstream, causing severe infection that sometimes proves fatal.
How is Salmonella infection diagnosed?
Salmonella infection is detected by stool culture performed in a medical test laboratory. If the infection is severe, a blood culture can also be performed to check for Salmonella in the blood.
What treatments are available?
Healthy adults normally recover from gastroenteritis without treatment after 3 to 5 days on average. But antibiotics may be required for patients with a more severe infection or for the most vulnerable individuals (elderly people, infants, immunocompromised individuals, pregnant women, etc.), who can develop complications more quickly.
How can Salmonella infection be prevented?
The best protection against Salmonella infection is to make sure that food, especially meat, is properly cooked, to at least 65°C for 5 to 6 minutes. Frozen burgers should be cooked from frozen, as defrosting increases the risk of bacterial proliferation – the cold stops the bacteria from developing but does not kill them.
In the late 1980s, the number of infections in Europe linked to Salmonella serotype Enteritidis rose steeply after egg farms were contaminated by this serotype. Bacteria of this serotype are characterized by their ability to colonize not only the surface of eggshells, but also the content of intact eggs. Since then consumers have therefore been advised to keep eggs refrigerated and to chill any food containing raw eggs (mayonnaise, chocolate mousse, creams, cakes, desserts, etc.) and eat it as soon as possible after it has been prepared. Vulnerable groups (the elderly or sick, infants and pregnant women) should avoid eating raw or undercooked eggs altogether.
Finally, experts recommend hand washing after coming into contact with any live animals (especially reptiles) and advise vulnerable people (infants, pregnant women, immunocompromised individuals, etc.) to avoid touching pet reptiles.
How many people are affected? Who is affected?
Foodborne Salmonella infection can give rise to major outbreaks, which can reach national or even international proportions if a specific food product sold widely on the mass market is contaminated. In 1994 in the United States, an outbreak traced back to a brand of ice cream is thought to have affected 224,000 people. In France, one of the most severe Salmonella outbreaks occurred in 1985. The food source of the outbreak was never identified, but even the lowest estimates suggest that it affected at least 25,000 people.
Salmonellosis is the second leading cause of gastrointestinal infections in humans after campylobacteriosis, and is a major cause of foodborne infection in Europe, with 60,050 confirmed cases in 2021, representing a rate of 15.7 cases/100,000 population.(1) In 2021, 1,309 foodborne illness outbreaks were reported in France, affecting 11,056 people, with 512 hospitalizations (5%) and 16 deaths (0.14%).(2) As in previous years, the most commonly identified pathogen was the Salmonella bacterium (in 44% of foodborne illness outbreaks).
The French National Reference Center (CNR) for Escherichia coli, Shigella and Salmonella at the Institut Pasteur, which is responsible for microbiological surveillance of human Salmonella infection, isolates between 8,000 and 13,000 cases of Salmonella in humans each year (12,118 in 2023). The CNR is able to distinguish between over 2,700 serotypes of Salmonella bacteria using conventional techniques, as well as more than 400,000 types using genome sequencing. The most common serotype is S. Enteritidis, commonly found in the poultry industry, followed by the monophasic variant of S. Typhimurium (with the antigenic formula 1,4,[5],12:i:-), which is probably spread through the pig industry, and in third place S. Typhimurium (ubiquitous). These three serotypes represent 63% of all Salmonella isolations in France.(3)
November 2024
- European Food Safety Authority and European Centre for Disease Prevention and Control (EFSA and ECDC).The European Union One Health 2021 Zoonoses Report
- Santé publique France. Toxi-infections alimentaires collectives en France : les chiffres 2021 (in French)
- Annual reports of the CNR for Escherichia coli, Shigella and Salmonella
FAQ
Salmonellosis is an infection caused by bacteria of the genus Salmonella.
Salmonella are bacteria that live in the digestive tract of animals, from reptiles to elephants. Their presence in the digestive tract of farm animals can have repercussions for human food and health.
There are more than 2,700 types of Salmonella. Some have evolved to certain animals and are only found in poultry, sheep or humans, for example. This is the case of the Salmonella bacteria that cause typhoid and paratyphoid fever.
Salmonella are naturally found in the digestive tract of farm animals. The feces of these animals can come into contact with milk or meat carcasses and contaminate them. If we consume this food – raw or undercooked meat, raw (unpasteurized) milk or raw-milk cheese –, we can be infected by these live bacteria.
There are two modes of transmission: via animals or directly between humans.
- Most Salmonella are zoonotic bacteria. In 90% of cases, they are spread through contaminated food. In the remaining 10% of cases, infection occurs through direct contact with an animal. For example, children on an educational farm visit might touch a calf, not wash their hands and then touch their mouth.
- Some Salmonella, like Salmonella Typhi and Paratyphi (A, B and C), responsible for typhoid and paratyphoid fever, are only carried by humans and can spread between people through dirty hands (for example if people fail to wash their hands properly after going to the toilet) or through water contaminated with feces or urine. For other types of Salmonella, human-to-human transmission is relatively rare. It generally occurs in communities of young children (nurseries, families, etc.) where there is a problem with hand hygiene.
Cross-contamination with Salmonella occurs when bacteria are transferred from a contaminated product to a non-contaminated product.
This may happen, for example, if a chopping board is used to prepare a contaminated raw chicken and then the same board is used to prepare a tomato salad. The bacteria from the raw chicken carcass infect the tomato salad. This also applies if the same knife is used to cut the raw chicken and the tomatoes that are eaten uncooked.
To avoid Salmonella infection, follow these best preventive practices:
- Wash your hands. This is the most important thing to do, especially after going to the toilet and before preparing meals.
- Keep eggs in the fridge: Salmonella bacteria cannot multiply under 9°C. Refrigerating eggs significantly reduces the risk of salmonellosis.
- Refrigerate dishes made with raw eggs: prepare them at the last minute and keep them in the fridge. A mayonnaise or chocolate mousse that has been out of the fridge for several hours should be thrown away as the bacteria can multiply rapidly (especially in hot weather) and make the product dangerous to eat.
- Cook food thoroughly. Bacteria like Salmonella or Escherichia coli do not tolerate heat. They are destroyed as soon as they reach a temperature of between 60 and 90°C. Salmonella cannot be spread through pasteurized food.
- Peel and wash vegetables. Vegetables can be contaminated by irrigation water or manure containing pathogenic bacteria, so they should be peeled and cleaned to remove any bacteria.
The symptoms of Salmonella infection are like those of gastroenteritis: digestive problems, nausea, vomiting, diarrhea and often fever. Salmonella infection is detected by stool culture performed in a medical test laboratory, which identifies bacterial colonies typical of Salmonella.
The first symptoms of Salmonella infection occur on average 12 to 36 hours after eating a contaminated product. This may vary depending on the quantity of bacteria absorbed. Symptoms may begin just 6 hours after ingesting a large quantity of bacteria, or up to 72 hours after eating a small amount of contaminated food.
Salmonellosis is a foodborne infection, so anyone who ingests contaminated food can fall ill. But some people are more likely to develop severe symptoms: elderly people, young children, pregnant women and people under heavy medication may develop a more severe form of illness.
Salmonella infection is detected by stool culture performed in a medical test laboratory, which identifies the bacterial colonies typical of Salmonella.
It is thought that there are around 100,000 to 200,000 cases of Salmonella infection in France each year, leading to 4,000 hospitalizations.
Estimates suggest that between 50 and 100 deaths are caused by Salmonella infection each year.
Eating a product that has been contaminated by bacteria does not automatically make us ill. Several natural defenses help ward off salmonellosis:
- The stomach and the gastric acid it produces,
- Immunity.
Our propensity to fall ill therefore depends on both the quality of our defenses and the quantity of bacteria ingested. We can use the metaphor of an attack on a fortress. The attackers are the live bacteria consumed. The gastric acid is the moat, and our immune defenses are the walls. Depending on the number of attackers and the quality of the defenses, the fortress will either be taken (we will fall ill) or defended (we will stay healthy).
Most of the time, people with Salmonella infection recover on their own and do not require specific treatment.
But for people with severe infection or most at risk of severe infection (when the bacteria passes into the bloodstream), including elderly people, infants, pregnant women or individuals under heavy medication, antibiotics may be necessary. In this case, the medical test laboratory will perform an antibiogram, a test to determine which antibiotics will act on the Salmonella bacteria. The patient is prescribed the most effective antibiotic.