Most infections caused by Salmonella bacteria present as gastroenteritis. Symptom onset generally takes 1 to 2 days and depends on the bacteria dose ingested, the health of the host and the characteristics of the Salmonella strain. The main symptoms of infection are fever, diarrhea, vomiting and abdominal pain.
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 including cured meat, egg-based products, and raw (unpasteurized) milk-based products including some cheeses may potentially pose a risk. More rarely, individuals can be infected by directly handling an animal that is either sick or a healthy carrier. It is worth noting that the vast majority of reptiles are healthy carriers of Salmonella. The new trend for keeping reptiles as pets can lead to severe infections in infants. Finally, contaminated plant-based foods that are not properly washed or cooked prior to consumption may also cause Salmonella infection.
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 affected 224,000 people. In France, one of the largest outbreaks, whose source was never identified, occurred in late 1985 and infected 25,000 people, according to the lowest estimates.
Salmonellosis is the second biggest cause of gastrointestinal infections in humans after campylobacteriosis, and is a major cause of foodborne infection in Europe with 91,857 confirmed cases in 2018, representing a rate of 20.1 cases/100,000 population (1). In 2018, 1,630 collective foodborne infections were reported in France, affecting 14,742 people, with 777 (5%) hospitalizations and two deaths (2). As in previous years, the most commonly identified pathogen was the Salmonella bacterium (in 51% of collective foodborne infections).
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 10,000 and 11,000 cases of Salmonella each year (10,926 in 2018). The CNR is able to distinguish between over 2,600 serotypes of these bacteria. The most common serotype is S. Enteritidis (poultry industry), followed by the monophasic variant of S. Typhimurium (with the antigenic formula 1,4,,12:i:-), which is probably spread through the pig industry, and in third place, the S. Typhimurium serotype (ubiquitous). These three serotypes represent 61% of all Salmonella isolations in France (3).
Healthy adults normally recover without treatment after 3 to 5 days on average. However, in elderly people, infants and immunocompromised individuals, the infection may be more severe and even life-threatening, and these patients may be prescribed antibiotics.
The best protection against Salmonella infection is to make sure that food, especially meat, is properly cooked at a temperature of 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 the S. Enteritidis serotype 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 contents of intact eggs. Consumers have therefore since been advised to keep their eggs refrigerated, to chill any food containing raw eggs (mayonnaise, creams, cakes, desserts, etc.) and to 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.
Typhoid and paratyphoid fever
Infections caused by Salmonella may also present as typhoid and paratyphoid fever.
At the Institut Pasteur
The National Reference Center for Escherichia coli, Shigella and Salmonella (CNR), hosted in the Enteric Bacterial Pathogens Research and Expertise Unit at the Institut Pasteur (both entities are led by Professor François-Xavier Weill), is responsible for microbiological surveillance of human Salmonella infection. Every year, the CNR analyzes 8,000 to 10,000 Salmonella strains from both medical test and hospital laboratories.
It plays a key role in outbreaks, alerting the General Directorate of Health and Santé publique France if high case numbers are detected and contributing to epidemiological investigations by typing bacterial strains. The CNR also monitors the susceptibility of Salmonella bacteria to antibiotics over time. Since 2018, all Salmonella strains sent to the CNR have been analyzed by whole-genome sequencing to improve detection, investigation and monitoring of bacterial populations involved in outbreaks.
The CNR collaborates with the national network of the French Agency for Food, Environmental and Occupational Health and Safety (ANSES) and the European Food- and Waterborne Diseases and Zoonoses Network (EPIS-FWD) of the European Center for Disease Prevention and Control (ECDC).
The teams involved
Enteric Bacterial Pathogens Unit led by François-Xavier Weill
National Reference Center for Escherichia coli, Shigella and Salmonella led by François-Xavier Weill
1. EFSA and ECDC (European Food Safety Authority and European Centre for Disease Prevention and Control), 2019. The European Union One Health 2018 Zoonoses Report. EFSA Journal 2019;17(12):5926, 276 pp.
2. Fournet N, et al. Surveillance des toxi-infections alimentaires collectives. Données de la déclaration obligatoire, 2018. Le point épidémio/Santé publique France. Janvier 2019. https://www.santepubliquefrance.fr/maladies-et-traumatismes/maladies-infectieuses-d-origine-alimentaire/toxi-infections-alimentaires-collectives/documents/bulletin-national/surveillance-des-toxi-infections-alimentaires-collectives.-donnees-de-la-declaration-obligatoire-2018.