Study findings highlight the need to more closely monitor Clostridioides difficile in horses

Clostridioides difficile is a bacterium that can  colonise the intestinal tract  of human beings without causing symptoms or disease. When symptoms do occur, they are caused by the toxins produced by the bacterium and include diarrhoea, which may be severe and can even be fatal in the most serious cases. The bacterium is also found in animals and can cause infections – especially in horses – although it has been studied less extensively in animal populations.

For the first time in France, a systematic study on C. difficile in horses was carried out by scientists from ANSES (Laboratory for Animal Health, Normandy site), in collaboration with Institut Micalis (Paris-Saclay University, INRAE, AgroParisTech), the LABÉO testing laboratory and the National Reference Centre for Clostridioides difficile (Saint-Antoine Hospital, AP-HP). It focused on animals necropsied in Normandy between 2019 and 2021, regardless of the cause of death or the symptoms shown by the animal.

The scientists looked for the bacterium and the toxins it produces in the digestive tract of the horses. They measured the prevalence and virulence of C. difficile in the equine population, described the diversity of the isolated strains and assessed the risk of transmission, especially to humans. The results were published in the journal Microbiology Spectrum in December 2025.

Twenty percent of horses carry a pathogenic strain

“Of the 100 horses studied, 27 were found to carry C. difficile, including 20 that were carrying pathogenic strains, most often without any clinical signs”, notes Sandrine Petry, a scientist at ANSES. All the pathogenic strains found belonged to types that are commonly found in human patients. Close contact between horses and humans may allow the bacterium to spread in either direction.

The scientists also investigated whether the toxins produced by C. difficile were present. Testing for these toxins is necessary to demonstrate infection but is seldom carried out in animals. The toxins were detected in four animals from which a pathogenic C. difficile strain had been isolated. These horses had presented with diarrhoea prior to death, suggesting infection with C. difficile. Three of them had recently been given antibiotics, which likely increased their risk of infection: “In both humans and animals, C. difficile infections often occur following a course of antibiotics. These treatments disrupt the intestinal microbiota, promoting the growth of this bacterium and ultimately leading to infection”, explains the scientist.

This study provided insight into the prevalence of C. difficile in horses and how it affects their health. It also demonstrated the need to more closely monitor C. difficile in animals in order to prevent it from spreading to humans.