The Klebsiella pneumoniae bacterium is a major concern for both human and equine health. Certain human strains have become resistant to multiple antibiotics, including treatments used as a last resort. Other strains are hypervirulent, i.e. they multiply rapidly and can cause potentially fatal symptoms such as pneumonia. In horses, this bacterium can induce abortions and cause mortality, among other things. It must be monitored in some breeds to prevent sexual transmission.

Earlier this year, this bacterium was the subject of a scientific article in Frontiers in Microbiology, resulting from a collaboration between the Pathophysiology and Epidemiology of Equine Diseases Unit of ANSES’s Laboratory for Animal Health (Normandy site), the Dynamicure joint research unit and Caen University Hospital.

The study sought to analyse the antibiotic resistance of K. pneumoniae strains isolated from horses and the presence of genes in the genome of these strains that favour greater virulence. One of the questions was to determine whether horses carried strains of concern for humans.

ANSES has a collection of strains isolated from necropsies carried out on horses since 1996. All the strains of K. pneumoniae in this collection were included in the study, regardless of the animals' cause of death. Strains isolated from samples taken for screening or confirmation of infection by the Labéo laboratory in Normandy were also included.

A population of bacteria specific to horses

"The university hospital teams were surprised by the wide diversity of strains in the equine samples, compared with those usually found in humans: 83 different groups were found among the 119 strains analysed. Although some can also be found in humans, most are specific to Equidae," explains Sandrine Petry, head of the Bacteriology team in the Pathophysiology and Epidemiology of Equine Diseases Unit at ANSES. This therefore shows that there is little transmission of bacteria between horses and humans.

Resistance to 35 antibiotics used in human or veterinary medicine was tested. This found that 39% of Klebsiella pneumoniae strains were resistant to at least three antibiotic classes (multi-drug resistant). This proportion has increased over time: while 18.8% of strains taken from necropsies carried out between 1996 and 2007 were resistant, this proportion rose to 39.1% between 2008 and 2020. This increase is probably due to the growth in the use of antibiotics in the horse-breeding sector over the same period.

New strains of bacteria to be monitored

Hypervirulent strains accounted for 9% of the strains analysed. Half of these were also multi-drug resistant. Some strains of K. pneumoniae secrete a capsule that protects them from the host immune system. They are therefore usually regarded as more virulent than others. "Only three capsular types of K. pneumoniae are considered to be sexually transmitted in horses, and are subject to mandatory monitoring in certain breeds," explains Sandrine Petry. "However, the results of the study show that the hypervirulent and/or multi-drug resistant strains of K. pneumoniae are not limited to these three capsular types". Surveillance therefore needs to be extended to these bacteria by improving current detection tools, which only take account of the three capsular types and not any hypervirulence marker genes.

This study, conducted on a collection of bacteria mainly from horses living in Normandy, provides an initial review of the K. pneumoniae strains affecting horses, although this work needs to be extended to provide a more comprehensive view of the situation.