Clostridioides difficile in feral horse populations in Australia

Clostridioides difficile is a known cause of diarrhea and colitis in human and non-human animals. While C. difficile is regularly isolated from domesticated horses, little is known about its prevalence in wild or feral populations. In Australia, the horse population encompasses a mix of both domesticated and feral animals, with the feral population of 400,000 estimated to be the largest in the world. This study investigated the presence and characteristics of C. difficile in Australian feral horses and evaluated their potential as a source or reservoir of C. difficile in the wider community. Fecal samples (n = 380) were collected from free-roaming feral horses from five Australian jurisdictions and cultured for C. difficile. Isolates were characterized by PCR ribotyping and toxin profiling. Antimicrobial susceptibility testing was performed for fidaxomicin, vancomycin, metronidazole, rifaximin, clindamycin, erythromycin, amoxicillin-clavulanate, moxifloxacin, meropenem, and tetracycline. C. difficile was isolated from 45 of the 380 samples (11.8%)-one-third of that seen in recent studies on Australian domesticated horses but consistent with wild animal species worldwide. Forty ribotypes (RTs) were identified, 28 of which (70%) were novel; other RTs had been previously reported in humans, livestock, and soils. Eighteen toxigenic C. difficile strains were isolated, of which eight contain binary toxin genes. Strains were largely susceptible to the antimicrobial agents tested. This investigation provides preliminary information on C. difficile in feral horses in Australia and allows a comparison with their domestic counterparts. The findings support the hypothesis that all horse feces represent a potential source of C. difficile in the community.

Clostridioides difficile poses an ongoing threat to healthcare in the community, with increasing evidence of transmission outside the hospital setting. In keeping with a One Health model of dispersion, investigations into this microorganism within the wider environment are vital to understanding this evolving epidemiology. Australia has the biggest population of feral horses in the world, and this study of C. difficile in feral horses provides insight into the role of non-domesticated animals in the dissemination of C. difficile. Examination of prevalence and characterization of isolates provides a baseline for evaluating the effect of antimicrobials and other factors associated with domestication on equine C. difficile infection.