Role of anaerobic bacteria in equine endometritis

This study, performed over 3 breeding seasons, surveyed anaerobic and aerobic bacterial isolates from 362 clitoral and endometrial swabs and uterine washes from 263 Thoroughbred maiden, foaling, foal heat and barren mares, and from 113 urethral, urethral fossa, preputial and pre-ejaculatory fluid swabs from 29 Thoroughbred stallions. The significance of isolates was determined by their association with acute endometritis, as determined by concurrent endometrial smear results and by consideration of age and reproductive status before and after the survey. The results suggest that the horse uterus may harbour obligate anaerobes as surface commensals. These organisms normally inhabit the external genital surfaces of mares and stallions and are periodically introduced into the uterus at coitus or in association with genital pathology, e.g. pneumovagina or vagino/cervical injury. They may act as opportunist pathogens when there is epithelial damage, e.g. during the post-partum involutionary period. Synergism with aerobic bacteria may result in mixed infection and active endometritis. In the mare, the predominant uterine anaerobic species is Bacteroides fragilis. This species is predominantly penicillin- and aminoglycoside-resistant, a feature which is pertinent when intrauterine antimicrobial therapy for endometritis is considered. Detailed anaerobic diagnoses are unavoidably time consuming and as mixed infections are common, nitrofurantoin or metronidazole should be included in antibiotic mixtures. Failure to do this may account for some cases of aerobe-negative persistent endometritis.