Study links profuse E. coli growth to lower live foal rates

A new retrospective cohort study in Equine Veterinary Journal examined whether endometrial swab cytology and bacteriology findings were linked to live foal outcomes in UK Thoroughbred broodmares. Using 7,691 last-of-season swabs from 3,579 mares on 196 farms between 2014 and 2020, the researchers found that mares with profuse Escherichia coli growth had significantly lower predicted live foal rates than mares with no bacterial growth, 59.1% versus 80.9%. They also found that marked endometrial inflammation on cytology, defined as more than 30% polymorphonuclear cells, was associated with reduced live foal rates in mares older than 12 years, but not in younger mares. The study was led by investigators from Rossdales Veterinary Surgeons, the Royal Veterinary College, Cornell, and Université de Montréal. (pubmed.ncbi.nlm.nih.gov)

Why it matters: For equine veterinarians, the findings suggest that not all positive swab results carry the same prognostic weight. In this dataset, profuse E. coli stood out, while other bacterial growth categories did not show the same association with poorer live foal outcomes. The age-specific cytology signal is also clinically useful: inflammation on a swab may deserve more concern in older broodmares, which fits broader evidence that age-related uterine clearance and endometrial changes can worsen fertility. The authors also note important stewardship implications, because single swabs can have limited specificity and may detect contamination or clinically insignificant growth, especially when unguarded swabs are used. (pmc.ncbi.nlm.nih.gov)

What to watch: Further work will likely focus on whether profuse E. coli is acting as a true pathogen, a marker of dysbiosis, or a signal of treatment-resistant disease, and on how that should change diagnostic and antimicrobial protocols in stud practice. (pmc.ncbi.nlm.nih.gov)