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

A new retrospective cohort study in Equine Veterinary Journal links certain endometrial swab findings with lower live foal rates in UK Thoroughbred broodmares. Researchers analyzed 7,691 last-of-season swabs from 3,579 mares across 196 farms, using laboratory data from Newmarket collected between 2014 and 2020. The standout finding was that mares with profuse Escherichia coli growth had a significantly lower predicted live foal rate than mares with no bacterial growth, 59.1% versus 80.9%. The study also found an age-related cytology effect: in mares older than 12 years, swabs showing more than 30% polymorphonuclear cells were associated with lower live foal rates, while that pattern wasn’t seen in mares 12 and younger. The authors note important limitations, including use of unguarded swabs and a lack of clinical information. (pubmed.ncbi.nlm.nih.gov)

Why it matters: For equine veterinarians, the paper adds real-world outcome data to a diagnostic workflow that’s widely used but often difficult to interpret. The findings suggest that not all positive bacteriology results carry the same fertility signal, and that heavy E. coli growth may deserve particular attention. They also reinforce that cytology results may need to be read through the lens of mare age, especially in older broodmares. That matters in a setting where culture and cytology remain standard tools for diagnosing infectious endometritis, but experts continue to caution against reflexive antibiotic use without evidence of true infection. (pubmed.ncbi.nlm.nih.gov)

What to watch: Watch for follow-up work clarifying why profuse E. coli growth tracks with poorer fertility, and whether those mares represent a distinct clinical subgroup that needs different management. (pubmed.ncbi.nlm.nih.gov)