Associations between endometrial swab bacteriology and cytology findings and live foal rates in Thoroughbred broodmares in the United Kingdom.
Authors: Fehin Billy, Scott Camilla J, Arango-Sabogal Juan Carlos, de Mestre Amanda M, Mouncey Rebecca
Journal: Equine veterinary journal
Summary
# Editorial Summary A retrospective analysis of 7,691 endometrial swabs from 3,579 UK Thoroughbred broodmares across 196 farms between 2014 and 2020 has identified important associations between pre-breeding microbial and cytological findings and live foal rates, offering valuable guidance for fertility management decisions. Profuse growth of *Escherichia coli* emerged as the sole bacterial isolate category with significantly compromised fertility outcomes, reducing live foal rates to 59.1% compared with 80.9% in mares with no bacterial growth—a clinically substantial 22-percentage-point difference that warrants investigation into underlying mechanisms. Age-dependent cytological thresholds also proved important: older mares (>12 years) showed markedly reduced live foal rates when polymorphonuclear neutrophil counts exceeded 30% of endometrial cells per high power field, whereas this cytological finding had no predictive value in younger mares (≤12 years), highlighting the complexity of interpreting swab results without considering mare demographics. These findings should encourage practitioners to contextualise laboratory results within mare age and clinical history, whilst recognising that current knowledge gaps surrounding *E. coli* infections in particular justify further investigation into why some mares clear this pathogen whilst others experience persistent subfertility despite apparent bacterial clearance.
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Practical Takeaways
- •Profuse E. coli growth on endometrial swabs warrants serious consideration as a fertility risk factor—mares with this finding had ~21% lower live-foal rates and may benefit from targeted management strategies
- •Cytology interpretation in older mares (>12 years) requires different clinical thresholds than younger mares; elevated PMN levels (>30%) are more predictive of reduced fertility in this age group
- •While endometrial swabbing provides useful diagnostic information, interpretation must account for mare age and specific bacterial species; blanket recommendations based solely on general bacterial growth or cytology findings may lead to inappropriate clinical decisions
Key Findings
- •Mares with profuse E. coli growth had significantly lower live-foal rates (59.1%) compared to those with no growth (80.9%, p=0.005)
- •Age-dependent interaction observed: mares >12 years with >30% polymorphonuclear cells showed significantly reduced live-foal rates compared to those with ≤0.5% PMN, but this effect was absent in mares ≤12 years
- •Study included 7,691 last endometrial swabs from 3,579 mares across 196 UK farms submitted between 2014-2020
- •Other bacterial isolate categories did not show significant associations with reduced live-foal rates, suggesting E. coli represents a specific fertility concern