Associations between endometrial swab bacteriology and cytology findings and live foal rates in Thoroughbred broodmares in the United Kingdom.

Background: To date, relationships between pre-covering endometrial swab cytology and bacteriology and fertility outcomes in Thoroughbred broodmares in the United Kingdom are unknown and could aid clinical decision making.

Objectives: To investigate associations between cytology and bacteriology findings from the last endometrial swab taken in the breeding season (15 February to 15 July) and live-foal rates (predicted mean probability of producing a live foal) in UK Thoroughbred broodmares.

Study Design: Retrospective cohort study.

Methods: Endometrial cytology and bacteriology findings were extracted from laboratory records for all last-swabs submitted between 2014 and 2020. Mares' status, age and foaling outcome were collected from publicly available data sources. Live-foal rates were estimated for reported categories of cytology and bacteriology findings using a multivariable logistic regression model with mare and farm fitted as random effects, while adjusting for mares' age, status, number of previous swabs submitted in that season and any interactions. Pairwise comparisons with Bonferroni correction evaluated between-category live-foal rate differences.

Results: Data were available from 7691 last swabs from 3579 mares on 196 farms. In contrast to other categories of isolate, mares with a profuse growth of Escherichia coli had significantly lower (p = 0.005) live-foal rates (59.1%; 95% confidence interval (CI) 43.7-74.5) compared to those with no growth (80.9%; 95% CI 79.2-82.6). There was interaction between mares' age and cytology. In mares >12 years, significant reductions in live-foal rates (p < 0.05 in pairwise comparisons) were observed between mares with >30% polymorphonuclear: endometrial cells/high power field at cytological examination and mares with ≤0.5% PMN, a finding absent in mares ≤12 years.

Main Limitations: The use of unguarded swabs and absence of clinical information.

Conclusions: Results highlight complexities to consider when interpreting endometrial swab cytology findings and a subset of mares with a profuse growth of E. coli in which knowledge gaps exist around the aetiologies underlying their poorer fertility outcomes.