Predisposition to persistent breeding-induced endometritis in warmblood mares

Age, embryo donor status, and insemination with chilled semen all predispose to persistent breeding-induced endometritis in warmblood mares

Equine Vet J. 2026 Apr 24. doi: 10.1002/evj.70175. Online ahead of print.
Authors
Tom Stout 1 2 , Rik Serrarens 2 , Bart Leemans 2 , Anthony Claes 2
Affiliations

1 Maxwell H. Gluck Equine Research Center, University of Kentucky, Lexington, Kentucky, USA.
2 Faculty of Veterinary Medicine, Department of Clinical Sciences, Utrecht University, Utrecht, the Netherlands.

PMID: 42033022
DOI: 10.1002/evj.70175

Abstract
in English, Chinese

Background: Persistent breeding-induced endometritis (PBIE) is a common reason for mares failing to establish pregnancy.

Objective: To examine the hypothesis that advancing mare age and embryo donor status predispose to PBIE and compromise fertility.

Study design: Retrospective analysis of the impact of mare age, breeding system (embryo donor versus broodmare), and semen type (chilled versus frozen-thawed) on the likelihood of PBIE and subsequent pregnancy/embryo recovery.

Methods: Warmblood mares (n = 769) inseminated during 1745 oestrous cycles were divided into three age categories (≤6, 7-13 and ≥14 years) and as embryo donors versus broodmares. PBIE was defined as the detection of >2 cm intrauterine fluid from the day after insemination. Multivariable analysis was used to examine the effect of mare age, breeding system, semen type, inseminated cycle number, ovulation induction, oestrus induction, reproductive status (barren, foaling, maiden), treatment for fluid accumulation (oxytocin, uterine lavage, post-ovulation antibiotic infusion, pre-breeding corticosteroid administration) and time of year (month) on the incidence of PBIE and pregnancy/embryo recovery.

Results: The likelihood of developing PBIE (27.6% of all cycles) was higher in older mares (≥14 years: odds ratio (OR) 6.77: relative risk (RR) 3.28) and ET donors (OR 1.58: RR 1.3) but reduced in foaling mares (OR 0.16: RR 0.37) or when using frozen semen (OR 0.4: RR 0.61). Pregnancy or embryo recovery was lower (OR 0.35: RR 0.68) for frozen-thawed (39.8%) than chilled-transported (58.3%) semen but was not significantly affected by PBIE (p = 0.86), suggesting that treatment was usually sufficient to mitigate the negative effects of PBIE.

Main limitation: Retrospective data from a multi-veterinarian clinic; mare status and pregnancy data are missing from some cases.

Conclusions: Advancing mare age increases the likelihood of PBIE; although embryo donor status and chilled semen use also increase the odds of PBIE, effects on fertility are minimised by simple treatments.

Keywords: broodmare; embryo donor; horse; persistent breeding‐induced endometritis; pregnancy; uterine fluid.

© 2026 The Author(s). Equine Veterinary Journal published by John Wiley & Sons Ltd on behalf of EVJ Ltd.