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Equine veterinary journal2026; doi: 10.1002/evj.70175

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

Abstract: 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. Methods: 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. Conclusions: 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. 背景: 持续性配种诱导性子宫内膜炎(Persistent breeding‐induced endometritis, PBIE)是导致母马未能成功妊娠的常见原因之一。 目的: 检验母马年龄增长及供胚母马状态是否会增加PBIE发生风险并降低繁殖力的假设。 研究设计: 回顾性分析母马年龄、繁殖方式(供胚母马对比繁殖母马)及精液类型(冷藏精液对比冷冻解冻精液)对PBIE发生概率及后续妊娠/胚胎回收的影响。 方法: 本研究纳入温血母马769匹，共1745个发情周期。根据年龄分为三组(≤6岁、7–13岁、≥14岁)，并按供胚母马与繁殖母马分类。PBIE定义为授精后次日起检测到子宫内液体>2 cm。采用多变量分析评估以下因素对PBIE发生率及妊娠/胚胎回收的影响:母马年龄、繁殖方式、精液类型、授精次数、排卵诱导、发情诱导、繁殖状态(空怀、带驹、初配)、液体潴留治疗(催产素、子宫冲洗、排卵后抗生素灌注、配种前皮质类固醇应用)及时间(月份)。 结果: 总发情周期的PBIE发生率为27.6%。老龄母马(≥14岁:风险因素 (OR) 6.77；相对风险 (RR) 3.28)及供胚母马(OR 1.58；RR 1.30)发生PBIE的风险较高，而带驹母马(OR 0.16；RR 0.37)或使用冷冻精液(OR 0.4；RR 0.61)时风险降低。使用冷冻解冻精液的妊娠或胚胎回收率(39.8%)低于冷藏运输精液(58.3%)(OR 0.35；RR 0.68)。PBIE对妊娠或胚胎回收率结果无显著影响(p=0.86)，提示常规治疗通常足以减轻PBIE对其负面影响。 主要局限性: 研究为多兽医诊所的回顾性数据分析，部分病例缺失母马状态及妊娠数据。 结论: 母马随年龄增长增加PBIE发生风险；尽管供胚母马状态及使用冷藏精液也会提高PBIE发生概率，但通过简单治疗措施，其对繁殖力的负面影响可被有效降低。.
© 2026 The Author(s). Equine Veterinary Journal published by John Wiley & Sons Ltd on behalf of EVJ Ltd.
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Publication Date: 2026-04-24 PubMed ID: 42033022DOI: 10.1002/evj.70175Google Scholar: Lookup
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Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

Overview

Background

  • Persistent breeding-induced endometritis (PBIE) is a condition where inflammation of the uterus persists after breeding, often causing failure to establish pregnancy in mares.
  • PBIE detection is typically based on finding intrauterine fluid accumulation greater than 2 cm following insemination.
  • The study aims to assess whether mare age, embryo donor status, and type of semen used (chilled vs. frozen-thawed) influence the likelihood of developing PBIE and how this affects fertility measures such as pregnancy rates or embryo recovery success.

Methods

  • The analysis was retrospective, utilizing breeding records from 769 Warmblood mares across 1745 oestrous cycles.
  • Mares were classified into three age groups: ≤6 years, 7–13 years, and ≥14 years.
  • Mares were further categorized as either embryo donors or broodmares (non-donor mares used for breeding).
  • PBIE was defined by ultrasound detection of more than 2 cm of intrauterine fluid the day after insemination.
  • The study investigated multiple variables including:
    • Mare age category
    • Breeding system (embryo donor vs. broodmare)
    • Semen type (chilled vs. frozen-thawed)
    • Number of inseminated cycles
    • Ovulation and estrus induction treatments
    • Reproductive status (barren, foaling, maiden)
    • Treatment for fluid accumulation (oxytocin, uterine lavage, antibiotic infusion post-ovulation, pre-breeding corticosteroids)
    • Month of the year (seasonal effect)
  • Multivariable statistical models were used to determine which factors significantly influenced the incidence of PBIE and pregnancy or embryo recovery outcomes.

Results

  • Overall, PBIE occurred in 27.6% of the analyzed breeding cycles.
  • Key findings regarding PBIE risk:
    • Older mares (≥14 years) had a markedly increased risk of developing PBIE (Odds Ratio [OR] 6.77; Relative Risk [RR] 3.28).
    • Embryo donor mares were at higher risk than broodmares (OR 1.58; RR 1.3).
    • Foaling mares (mares that had recently foaled) had a significantly reduced risk of PBIE (OR 0.16; RR 0.37).
    • The use of frozen-thawed semen was associated with a lower risk of PBIE compared to chilled semen (OR 0.4; RR 0.61).
  • Pregnancy or embryo recovery rates:
    • Frozen-thawed semen yielded lower pregnancy or embryo recovery success (39.8%) compared to chilled semen (58.3%) (OR 0.35; RR 0.68).
    • Interestingly, presence of PBIE did not significantly reduce pregnancy or embryo recovery success (p=0.86), suggesting that treatments implemented were generally effective in mitigating negative effects.

Conclusions

  • Advancing age in mares is a strong risk factor for developing PBIE.
  • Embryo donor mares and use of chilled semen also increase the likelihood of PBIE, but these effects do not drastically compromise fertility when proper treatments are applied.
  • Foaling mares appear less susceptible to PBIE, possibly due to enhanced uterine clearance or other physiological factors post-foaling.
  • The reduced pregnancy rates with frozen-thawed semen indicate that semen type affects fertility independent of PBIE risk.
  • Common veterinary treatments such as oxytocin administration, uterine lavage, and corticosteroids are effective at managing PBIE and supporting successful pregnancy outcomes.

Limitations

  • The study was retrospective, based on records from multiple veterinarians and clinics, which may introduce variability.
  • Some cases had missing data on mare reproductive status and pregnancy outcomes.
  • The findings may not be fully generalizable to other breeds or management systems outside Warmblood mares or the studied clinics.

Cite This Article

APA
Stout T, Serrarens R, Leemans B, Claes A. (2026). Age, embryo donor status, and insemination with chilled semen all predispose to persistent breeding-induced endometritis in warmblood mares. Equine Vet J. https://doi.org/10.1002/evj.70175

Publication

Equine veterinary journal
ISSN: 2042-3306
NlmUniqueID: 0173320
Country: United States
Language: English

Researcher Affiliations

Stout, Tom
  • Maxwell H. Gluck Equine Research Center, University of Kentucky, Lexington, Kentucky, USA.
  • Faculty of Veterinary Medicine, Department of Clinical Sciences, Utrecht University, Utrecht, the Netherlands.
Serrarens, Rik
  • Faculty of Veterinary Medicine, Department of Clinical Sciences, Utrecht University, Utrecht, the Netherlands.
Leemans, Bart
  • Faculty of Veterinary Medicine, Department of Clinical Sciences, Utrecht University, Utrecht, the Netherlands.
Claes, Anthony
  • Faculty of Veterinary Medicine, Department of Clinical Sciences, Utrecht University, Utrecht, the Netherlands.

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