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Equine veterinary journal2025; 58(2); 348-358; doi: 10.1111/evj.70086

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

Abstract: 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. Objective: 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. Methods: 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. Conclusions: 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. Background: À ce jour, la relation entre les cytologies et bactériologies sur écouvillons endométriaux pré‐saillie et les taux de fertilité chez les juments poulinières au Royaume‐Uni demeurent inconnus et pourrait contribuer au raisonnement clinique. Objective: Investiguer les associations entre les résultats cytologique et bactériologique provenant du dernier écouvillon prélevé en saison de reproduction (15 Février au 15 Juillet) et les taux de naissance de poulains vivants (probabilité moyenne prédite de produire un poulain vivant) chez les juments poulinières au Royaume‐Uni. MODÈLE D’ÉTUDE: Étude de cohorte rétrospective. MÉTHODES: Les résultats de cytologie et bactériologie endométriales ont été retrouvés parmi les dossiers de laboratoire pour tous les derniers écouvillons soumis entre 2014 et 2020. Le statut des juments, leur âge et leur succès au poulinage out été recueillis à partir de données disponibles au public. Les taux de poulains vivants ont été estimés pour les catégories de résultats cytologiques et bactériologiques en utilisant un modèle de régression linéaire avec jument et ferme identifiés comme effets aléatoires et en ajustant pour le statut des juments, leur âge, le nombre d’écouvillons soumis précédemment dans la saison de reproduction et toutes interactions. Des comparaisons en paires avec correction de Bonferroni ont été utilisées pour évaluer les différences entre les catégories pour les taux de poulains vivants. RÉSULTATS: Les données de 7691 écouvillons provenant de 3579 juments sur 196 fermes ont été retrouvées. Au contraire des autres catégories de souches, les juments avec croissance profuse d’Escherichia coli avait un taux de poulains vivants significativement inférieur (P = 0.005, 59.1%; 95% confidence interval (CI) 43.7–74.5) comparativement aux juments avec aucune croissance (80.9%; 95% CI 79.2–82.6). Une interaction entre l’âge des juments et la cytologie a été identifiée. Pour les juments de plus de 12 ans, des réductions significatives du taux de poulains vivants ont été observées entre les juments ayant plus de 30% de cellules endométriales polymorphonucléaires/haut champs à l’examen cytologique et celles avec ≤0.5% PMN, un résultat absent chez les juments de ≤12 ans. Unassigned: Utilisation d’écouvillon sans gaine protectrice et absence d’information clinique. Conclusions: Les résultats suggèrent certaines complexités lors d’interprétation des résultats de cytologie pour les écouvillons endométriaux et une sous‐population de juments avec croissance profuse d’Escherichia coli pour qui des incompréhensions persistent quant aux étiologies potentielles pour leur fertilité réduite.
© 2025 The Author(s). Equine Veterinary Journal published by John Wiley & Sons Ltd on behalf of EVJ Ltd.
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Publication Date: 2025-09-01 PubMed ID: 40888035PubMed Central: PMC12892376DOI: 10.1111/evj.70086Google 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

  • This study investigated how findings from endometrial swabs—specifically cytology and bacteriology—relate to the likelihood of Thoroughbred broodmares in the UK producing live foals.
  • The research analyzed data from thousands of mares to identify which infections or cellular changes might predict reduced fertility, thereby informing veterinary clinical decisions.

Background and Objective

  • Thoroughbred broodmare fertility is critical for the horse breeding industry, yet the relationship between uterine bacterial infections or inflammatory cell presence (detected via endometrial swabs before breeding) and reproductive success was previously unclear in UK horses.
  • The objective was to explore associations between last endometrial swab results within the breeding season and live foal rates, measured as the predicted probability of producing a live foal.

Methods

  • Study Design: Retrospective cohort study analyzing existing clinical laboratory and breeding data.
  • Data Collection:
    • Endometrial cytology (cell types and counts) and bacteriology (type and abundance of bacteria grown) were extracted from lab records covering swabs taken between 2014 and 2020.
    • Mares’ status, age, and foaling outcomes were gathered from public databases related to Thoroughbred horses.
    • The “last swab” of each mare within a breeding season was the focus to assess her endometrial environment closest to breeding.
  • Statistical Analysis:
    • A multivariable logistic regression model predicted live-foal rates according to cytology and bacteriology results.
    • Mare and farm effects were treated as random effects to account for individual and environmental variability.
    • The model adjusted for confounders such as mare’s age, reproductive status, number of swabs taken that season, and interaction effects (e.g., age by cytology).
    • Pairwise comparisons between categories were evaluated using Bonferroni correction to reduce false-positive findings.

Results

  • Data included 7691 last swabs from 3579 mares across 196 farms, providing a large and diverse sample.
  • Bacterial Findings:
    • Mares with profuse growth of Escherichia coli on swabs had significantly lower live-foal rates (59.1%; 95% CI: 43.7–74.5%) than mares with no bacterial growth (80.9%; 95% CI: 79.2–82.6%).
    • This indicates a particular negative association of heavy E. coli infection with fertility compared to other bacteria or no growth.
  • Cytology Findings and Age Interaction:
    • Cytology measured the percentage of polymorphonuclear cells (PMNs), which are inflammatory cells indicating uterine inflammation.
    • In mares older than 12 years, those with more than 30% PMNs in endometrial cells had significantly reduced live-foal rates compared to mares with ≤0.5% PMNs.
    • This effect was not seen in younger mares (12 years or younger), suggesting that age modifies the impact of uterine inflammation on fertility.
  • Other bacterial isolates did not show significant associations with live-foal rates, highlighting the unique effect of heavy E. coli infection.

Conclusions and Clinical Implications

  • The study highlights the complexity in interpreting endometrial cytology results, especially considering mare age and infection status.
  • Heavy E. coli growth might be an important marker for poorer fertility outcomes, but further research is needed to understand the underlying causes and whether this represents active infection, contamination, or other pathological mechanisms.
  • The interaction of inflammation markers with age suggests that older mares may be more susceptible to the negative reproductive effects of uterine inflammation.
  • Veterinary practitioners could use these findings to better diagnose and manage fertility problems in broodmares by focusing on specific bacterial infections and inflammatory changes.
  • Limitations include the use of unguarded swabs (which may be prone to contamination) and lack of detailed clinical data on mare health or treatment, which may affect the interpretation of results.
  • Overall, this research supports targeted fertility assessments and tailored interventions in Thoroughbred broodmares to improve live foal rates.

Cite This Article

APA
Fehin B, Scott CJ, Arango-Sabogal JC, de Mestre AM, Mouncey R. (2025). Associations between endometrial swab bacteriology and cytology findings and live foal rates in Thoroughbred broodmares in the United Kingdom. Equine Vet J, 58(2), 348-358. https://doi.org/10.1111/evj.70086

Publication

Equine veterinary journal
ISSN: 2042-3306
NlmUniqueID: 0173320
Country: United States
Language: English
Volume: 58
Issue: 2
Pages: 348-358

Researcher Affiliations

Fehin, Billy
  • Rossdales Veterinary Surgeons, Beaufort Cottage Stables, Suffolk, UK.
Scott, Camilla J
  • Rossdales Veterinary Surgeons, Beaufort Cottage Stables, Suffolk, UK.
Arango-Sabogal, Juan Carlos
  • Département de pathologie et microbiologie, Faculté de médecine vétérinaire, Université de Montréal, Saint-Hyacinthe, Québec, Canada.
  • Department of Pathobiology and Population Sciences, The Royal Veterinary College, University of London, Hertfordshire, UK.
de Mestre, Amanda M
  • Baker Institute for Animal Health, College of Veterinary Medicine, Cornell University, Ithaca, New York, USA.
Mouncey, Rebecca
  • Department of Pathobiology and Population Sciences, The Royal Veterinary College, University of London, Hertfordshire, UK.

MeSH Terms

  • Animals
  • Horses
  • Female
  • Horse Diseases / microbiology
  • Horse Diseases / epidemiology
  • Retrospective Studies
  • United Kingdom / epidemiology
  • Endometrium / microbiology
  • Endometrium / cytology
  • Pregnancy
  • Cohort Studies

Conflict of Interest Statement

The authors declare no conflicts of interest.

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