FREE SHIPPING over $40
OVER 10000 FIVE-STAR REVIEWS
5% OFF ALL SUBSCRIPTIONS
100% HAPPINESS GUARANTEE
Analyze Diet
0
PloS one2026; 21(8); e0356822; doi: 10.1371/journal.pone.0356822

Evidence-based interpretation of uterine cultures in mares: Linking bacterial growth patterns to endometrial inflammation.

Abstract: We evaluated uterine bacterial growth patterns and their association with endometrial inflammation (EI) in equine samples collected using uterine swabs (US) or low-volume uterine lavage (UL). A database including 1,545 US and 2,066 UL with cytological and bacteriological results was retrospectively analysed. Endometrial inflammation was defined as ≥2 polymorphonuclear cells per high-power field, and bacterial culture was considered positive when aerobic growth occurred within 48 h, with the number of bacterial isolates per sample recorded (0, 1, 2, or ≥3). Data were analysed using generalized mixed-effects models including the number of isolates and bacterial species as fixed effects. The prevalence of EI was 4.8% for US and 36.6% for UL. For both techniques, EI prevalence was lower in samples with negative bacterial cultures (1.0 ± 0.3% for US and 10.7 ± 1.7% for UL) compared with samples yielding ≥1 bacterial isolate. In UL samples, isolation of a single bacterial species was associated with greater probability of EI (45.1 ± 2.6%) compared with samples yielding ≥3 isolates (33.3 ± 3.1%). For UL, the presence of Streptococcus sp. (β-haemolytic) and Staphylococcus aureus increased the probability of EI compared with their absence, whereas in US samples only Streptococcus sp. (β-haemolytic) increased EI probability. In conclusion, bacterial growth increased the likelihood of EI for both sampling techniques, with Streptococcus sp. (β-haemolytic) as the primary bacteria associated with evidence of EI. The integration of endometrial cytology with bacterial culture and pathogen identification improves interpretation of bacteriological findings and supports responsible antimicrobial use in mares.
Copyright: © 2026 Yáñez et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Get Full Text
Publication Date: 2026-08-25 PubMed ID: 42640952PubMed Central: PMC13505902DOI: 10.1371/journal.pone.0356822Google Scholar: Lookup
The Equine Research Bank provides access to a large database of publicly available scientific literature. Inclusion in the Research Bank does not imply endorsement of study methods or findings by Mad Barn.
LinkedInCopy
  • Journal Article

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 different patterns of bacterial growth in the uterus of mares relate to inflammation of the endometrium (the uterine lining).
  • The research compared two sampling methods and linked specific bacterial infections to the presence of inflammation, aiming to improve interpretation of uterine cultures and promote responsible antibiotic use.

Background and Purpose

  • Endometrial inflammation (EI) in mares can negatively affect fertility and reproductive health.
  • Bacterial infections are a common cause of EI, but interpreting uterine bacterial cultures is challenging due to variability in sampling and bacterial growth patterns.
  • The research aimed to analyze the relationship between bacterial growth patterns from uterine samples and the presence of EI to guide better diagnosis and treatment.

Methods

  • Two uterine sampling methods were used on mares: uterine swabs (US) and low-volume uterine lavage (UL).
  • A large retrospective database was used comprising:
    • 1,545 uterine swab samples (US)
    • 2,066 uterine lavage samples (UL)
  • For each sample, cytology was performed to detect inflammation; EI was defined as having ≥2 polymorphonuclear cells per high-power field.
  • Bacterial cultures were considered positive if aerobic bacteria grew within 48 hours.
  • The number of bacterial isolates per sample was recorded: 0, 1, 2, or ≥3 different bacteria.
  • Statistical analysis involved generalized mixed-effects models assessing the impact of number and species of bacterial isolates on EI presence.

Key Findings

  • Prevalence of endometrial inflammation differed by sampling method:
    • 4.8% prevalence in uterine swab (US) samples
    • 36.6% prevalence in uterine lavage (UL) samples
  • Samples without bacterial growth had lower EI prevalence:
    • US: about 1.0% EI in culture-negative samples
    • UL: about 10.7% EI in culture-negative samples
  • Presence of bacterial growth increased likelihood of inflammation:
    • For UL samples, isolating a single bacterial species was more strongly associated with EI (approx. 45.1%) compared to samples with ≥3 different isolates (approx. 33.3%).
  • Specific bacteria showed stronger links to inflammation:
    • In UL samples, β-haemolytic Streptococcus species and Staphylococcus aureus significantly increased EI probability.
    • In US samples, only β-haemolytic Streptococcus significantly increased EI risk.

Interpretation and Implications

  • Bacterial growth in the uterus is associated with endometrial inflammation regardless of sampling technique.
  • The finding that β-haemolytic Streptococcus species are the primary bacteria linked with EI informs veterinarians to focus on these pathogens when interpreting culture results.
  • The higher inflammation rate in UL samples may be due to this method’s ability to collect a more comprehensive sample of the uterine environment compared to swabs.
  • Samples with multiple bacterial species were less likely to be associated with severe inflammation compared to samples with a single predominant pathogen, which could reflect contamination or non-pathogenic colonization in polymicrobial cultures.
  • Combining cytological analysis (detecting inflammatory cells) with bacteriological culture and bacterial species identification improves clinical interpretation by distinguishing true infections from contaminations or non-pathogenic flora.
  • Improved interpretation supports responsible use of antimicrobials by targeting treatment only when there is evidence of bacterial infection linked to inflammation, potentially reducing unnecessary antibiotic administration.

Conclusion

  • This study provides evidence-based guidance for interpreting uterine bacterial cultures in mares.
  • It highlights the importance of sampling method, bacterial growth patterns, and pathogen identification in assessing endometrial inflammation and making informed treatment decisions.
  • The integration of cytology and bacteriology aids in the responsible application of antimicrobials in equine reproductive management.

Cite This Article

APA
Yáñez U, Krupa M, Gibbons J, Storme J, Lewis N, Pascottini OB. (2026). Evidence-based interpretation of uterine cultures in mares: Linking bacterial growth patterns to endometrial inflammation. PLoS One, 21(8), e0356822. https://doi.org/10.1371/journal.pone.0356822

Publication

PloS one
ISSN: 1932-6203
NlmUniqueID: 101285081
Country: United States
Language: English
Volume: 21
Issue: 8
Pages: e0356822
PII: e0356822

Researcher Affiliations

Yáñez, Uxía
  • School of Veterinary Medicine, University College Dublin, Belfield, Dublin, Ireland.
  • Unit of Reproduction and Obstetrics, Department of Animal Pathology, Faculty of Veterinary Medicine, Universidade de Santiago de Compostela, Lugo, Spain.
Krupa, Milena
  • School of Veterinary Medicine, University College Dublin, Belfield, Dublin, Ireland.
Gibbons, James
  • School of Veterinary Medicine, University College Dublin, Belfield, Dublin, Ireland.
  • Irish Equine Centre, Johnstown, Naas, Co. Kildare, Ireland.
Storme, Julie
  • Waterside Equine Repro Services, Co. Meath, Ireland.
Lewis, Niamh
  • School of Veterinary Medicine, University College Dublin, Belfield, Dublin, Ireland.
Pascottini, Osvaldo Bogado
  • School of Veterinary Medicine, University College Dublin, Belfield, Dublin, Ireland.

MeSH Terms

  • Animals
  • Female
  • Horses
  • Uterus / microbiology
  • Uterus / pathology
  • Endometritis / microbiology
  • Endometritis / veterinary
  • Endometritis / pathology
  • Horse Diseases / microbiology
  • Endometrium / microbiology
  • Endometrium / pathology
  • Bacteria / growth & development
  • Bacteria / isolation & purification
  • Staphylococcus aureus / isolation & purification

Conflict of Interest Statement

I have read the journal’s policy, and the authors of this manuscript have the following competing interests: James Gibbons was employed as a veterinary microbiologist at the Irish Equine Centre during the period of data collection. Julie Storme works for Waterside Equine Repro Services. This does not alter our adherence to PLOS ONE policies on sharing data and materials. Both institutions did not have a role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. The specific roles of these authors are articulated in the ‘author contributions’ section. The remaining authors have declared that no competing interests exist.

References

This article includes 42 references
  1. LeBlanc MM. Advances in the diagnosis and treatment of chronic infectious and post-mating-induced endometritis in the mare.. Reprod Domest Anim 2010;45 Suppl 2:21–7.
    doi: 10.1111/j.1439-0531.2010.01634.xpubmed: 20591061google scholar: lookup
  2. Samper JC. Anatomy and physiology of the mare.. In: Samper JC, editor. Equine breeding management and artificial insemination. St. Louis: Saunders Elsevier; 2009. 117–8.
  3. Diel de Amorim M, Gartley CJ, Foster RA, Hill A, Scholtz EL, Hayes A. Comparison of clinical signs, endometrial culture, endometrial cytology, uterine low-volume lavage, and uterine biopsy and combinations in the diagnosis of equine endometritis.. J Equine Vet Sci 2016;44:54–61.
    doi: 10.1016/j.jevs.2015.10.012google scholar: lookup
  4. Christoffersen M, Troedsson M. Inflammation and fertility in the mare.. Reproduction in Domestic Animals 2017;52: 14–20.
    doi: 10.1111/rda.13013pubmed: 28815848google scholar: lookup
  5. H A Morris L, M McCue P, Aurich C. Equine endometritis: a review of challenges and new approaches.. Reproduction 2020;160(5):R95–110.
    doi: 10.1530/REP-19-0478pubmed: 32805710google scholar: lookup
  6. Scholtz M, Guthrie AJ, Newton R, Schulman ML. Review of and as venereal pathogens in horses.. Equine Vet J 2025;57(3):587–97.
    doi: 10.1111/evj.14201pmc: PMC11982429pubmed: 39103748google scholar: lookup
  7. Prete CD, Nocera FP, Piegari G, Palumbo V, Martino LD, Cocchia N. Use of cytobrush for bacteriological and cytological diagnosis of endometritis in mares.. Vet World 2024;17(2):398–406.
    doi: 10.14202/vetworld.2024.398-406pmc: PMC11000473pubmed: 38595673google scholar: lookup
  8. Overbeck W, Witte TS, Heuwieser W. Comparison of three diagnostic methods to identify subclinical endometritis in mares.. Theriogenology 2011;75(7):1311–8.
    doi: 10.1016/j.theriogenology.2010.12.002pubmed: 21251703google scholar: lookup
  9. Aguilar J, Hanks M, Shaw DJ, Else R, Watson E. Importance of using guarded techniques for the preparation of endometrial cytology smears in mares.. Theriogenology 2006;66(2):423–30.
    doi: 10.1016/j.theriogenology.2005.12.007pubmed: 16427691google scholar: lookup
  10. Cocchia N, Paciello O, Auletta L, Uccello V, Silvestro L, Mallardo K. Comparison of the cytobrush, cottonswab, and low-volume uterine flush techniques to evaluate endometrial cytology for diagnosing endometritis in chronically infertile mares.. Theriogenology 2012;77(1):89–98.
    doi: 10.1016/j.theriogenology.2011.07.020pubmed: 21855980google scholar: lookup
  11. European Union. Regulation (EU) 2019/6 of the European Parliament and of the Council of 11 December 2018 on veterinary medicinal products and repealing Directive 2001/82/EC.. 2019.
  12. Walter J, Neuberg KP, Failing K, Wehrend A. Cytological diagnosis of endometritis in the mare: investigations of sampling techniques and relation to bacteriological results.. Anim Reprod Sci 2012;132(3–4):178–86.
    doi: 10.1016/j.anireprosci.2012.05.012pubmed: 22727031google scholar: lookup
  13. Katila T. Evaluation of diagnostic methods in equine endometritis.. Reprod Biol 2016;16(3):189–96.
    doi: 10.1016/j.repbio.2016.06.002pubmed: 27692361google scholar: lookup
  14. Bohn AA, Ferris RA, McCue PM. Comparison of equine endometrial cytology samples collected with uterine swab, uterine brush, and low-volume lavage from healthy mares.. Vet Clin Pathol 2014;43(4):594–600.
    doi: 10.1111/vcp.12194pubmed: 25208767google scholar: lookup
  15. . Prevalence of microbial isolates cultured from endometrial swab samples collected from United kingdom thoroughbred mares from 2014 to 2020.. Vet Sci 2024;11(2):82.
    doi: 10.3390/vetsci11020082pmc: PMC10891641pubmed: 38393100google scholar: lookup
  16. Davies Morel MCG, Lawlor O, Nash DM. Equine endometrial cytology and bacteriology: effectiveness for predicting live foaling rates.. Vet J 2013;198(1):206–11.
    doi: 10.1016/j.tvjl.2013.08.002pubmed: 24035467google scholar: lookup
  17. Davis HA, Stanton MB, Thungrat K, Boothe DM. Uterine bacterial isolates from mares and their resistance to antimicrobials: 8,296 cases (2003-2008).. J Am Vet Med Assoc 2013;242(7):977–83.
    doi: 10.2460/javma.242.7.977pubmed: 23517211google scholar: lookup
  18. Christoffersen M, Söderlind M, Rudefalk SR, Pedersen HG, Allen J, Krekeler N. Risk factors associated with uterine fluid after breeding caused by .. Theriogenology 2015;84(8):1283–90.
    doi: 10.1016/j.theriogenology.2015.07.007pubmed: 26300275google scholar: lookup
  19. Pascottini OB, Aurich C, England G, Grahofer A. General and comparative aspects of endometritis in domestic species: a review.. Reprod Domest Anim 2023;58 Suppl 2:49–71.
    doi: 10.1111/rda.14390pubmed: 37191856google scholar: lookup
  20. Riddle WT, LeBlanc MM, Stromberg AJ. Relationships between uterine culture, cytology and pregnancy rates in a Thoroughbred practice.. Theriogenology 2007;68(3):395–402.
    doi: 10.1016/j.theriogenology.2007.05.050pubmed: 17583785google scholar: lookup
  21. Gil-Miranda A, Macnicol J, Orellana-Guerrero D, Samper JC, Gomez DE. Reproductive Tract Microbiota of Mares.. Vet Sci 2024;11:324.
    doi: 10.3390/vetsci11070324pmc: PMC11281493pubmed: 39058008google scholar: lookup
  22. Donato GG. New insights into the diagnosis and treatment of endometritis in the mare.. .
  23. Casagrande Proietti P, Bietta A, Coppola G, Felicetti M, Cook RF, Coletti M. Isolation and characterization of β-haemolytic-Streptococci from endometritis in mares.. Vet Microbiol 2011;152(1–2):126–30.
    doi: 10.1016/j.vetmic.2011.04.009pubmed: 21570219google scholar: lookup
  24. Ravaioli V, Raffini E, Tamburini M, Galletti G, Frasnelli M. Infectious endometritis in mares: microbiological findings in field samples.. J Equine Vet Sci 2022;112:103913.
    doi: 10.1016/j.jevs.2022.103913pubmed: 35196546google scholar: lookup
  25. Rasmussen CD, Haugaard MM, Petersen MR, Nielsen JM, Pedersen HG, Bojesen AM. Streptococcus equi subsp. zooepidemicus isolates from equine infectious endometritis belong to a distinct genetic group.. Vet Res 2013;44(1):26.
    doi: 10.1186/1297-9716-44-26pmc: PMC3640914pubmed: 23597033google scholar: lookup
  26. Causey RC. Making sense of equine uterine infections: the many faces of physical clearance.. Vet J 2006;172(3):405–21.
    doi: 10.1016/j.tvjl.2005.08.005pubmed: 16169264google scholar: lookup
  27. Yi L, Wang Y, Ma Z, Zhang H, Li Y, Zheng J. Biofilm formation of Streptococcus equi ssp. zooepidemicus and comparative proteomic analysis of biofilm and planktonic cells.. Curr Microbiol 2014;69(3):227–33.
    doi: 10.1007/s00284-014-0574-zpubmed: 24696150google scholar: lookup
  28. Díaz-Bertrana ML, Deleuze S, Pitti Rios L, Yeste M, Morales Fariña I, Rivera Del Alamo MM. Microbial prevalence and antimicrobial sensitivity in equine endometritis in field conditions.. Animals (Basel) 2021;11(5):1476.
    doi: 10.3390/ani11051476pmc: PMC8160901pubmed: 34065566google scholar: lookup
  29. Blanc FC, Dombrowski ISA, de Oliveira NF, da Silva SMF, Moreira JC, Santos CO. Antimicrobial and antibiofilm in vitro effect of oxygen-ozone gas against .. Res Vet Sci 2025;194:105814.
    doi: 10.1016/j.rvsc.2025.105814pubmed: 40749316google scholar: lookup
  30. Moormeier DE, Bayles KW. biofilm: a complex developmental organism.. Mol Microbiol 2017;104(3):365–76.
    doi: 10.1111/mmi.13634pmc: PMC5397344pubmed: 28142193google scholar: lookup
  31. Zhao J-L, Ding Y-X, Zhao H-X, He X-L, Li P-F, Li Z-F. Presence of superantigen genes and antimicrobial resistance in isolates obtained from the uteri of dairy cows with clinical endometritis. Vet Rec 2014;175(14):352.
    doi: 10.1136/vr.102302pubmed: 24989035google scholar: lookup
  32. Nocera FP, Maurizi L, Masullo A, Nicoletti M, Conte AL, Brunetti F. Genotypic and phenotypic characterization of Escherichia coli isolates recovered from the uterus of mares with fertility problems. Animals (Basel) 2023;13(10):1639.
    doi: 10.3390/ani13101639pmc: PMC10215218pubmed: 37238068google scholar: lookup
  33. Bicalho RC, Machado VS, Bicalho MLS, Gilbert RO, Teixeira AGV, Caixeta LS. Molecular and epidemiological characterization of bovine intrauterine Escherichia coli. J Dairy Sci 2010;93(12):5818–30.
    doi: 10.3168/jds.2010-3550pubmed: 21094754google scholar: lookup
  34. Barba M, Martínez-Boví R, Quereda JJ, Mocé ML, Plaza-Dávila M, Jiménez-Trigos E. Vaginal microbiota is stable throughout the estrous cycle in Arabian Maress. Animals (Basel) 2020;10(11):2020.
    doi: 10.3390/ani10112020pmc: PMC7692283pubmed: 33153053google scholar: lookup
  35. Virendra A, Gulavane SU, Ahmed ZA, Reddy R, Chaudhari RJ, Gaikwad SM. Metagenomic analysis unravels novel taxonomic differences in the uterine microbiome between healthy mares and mares with endometritis. Vet Med Sci 2024;10(2):e1369.
    doi: 10.1002/vms3.1369pmc: PMC10867593pubmed: 38357732google scholar: lookup
  36. Guo L, Holyoak GR, DeSilva U. Endometrial microbiome in mares with and without clinical endometritis. Front Vet Sci 2025;12:1588432.
    doi: 10.3389/fvets.2025.1588432pmc: PMC12355929pubmed: 40822659google scholar: lookup
  37. World Health Organization. WHO bacterial priority pathogens list, 2024: Bacterial pathogens of public health importance to guide research, development and strategies to prevent and control antimicrobial resistance. 2024.
    pmc: PMC12367593pubmed: 40245910
  38. Canisso IF, Segabinazzi LGTM, Fedorka CE. Persistent breeding-induced endometritis in mares - a multifaceted challenge: from clinical aspects to immunopathogenesis and pathobiology. Int J Mol Sci 2020;21(4):1432.
    doi: 10.3390/ijms21041432pmc: PMC7073041pubmed: 32093296google scholar: lookup
  39. Friso AM, Segabinazzi LGTM, Cyrino M, Correal SB, Freitas-Dell’Aqua CP, Teoro do Carmo M. Periovulatory administration of firocoxib did not alter ovulation rates and mitigated post-breeding inflammatory response in mares. Theriogenology 2019;138:24–30.
    doi: 10.1016/j.theriogenology.2019.06.045pubmed: 31280182google scholar: lookup
  40. Liu IKM, Troedsson MHT. The diagnosis and treatment of endometritis in the mare: yesterday and today. Theriogenology 2008;70(3):415–20.
    doi: 10.1016/j.theriogenology.2008.05.040pubmed: 18513792google scholar: lookup
  41. Del Prete C, Montano C, Cocchia N, de Chiara M, Gasparrini B, Pasolini MP. Use of regenerative medicine in the treatment of endometritis in mares: a systematic review and meta-analysis. Theriogenology 2024;227:9–20.
    doi: 10.1016/j.theriogenology.2024.07.006pubmed: 38991434google scholar: lookup
  42. Silva JA, Castañares M, Mouguelar H, Valenciano JA, Pellegrino MS. Isolation of lactic acid bacteria from the reproductive tract of mares as potentially beneficial strains to prevent equine endometritis. Vet Res Commun 2024;48(3):1353–66.
    doi: 10.1007/s11259-024-10295-2pubmed: 38233700google scholar: lookup

Citations

This article has been cited 0 times.
Subscribe to our newsletter
Join 106,644 horse owners like you who receive our email updates on horse health and nutrition.