Endometritis: What the Research Says
Evidence from 29 peer-reviewed studies
What Professionals Should Know
- •Pre-breeding screening for P. aeruginosa and K. pneumoniae may need reassessment, as evidence for venereal transmission is lacking; current protocols may not reflect true disease risk
- •Environmental contamination (water, equipment, fomites) should be considered as primary transmission sources when these organisms are isolated from breeding stock
- •Isolation of these bacteria from genital swabs should not automatically be interpreted as a venereal disease risk; further investigation of transmission routes is needed before excluding horses from breeding
- •NGS can enhance detection of fungal and bacterial pathogens in equine endometritis cases where culture results are negative or inconclusive, potentially improving treatment outcomes
- •While NGS shows good correlation with culture (74-80% agreement), it should be considered complementary to rather than a direct replacement for culture in diagnostic protocols
- •NGS may identify polymicrobial infections and organisms that culture methods miss, providing more comprehensive pathogen profiles for treatment planning in subfertile mares
- •Endometritis in German mares remains highly responsive to traditional antibiotics, particularly penicillins for streptococci and gentamicin for E. coli, supporting current treatment protocols
- •Multi-drug resistant bacteria are relatively uncommon (3.1% of infections), suggesting routine culture and sensitivity testing may help optimize antibiotic selection without widespread MDR concerns
- •Continued surveillance of antibiotic resistance patterns is warranted to maintain the currently favorable situation and guide prudent antimicrobial stewardship in equine reproduction
- •Returning flushed embryos to donor mares immediately after collection is contraindicated—the procedure causes luteal dysfunction, endometritis, and complete pregnancy failure in this study population
- •Embryo flushing itself may damage embryos regardless of reinsertion strategy; embryos showed significantly higher degeneration rates when collected 24 hours post-transfer compared to day 8 collection
- •Use recipient mares for embryo transfer rather than attempting same-mare reinsertion to maximize pregnancy success rates (75% vs 0%)
- •Serological and uterine fluid biomarkers (particularly FRAP, thiol levels, NOx, and protease activity) may improve endometritis detection, though cytology appears more discriminatory than culture alone
- •Despite biomarker differences between endometritis-positive and -negative mares, pregnancy outcomes were similar, suggesting endometritis detection alone may not predict fertility outcomes in AI programs
- •Current diagnostic approaches for endometritis (cytology, culture) should be interpreted cautiously when predicting breeding success, as biomarker presence does not necessarily correlate with reduced fertility
- •When treating suspected endometritis in UAE-based mares, be aware that P. aeruginosa and K. pneumoniae are more prevalent locally than in other regions; avoid relying solely on trimethoprim-sulphonamide antibiotics due to high resistance rates
- •Culture and sensitivity testing is critical for guiding treatment decisions, as antibiotic resistance patterns vary significantly by bacterial species and geographic location
- •Amikacin and amoxicillin with clavulanic acid demonstrated better susceptibility profiles in this population and may be preferred empirical choices pending culture results
- •Intrauterine ozone gas therapy is an effective, low-cost alternative to conventional treatments for endometritis in mares with documented uterine infection
- •O3-gas treatment achieves superior results compared to simple lavage alone, reducing both inflammatory cell counts and positive bacterial cultures significantly within a single treatment cycle
- •Consider ozone therapy as a viable option for mares with poor response to standard antimicrobial protocols or in resource-limited settings where antimicrobials are unavailable
- •Culture and sensitivity testing are essential before treating equine endometritis, as β-lactam antimicrobials show poor efficacy against prevalent organisms
- •Avoid empirical penicillin therapy; amikacin, cefoxitin, or gentamicin are more likely to be effective based on this prevalence data
- •Nearly 9 in 10 subfertile mares with endometrial cytology findings will have positive bacterial cultures, justifying microbiological investigation in repetitively infertile mares
- •Leukocyte esterase strips can be used as a quick stall-side screening tool to confirm suspected endometritis (high specificity), but negative results cannot exclude the disease and require follow-up cytology or culture
- •Use a cutoff point of trace or greater on the strip to optimize sensitivity-specificity balance for clinical decision-making
- •Combine LET with endometrial cytology rather than relying on culture results for more reliable endometritis diagnosis during breeding management
- •Streptomycin is no longer a reliable treatment option for contagious equine metritis in Czech populations; alternative antibiotic protocols should be reviewed and implemented
- •Continued antibiotic surveillance and genomic monitoring of T. equigenitalis is essential to track emerging resistance patterns and inform treatment recommendations
- •Mare screening protocols should remain rigorous as this pathogen continues to evolve; consider regional resistance patterns when choosing prophylactic or therapeutic approaches
- •These metabolite biomarkers could potentially provide a non-invasive blood test for diagnosing endometritis in mares, reducing reliance on endoscopic examination
- •Understanding the metabolic signature of endometritis (particularly in tryptophan and lipid metabolism) may inform future therapeutic approaches targeting inflammatory pathways
- •The identified metabolite panel warrants validation in larger populations before clinical implementation as a diagnostic tool
- •MPO presence in uterine fluid does not automatically indicate endometritis; baseline MPO levels vary with estrous cycle phase in healthy mares
- •Detection of MPO should be interpreted alongside other clinical and cytological findings when evaluating mare fertility or suspected endometrial disease
- •Understanding physiological MPO expression patterns may improve diagnostic accuracy in distinguishing inflammatory from non-inflammatory uterine conditions
- •This research establishes a validated laboratory model for testing anti-inflammatory and antibiotic therapies for equine endometritis, potentially accelerating development of clinical treatments
- •The activation of NF-κB pathway in endometritis suggests that dual targeting of bacterial load and inflammatory signaling may be beneficial in clinical treatment strategies
- •Practitioners should monitor elevated systemic leukocytes and uterine changes as key indicators of endometritis severity, as these correlate with inflammatory cytokine levels
- •Negative bacterial cultures in mares with endometritis do not rule out bacterial involvement; dormant bacteria and biofilms may persist despite negative results
- •E. coli-associated endometritis may involve surface biofilm rather than tissue invasion, suggesting traditional culture and histology may not detect the full extent of bacterial involvement
- •Current diagnostic and treatment approaches for endometritis may need revision to account for biofilm-forming bacteria that evade standard detection methods
- •Fluorescence spectroscopy of endometrial cells offers a quantitative diagnostic method for endometritis that could improve breeding soundness evaluations in mares before insemination
- •The assay's relatively low false negative rate (11.5%) makes it potentially useful as a screening tool, though interpretation should be combined with clinical findings given the 7.7% false positive rate
- •This technique measures immune cell activation rather than just pathogen presence, potentially identifying inflammation that may impair fertility even in culture-negative cases
- •Extracellular vesicle profiling (size, quantity, marker expression) may offer a novel diagnostic biomarker for endometritis that could supplement or potentially replace traditional diagnostic methods
- •This approach could enable earlier detection of subclinical endometritis, allowing earlier intervention during breeding season when fertility is critical
- •Further validation is needed before clinical application, but this represents a promising non-invasive or minimally-invasive diagnostic option
- •Do not rely solely on chromogenic agar plates for definitive identification of endometritis-causing bacteria in mares; additional confirmatory testing is warranted
- •Clinical laboratories should be aware that commercial rapid identification systems may produce inaccurate results for equine endometritis cases
- •Consider conventional culture and identification methods or molecular techniques alongside chromogenic agar when diagnosing bacterial endometritis
- •Endometritis involves oxidative stress and depletion of antioxidant systems; consider monitoring inflammatory markers in subfertile mares
- •Antioxidant supplementation may warrant investigation as an adjunctive therapy for mares with endometritis
- •Understanding the oxidative stress mechanism in endometritis could inform treatment strategies beyond standard antibiotic/anti-inflammatory protocols
- •Blood-based biomarkers could enable rapid screening for endometritis without invasive procedures, particularly valuable for identifying subclinical disease in breeding herds
- •A reliable non-invasive test would reduce diagnostic time and costs associated with case-by-case clinical examinations on large farms
- •Early detection through biomarker screening could help prevent reproductive complications and economic losses from endometritis in breeding mares
- •This uterine sanitizer shows promise as an adjunctive therapy for mares with endometritis, particularly those resistant to standard treatments, but clinical trials are needed before recommending it in practice.
- •The product is compatible with dilution of standard antibiotics without degradation, offering potential for combined antimicrobial therapy protocols.
- •Clinical efficacy and safety in living mares have not yet been demonstrated, so use should remain cautious and evidence-based once in vivo studies are completed.
- •Older, multiparous mares require targeted screening for endometritis due to increased susceptibility from age-related immune dysfunction and uterine clearance problems
- •Consider antimicrobial peptide expression as a diagnostic marker for detecting subclinical endometritis in breeding mares to improve fertility outcomes
- •Implement preventive management strategies for older mares to support uterine health and clearance mechanisms post-breeding
- •For subfertile mares, combine clinical examination (transrectal palpation and ultrasound) with laboratory confirmation using commercial cytobrush/swab collection and microbiological culture rather than relying on any single diagnostic method.
- •Endometrial histology alone is insufficient for endometritis diagnosis; microbiological culture should be prioritized as it shows superior sensitivity and specificity.
- •The commercial cytobrush/swab collector is the most practical sample collection technique for field use, making it suitable for routine diagnostic protocols in subfertile mare evaluations.
- •Poor pregnancy rates following frozen-thawed semen AI in donkeys may involve different immune mechanisms than in horses and cattle, suggesting species-specific handling protocols are needed
- •Understanding that seminal plasma components drive immune responses in donkeys could lead to improved semen processing or uterine management strategies for assisted reproduction
- •NETs may not be the primary factor limiting donkey fertility post-AI, warranting investigation of alternative sperm-immune interactions specific to this species
- •Elastase inhibition via sivelestat could represent a novel therapeutic strategy to reduce pathological collagen accumulation and fibrosis in equine endometritis cases
- •The cyclical differences between follicular and luteal phases suggest that treatment timing relative to reproductive cycle phase may influence therapeutic efficacy
- •Further in vivo studies are needed to translate these in vitro findings into clinical recommendations for mares with endometrial disease
- •Plan breeding programmes with awareness of natural seasonal reproductive patterns rather than relying solely on artificial breeding season definitions to optimise fertility outcomes
- •Screen mares for endometritis and monitor for multiple ovulation as these are preventable causes of reduced reproductive efficiency
- •Consider psychological and behavioural factors alongside physical pathology when investigating cycling abnormalities, as stress and behaviour influence reproductive performance
- •Endometrial disease is a major cause of subfertility in mares with significant economic impact on breeding operations; subclinical forms require biopsy for diagnosis
- •Glandular differentiation assessment must account for seasonal variation in the equine endometrium
- •Emerging molecular biomarkers may improve diagnostic accuracy and treatment decisions, though further validation for clinical use is needed
- •Cooling PRP to 4°C preserves platelet counts better than cryopreservation at -196°C for 14-day storage, potentially enabling wider distribution of PRP for field use
- •Even with storage, PRP retains significant growth factor release capacity (26% of fresh levels), suggesting clinical utility beyond immediate on-site application
- •DMSO and trehalose show promise as additives to improve PRP storage viability, which could expand clinical applications for treating bone, joint, tendon and other equine injuries
- •Recognize that post-breeding inflammation is normal and beneficial in most mares; focus management efforts on identifying and monitoring the susceptible subpopulation at risk for persistent endometritis
- •Current understanding of inflammatory pathways is still developing—work closely with your reproduction specialist to implement evidence-based treatments and remain alert for emerging therapeutic options targeting specific disease mechanisms
- •Consider endometrial gene expression patterns as a potential diagnostic tool to identify mares prone to persistent endometritis earlier, allowing for more timely intervention before pregnancy is threatened
- •Screen mares during oestrus or early post-breeding for intrauterine fluid to identify those susceptible to infection before breeding problems occur
- •Use combined oxytocin and antibiotic protocols for infected mares rather than single agents; consider intrauterine plasma as an adjunctive therapy when conventional treatments have failed
- •Avoid routine intrauterine antiseptics due to risk of endometrial damage; reserve uterine lavage and prostaglandin analogues for specific clinical scenarios
Key Research Findings
Literature definitively demonstrating venereal transmission of P. aeruginosa and K. pneumoniae in horses was not available in published sources
Molecular typing methods suggest common environmental sources such as water and fomites are likely transmission routes rather than venereal contact
Presence of pathogenic organisms on external genitalia does not predict venereal transmission, endometritis, or reduced fertility
Over 20,000 samples are tested annually in the United Kingdom for these pathogens despite unclear evidence of venereal transmission
Next-generation sequencing and fungal culture showed 80% agreement with moderate correlation (Kappa = 0.508) in detecting fungal pathogens from equine uterine samples
Next-generation sequencing identified the same bacterial organism in 78.1% (25/32) of culture-positive samples with 74% overall agreement (Kappa = 0.46)
Next-generation sequencing detected fungal agents in 11.9% (5/42) of culture-negative samples, suggesting potential for identifying organisms missed by culture
Next-generation sequencing identified bacterial DNA in 5 of 14 culture-negative samples, indicating enhanced detection capability beyond traditional culture methods
25.9% of endometrial samples showed growth of facultative pathogenic bacteria, with β-hemolytic streptococci dominant at 79.7%
E. coli was cultured in 4.3% of samples, followed by Klebsiella pneumoniae (3.9%), Candida species (2.9%), Pseudomonas aeruginosa (2.0%), and Staphylococcus aureus (1.5%)
β-hemolytic streptococci showed 99.5% sensitivity to penicillins and E. coli showed 96.2% sensitivity to gentamicin
Multi-drug resistant bacteria (ESBL-positive and MRSA) comprised 3.1% of positive culture results and remained stable from 2018-2022 in Germany
Embryo reinsertion in donor mares (EF-ET) resulted in 0% pregnancy rate compared to 75% in recipient mares (ET group) in Experiment 1
Progesterone concentration decreased significantly 72 hours after EF-ET (p=0.05) but not in EF or ET groups, with three mares showing full luteolysis and endometritis signs
In Experiment 2, more grade 1 embryos were recovered from ET cycles (5/6) versus EF-ET cycles (3/7), with 4 degenerated embryos (grade 3-4) only in EF-ET group
Evidence Base
Review of Pseudomonas aeruginosa and Klebsiella pneumoniae as venereal pathogens in horses.
Scholtz Melanie, Guthrie Alan John, Newton Richard et al. (2025) — Equine veterinary journal
Detection of fungal and bacterial organisms from clinical uterine samples of horses using next-generation sequencing in comparison with culture.
Cheong Soon Hon, Franklin-Guild Rebecca, Goodman Laura B et al. (2025) — Journal of equine veterinary science
Frequency of potentially pathogenic bacterial and fungal isolates among 28,887 endometrial samples from mares, with an emphasis on multi-drug resistant bacteria in Germany (2018-2022).
Köhne Martin, Hegger Anna, Tönissen Anna et al. (2024) — Journal of equine veterinary science
Pregnancy and Luteal Responses to Embryo Reinsertion following Embryo Flushing in Donor Mares.
Martínez-Boví Rebeca, Gaber Jana T H, Sala-Ayala Laura et al. (2024) — Animals : an open access journal from MDPI
Serological and Uterine Biomarkers for Detecting Endometritis in Mares.
Cecchini Gualandi Stefano, Di Palma Tommaso, Boni Raffaele (2023) — Animals : an open access journal from MDPI
Bacteria Isolated From Equine Uteri in The United Arab Emirates: A Retrospective Study.
Omar Hussein, Hambidge Margaret, Firmanes Bernadette et al. (2022) — Journal of equine veterinary science
Effectiveness of Ozone Therapy in The Treatment of Endometritis in Mares.
Ávila Ana Caroline Araújo, Diniz Natália Castro, Serpa Rafael Torres et al. (2022) — Journal of equine veterinary science
Microbial Prevalence and Antimicrobial Sensitivity in Equine Endometritis in Field Conditions.
Díaz-Bertrana María Luisa, Deleuze Stefan, Pitti Rios Lidia et al. (2021) — Animals : an open access journal from MDPI
Leukocyte Esterase Reagent Strips for Stall-Side Diagnosis of Endometritis in Mares.
Kelley Dale E, Schnobrich Maria R, Gayer Stephany et al. (2019) — Journal of equine veterinary science
Whole genome sequences of nine Taylorella equigenitalis strains isolated in the Czech Republic between 1982-2021: Molecular dating suggests a common ancestor at the time of Roman Empire.
Hrala Matěj, Andrla Petr, Bosák Juraj et al. (2025) — PloS one
Untargeted Metabolomic Analysis Reveals Plasma Differences between Mares with Endometritis and Healthy Ones.
Zhang Xijun, Gao Yujin, Mai Zhanhai et al. (2024) — Animals : an open access journal from MDPI
Characterization of Myeloperoxidase in the Healthy Equine Endometrium.
Parrilla Hernández Sonia, Franck Thierry, Munaut Carine et al. (2023) — Animals : an open access journal from MDPI
Construction of a model of endometritis in domestic rabbits using equine-derived pathogens and evaluation of therapeutic effect of sensitive drugs.
Ding Xuanpan, Cui Xiao, Shi Jinlian et al. (2023) — Frontiers in veterinary science
Tukia Elina, Hallman Isa, Penttilä Maarit et al. (2023) — Frontiers in veterinary science
Fluorescence Spectroscopy for the Diagnosis of Endometritis in the Mare.
D'Agostino Andrea, Di Palma Tommaso, Cecchini Gualandi Stefano et al. (2022) — Animals : an open access journal from MDPI
Extracellular vesicles in low volume uterine lavage and serum: novel and promising biomarker for endometritis in Arabian mares.
Ibrahim Sally, Hedia Mohamed, Taqi Mohamed O et al. (2022) — BMC veterinary research
Limitations of a chromogenic agar plate for the identifying bacteria isolated from equine endometritis samples.
Vera L, Boyen F, de Visscher A et al. (2019) — Equine veterinary journal
Changes in malondialdehyde concentrations and glutathione peroxidase activity in purebred Arabian mares with endometritis.
Yaralioglu-Gurgoze Sema, Cetin Hayrettin, Cen Osman et al. (2005) — Veterinary journal (London, England : 1997)
Selecting potential biomarkers of plasma proteins in mares with endometritis.
Zhang Xijun, Mai Zhanhai, Gao Yujin et al. (2024) — Equine veterinary journal
Equine uterine sanitizer: in vitro inhibition of endometritis-causing microorganisms and its stability to dilute antibiotics.
Mazzuchini Mariana P, Lisboa Fernando P, Segabinazzi Lorenzo G et al. (2024) — Journal of equine veterinary science
Show 9 more references
Endometrial expression of antimicrobial peptides as markers of subclinical endometritis in mares.
Moroni Rebecca, Fanelli Diana, Camillo Francesco et al. (2024) — Equine veterinary journal
Comparison Between Gynecological Examination Methods and Sample Collection Techniques for the Diagnosis of Endometritis in Subfertile Mares.
Teixeira-Soares Carlos Mattos, Viana Arabela Guedes de Azevedo, Ribeiro Iara Magalhães et al. (2022) — Journal of equine veterinary science
Seminal plasma, and not sperm, induces time and concentration-dependent neutrophil extracellular trap release in donkeys.
Mateo-Otero Yentel, Zambrano Fabiola, Catalán Jaime et al. (2022) — Equine veterinary journal
The In Vitro Inhibitory Effect of Sivelestat on Elastase Induced Collagen and Metallopeptidase Expression in Equine Endometrium.
Amaral Ana, Fernandes Carina, Rebordão Maria Rosa et al. (2020) — Animals : an open access journal from MDPI
Equine Reproduction: Seasonality, Endometritis, and Twinning in the Mare
A. Trundell David (2020) — Equine Science
The Healthy and Diseased Equine Endometrium: A Review of Morphological Features and Molecular Analyses.
Schöniger Sandra, Schoon Heinz-Adolf (2020) — Animals : an open access journal from MDPI
Cooling and Cryopreservation of Equine Platelet-Rich Plasma With Dimethyl Sulfoxide and Trehalose.
Kwirant Liomara Andressa do Amaral, De La Corte Flávio Desessards, Cantarelli Camila et al. (2019) — Journal of equine veterinary science
Inflammatory mechanisms of endometritis.
Woodward E M, Troedsson M H T (2015) — Equine veterinary journal
Making sense of equine uterine infections: the many faces of physical clearance.
Causey Robert C (2006) — Veterinary journal (London, England : 1997)