Subfertility: What the Research Says
Evidence from 20 peer-reviewed studies
What Professionals Should Know
- •Routine ultrasonographic screening for placental abnormalities on breeding farms can identify high-risk pregnancies early; intervention saves >90% of these pregnancies from abortion
- •Embryo transfer recipients require closer monitoring and may develop more severe placental pathology than mares carrying their own pregnancies
- •Most treated mares with placental abnormalities return to fertility in the next breeding season (61.1%), making early intervention economically justified on stud farms
- •Breeding decisions in Pura Raza Española and similar closed populations should prioritize reducing homozygosity and inbreeding coefficients to maintain mare fertility
- •Genomic testing for runs of homozygosity could help identify at-risk mares before fertility problems emerge, enabling early intervention or breeding plan adjustments
- •Genetic diversity management is critical in performance horse breeding programs to prevent inbreeding depression and associated subfertility
- •Use uterine lavage rather than swabs when sampling for endometritis diagnosis, as it significantly improves detection of pathogens in mares before breeding
- •Routine enrichment culturing is essential—it detects an additional 25% of infections that would be missed by direct smearing alone, improving early diagnosis and timely treatment initiation
- •Be aware that the most common causative organisms are α-haemolytic Streptococcus, E. coli, and β-haemolytic Streptococcus, which should inform empirical antibiotic selection pending culture results
- •Subclinical endometritis severity varies: subacute suppurative endometritis shows full activation of immune markers while chronic endometritis does not, suggesting different treatment approaches may be warranted based on inflammation type
- •Mares presenting with reduced conception rates or embryo loss warrant endometrial assessment, as subclinical inflammation can impair fertility without obvious clinical signs
- •Understanding that endometrial epithelial and stromal cells actively drive immune response may inform future therapeutic strategies targeting inflammatory pathways rather than just bacterial elimination
- •Monitor foaling dates annually—progressive delays signal declining reproductive efficiency and should trigger management or culling decisions
- •High-value mares are more likely to generate positive returns despite reproductive challenges; lower-value mares require near-perfect reproductive performance (foals in 6 of 7 years) to be profitable
- •Focus breeding management efforts on improving conception rates and ensuring early-season foaling to maximise cumulative lifetime profitability
- •Seminal vesiculitis with blocked ampullae can be successfully managed using combined oral and local antibiotic therapy delivered via video endoscopy guidance
- •Persistent low semen quality after infection resolution does not necessarily preclude fertility—this stallion remained capable of achieving pregnancies despite abnormal ejaculates
- •Multiple diagnostic approaches (ultrasound, cytology, bacteriology, alkaline phosphatase levels) are needed to fully characterize stallion reproductive tract disease
- •PRP sourced from mares with histories of PBIE resistance may be more therapeutically effective than PRP from susceptible mares due to enhanced immune-related protein profiles
- •Platelet count alone does not distinguish PRP quality; protein composition analysis may be needed to optimize PRP selection for treating PBIE in recipient mares
- •This foundational proteomics work requires follow-up clinical trials before changing current PRP procurement or treatment protocols
- •Amniotic microvesicles represent a novel regenerative approach for mares with chronic endometritis and recurrent implantation failure, though evidence is currently limited to a single case
- •This treatment may work by reducing inflammation and restoring endometrial function rather than treating infectious endometritis, making it applicable to degenerative rather than infectious cases
- •Further controlled trials are needed before adoption into clinical practice; this case warrants follow-up studies to assess reproducibility and optimal treatment protocols
- •Long-acting progesterone injections should be discontinued once pregnancy is confirmed, as continued treatment may trigger inappropriate lactation and adverse fetal outcomes in mares
- •Premature mammary development and galactorrhea in a pregnant mare should raise suspicion for progesterone exposure and warrant immediate investigation of medication history
- •Fetal death associated with progesterone treatment does not necessarily compromise future fertility or reproductive success, as demonstrated by this mare's subsequent normal pregnancy
- •For mares with unexplained subfertility after multiple cycles with known fertile stallions, hysteroscopic hydrotubation offers a minimally invasive diagnostic and therapeutic option without requiring surgery
- •The procedure can be performed standing under sedation, making it accessible and practical for field or clinic application with standard endoscopic equipment
- •High conception rates (93%) and rapid return to fertility post-treatment suggest this should be considered before pursuing more invasive surgical interventions for suspected oviductal pathology
- •Doppler ultrasound of the capsular artery offers a non-invasive diagnostic alternative to traditional invasive procedures for identifying testicular dysfunction in breeding stallions
- •Measuring blood flow parameters (particularly EDV, TAMV, and TABF) and vascular resistance indices (PI, RI) can help predict sperm quality and breeding potential before investing in breeding soundness exams
- •Establishing individual stallion Doppler baseline values and monitoring changes over time may enable early detection of testicular pathology and allow timely intervention to preserve fertility
- •Veterinarians and stallion owners must understand sperm quality parameters and processing requirements to provide suitable samples for IVP procedures, bridging clinical evaluation with laboratory standards
- •Selection of intact, high-quality sperm directly impacts fertilization success and embryo development, making proper evaluation and processing protocols essential for valuable breeding animals
- •IVP offers a practical solution for preserving genetics from subfertile, deceased, or emergency-euthanized stallions, but requires coordination between clinical practitioners and reproductive laboratories
- •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
- •Veterinarians working with breeding stallions should monitor testicular function in middle-aged and older males as degeneration becomes increasingly common with advancing age
- •Early recognition of age-related testicular changes is important to manage breeding expectations and prevent economic losses in breeding programs
- •Genetically valuable stallions may maintain breeding value into old age, but testicular degeneration should be factored into breeding career planning and management decisions
- •Choose semen collection method based on clinical question (volume/concentration needs, stallion cooperation) as it directly affects sperm parameters; document collection method when interpreting results
- •Prioritize motility analysis as the most clinically relevant fertility predictor, but ensure your laboratory's computer-assisted analyzer settings are standardized and validated for comparison across samples
- •Request seminal plasma analysis (pH, oxidative markers) when evaluating frozen semen quality or investigating unexplained poor cryopreservation, as these factors directly impact dose viability
- •Understanding that livestock reproductive microbiomes differ fundamentally from humans may inform development of species-specific interventions to enhance fertility
- •Microbiome profiling using 16S rRNA sequencing could become a diagnostic tool to assess reproductive health status in breeding animals
- •Management strategies targeting reproductive microbiota composition may offer new approaches to improve reproductive efficiency in herds
- •When evaluating horses with fertility problems or developmental issues, request cytogenetic analysis as chromosomal abnormalities are found in nearly 1 in 3 affected animals
- •Be aware that X-monosomy and sex reversal conditions are particularly common in horses and should be suspected in mares with reproductive failure or stallions with low fertility
- •Standard karyotyping remains the practical first-line diagnostic tool for breeding soundness evaluation and investigation of developmental problems
- •Use systematic diagnostics rather than trial-and-error approaches when dealing with problem breeders—it increases your success rate
- •Document your diagnostic pathway thoroughly; this protects you and the owner even if pregnancy doesn't result
- •Subfertility has multiple causes; don't assume one protocol fits all mares
- •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
- •Adopt a structured diagnostic protocol when evaluating subfertile mares rather than treating empirically—this increases success rates
- •Recognize that 'difficult mare' diagnosis encompasses multiple underlying causes; identify the specific problem before selecting treatment
- •Even when pregnancy cannot be achieved, systematic evaluation demonstrates due diligence to owners and helps refine future breeding decisions
Key Research Findings
Ultrasonographic placental abnormalities occurred in 4.2% (177/4192) of pregnancies on a commercial warmblood stud farm over 2017-2019
Embryo transfer recipient mares had significantly increased placental abnormality severity compared to mares with their own pregnancy
Early detection and treatment prevented abortion in >90% of high-risk pregnancies, with 61.1% of affected mares becoming pregnant in the subsequent season
Pregnancy establishment rates after placental abnormalities were 68.1% for PSc1, 53.4% for PSc2, and 53.8% for PSc3 severity scores, with 75.7% achieved in the first insemination cycle
Negative correlation between whole-genome homozygosity and fertility (ρ = -0.144), strengthening to -0.241 in the 10% most/least fertile individuals
Identified 13 genomic regions (ROHi) significantly associated with fertility after cross-validation, containing 17 candidate genes related to oocyte and embryo development
Least fertile mares showed approximately 30% increase in average inbreeding values compared to most fertile mares
Chromosome-level analysis revealed wide variability in homozygosity-fertility correlation across the genome
58% of uterine samples (230/394) were positive for bacterial pathogens, with 33% detected by direct smearing and an additional 25% only after enrichment culture
α-haemolytic Streptococcus and Escherichia coli were most frequently isolated (27% each), followed by β-haemolytic Streptococcus (26.1%) and Staphylococcus spp. (19.1%)
Uterine lavage demonstrated significantly higher sensitivity (80% positive) compared to uterine swabs (53% positive)
Gram-negative bacteria preferentially grew with direct smearing while Gram-positive bacteria were more frequently isolated following enrichment culture
mRNA expression of TLR2, IL-1β, IL-6, TLR4 and TNF-α was significantly upregulated in mares with subacute suppurative endometritis (SSE) compared to healthy controls (P<0.01 to P<0.0001)
IL-6 and TNF-α protein concentrations were elevated only in SSE mares, not in chronic endometritis, suggesting inflammation type determines cytokine activation
TNF-α and TLR2/4 immunolocalization was confirmed in endometrial epithelial, glandular and stromal cells in both healthy and diseased endometria
Evidence Base
Occurrence of ultrasonographic assessed placental abnormalities, treatments, pregnancy outcome, and subsequent fertility on a large warmblood stud farm: A retrospective field study.
Sielhorst J, Koether K, Volkmann N et al. (2024) — Journal of equine veterinary science
Fine-Scale Analysis of Runs of Homozygosity Islands Affecting Fertility in Mares.
Laseca Nora, Molina Antonio, Ramón Manuel et al. (2022) — Frontiers in veterinary science
Infectious Endometritis in Mares: Microbiological Findings in Field Samples.
Ravaioli Valentina, Raffini Elisabetta, Tamburini Marco et al. (2022) — Journal of equine veterinary science
Type of Inflammation Differentially Affects Expression of Interleukin 1β and 6, Tumor Necrosis Factor-α and Toll-Like Receptors in Subclinical Endometritis in Mares.
Siemieniuch Marta J, Szóstek Anna Z, Gajos Katarzyna et al. (2016) — PloS one
Impact of reproductive efficiency over time and mare financial value on economic returns among Thoroughbred mares in central Kentucky.
Bosh K A, Powell D, Neibergs J S et al. (2009) — Equine veterinary journal
Seminal vesiculitis and blocked ampullae occurring simultaneously in a stallion.
Huggins L, Dini P, Santos E et al. (2025) — Journal of equine veterinary science
Platelet-Rich Plasma Proteome of Mares Susceptible to Persistent-Breeding-Induced Endometritis Differs from Resistant Mares.
Novello Guilherme, Souza Fabiana F, Canisso Igor F (2024) — Animals : an open access journal from MDPI
Case Report: Use of Amniotic Microvesicles for Regenerative Medicine Treatment of a Mare With Chronic Endometritis.
Lange-Consiglio Anna, Funghi Federico, Cantile Carlo et al. (2020) — Frontiers in veterinary science
Fetal Death Associated With Premature Mammary Gland Development and Lactation in a Mare Treated With Weekly Injections of Long-Acting Progesterone.
McAfoos Jessie L, Ellerbrock Robyn E, Canisso Igor F (2019) — Journal of equine veterinary science
Clinical application of hysteroscopic hydrotubation for unexplained infertility in the mare.
Inoue Y, Sekiguchi M (2018) — Equine veterinary journal
Pulse Doppler ultrasound as a tool for the diagnosis of chronic testicular dysfunction in stallions.
Ortiz-Rodriguez Jose M, Anel-Lopez Luis, Martín-Muñoz Patricia et al. (2017) — PloS one
From clinic to lab: Sperm parameters for equine in vitro embryo production.
Martin-Pelaez Soledad, Orsolini Morgan, Verstraete Margo et al. (2026) — Journal of equine veterinary science
Endometrial expression of antimicrobial peptides as markers of subclinical endometritis in mares.
Moroni Rebecca, Fanelli Diana, Camillo Francesco et al. (2024) — Equine veterinary journal
Characterisation of the testicular transcriptome in stallions with age-related testicular degeneration.
Woodward Elizabeth, Schlingmann Karen, Tobias John et al. (2023) — Equine veterinary journal
Sperm Quality Assessment in Stallions: How to Choose Relevant Assays to Answer Clinical Questions.
Egyptien Sophie, Deleuze Stéfan, Ledeck Joy et al. (2023) — Animals : an open access journal from MDPI
Reproductive Microbiomes in Domestic Livestock: Insights Utilizing 16S rRNA Gene Amplicon Community Sequencing.
Poole Rebecca K, Soffa Dallas R, McAnally Brooke E et al. (2023) — Animals : an open access journal from MDPI
Horse Clinical Cytogenetics: Recurrent Themes and Novel Findings.
Bugno-Poniewierska Monika, Raudsepp Terje (2021) — Animals : an open access journal from MDPI
Breeding the ‘difficult mare’
Barker Kayleigh (2020) — UK-Vet Equine
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
Breeding the ‘difficult mare’
Barker Kayleigh (2020) — Equine Health