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EVJ embraces advances in equine reproduction with special focus edition

Stud Medicine BEVA Journals
06 Jun 2024 BEVA

The practice of equine reproduction is evolving rapidly. Advances have been driven by a combination of increasing integration of assisted reproductive technologies (ARTs) and novel diagnostic and therapeutic modalities to investigate or resolve sub-fertility. The July issue of the Equine Veterinary Journal (EVJ) is sharing the latest knowledge in these areas, through a Special Focus Edition that is free to view here until the end of August.

“ARTs bring many advantages such as addressing sub-fertility problems, accelerating genetic improvement by allowing more offspring to be produced from the best stallions and/or mares,” said EVJ guest editor Tom Stout. “The underlying theme of this collection is improving approaches to unresolved problems, while validating novel techniques to enhance ART outcomes.

The special collection is the result of a call for papers on equine reproduction. Submissions underwent EVJ’s usual peer review process led by Guest Editors Tom Stout, James Crabtree and Juan Cuervo-Arango. The 19 selected articles range from analytical clinical studies, descriptive clinical reports, surveys and population studies, and experimental and basic research studies.

Currently there is little up-to-date information on AI-led Standardbred breeding: Reproductive performance of a cohort of Standardbred mares under a commercial breeding system indicates that overall fertility is as good or better than in previous surveys, but also identifies mare age and use of chilled-transported or frozen–thawed semen as significant limiting factors.

The two papers; Post-cooling semen processing and sperm re-suspension as an alternative method to circumvent poor semen cooling in stallions and A simplified fixed-time insemination protocol using frozen-thawed stallion spermatozoa stored at 17°C for up to 24h before insemination show the progress being made in improving both the accessibility, applicability and success of AI with stored/shipped stallion semen.

Endometritis is a significant contributor to age-related decline in fertility of older mares but the diagnosis of the three major forms of the disease can be challenging. Three novel methods for detecting endometritis are included in the special focus edition: Selecting potential biomarkers of plasma proteins in mares with endometritis, Endometrial expression of antimicrobial peptides as markers of subclinical endometritis in mares and Inflammatory markers for differentiation of endometritis in the mare.

Managing twin pregnancies can be challenging. Twin management in the mare: a review emphasises that while early detection and manual crushing of supernumerary vesicles is the preferred method for managing dizygotic multiple pregnancy in the mare, a significant number of twins escape early resolution, for various reasons.

Success rates and factors influencing pregnancy outcome after 464 transvaginal ultrasound-guided twin reductions in the mare analyses the most extensive transvaginal ultrasound-guided aspiration (TUA) case series reported to date, showing that the method led to the birth of a live healthy singleton foal in 40.1% of cases.

Ultrasound-guided fetal thorax compression to reduce post-fixation twins in the mare describes the use this method as an alternative for twins that have passed the day 40 point after which TUA would not be recommended. The method resulted in 7 in 10 mares producing a live normal single foal.

An important side-effect of pregnancy loss is high circulating equine chorionic gonadotrophin (eCG) concentrations which commonly disrupt cyclicity and frustrate attempts to re-establish pregnancy. Using mycobacterium cell wall fraction to decrease equine chorionic gonadotropin after abortion provides a novel option for potentially rescuing the breeding season in mares suffering pregnancy loss after day 35.

In the Standardbred horse fertility survey, a cumulative 16% of pregnancies detected at the first scan failed to reach term, with conceptus aneuploidy (abnormal number of chromosomes) probably a major contributor. Detecting chromosomal abnormalities in live horses is also challenging and Molecular cytogenetic screening of sex chromosome abnormalities in young horse populations describes the use of fluorescent in situ hybridisation (FISH) to improve the reliability of detecting karyotypic abnormalities in live horses.

Inbreeding is also suspected, but not proven, to both compromise fertility and increase the risk of pregnancy loss in horses, primarily by increasing the risk of embryonic lethal homozygous mutations. Does inbreeding contribute to pregnancy loss in Thoroughbred horses? finds an association between an accepted index of inbreeding, runs of homozygosity, and mid to late gestation pregnancy but not early pregnancy losses.

One known cause of mid to late gestation pregnancy loss in horses is umbilical cord torsion. The equine umbilical cord in clinically healthy pregnancies presents novel information on umbilical cord length during gestation.

While there are clear advantages of cryopreserving embryos in terms of recipient mare selection and management, there are also potential disadvantages in terms of donor management and embryo survival. Successful vitrification of equine embryos >300 microns without puncture or aspiration reports that larger day 7–8 embryos can yield high pregnancy rates after vitrification and warming if the embryo is equilibrated for an extended period in the first vitrification solution.

Effects of repeated embryo flushing without PGF2a administration on luteal function, percentage of unwanted pregnancy and subsequent fertility in mares reports that even if flushing was performed on days 7–9, there was a significant risk of subsequent pregnancy (5/27: 18.5%) if a mare was not injected with a PGF2α analogue following an unsuccessful embryo recovery attempt.

Ovum pick up has clear potential benefits but it is an invasive procedure with risk of complications. Resolution of two cases of ovarian abscesses in mares subjected to ovum pick up concludes that peri-procedure antibiosis should be considered when performing OPU to reduce the low risk of ovarian abscess formation, which may not present until months later.

Ovarian abnormalities, in particular granulosa cell tumours, are often assumed to underlie undesirable behaviour in mares. Abnormal mare behaviour is rarely associated with changes in hormonal markers of granulosa cell tumours: a retrospective study shows that, with the exception of stallion-like behaviour, undesirable behaviour in mares is unlikely to be associated with a hormonally-active ovarian tumour.

In recent years mammary secretion pH has emerged as a quick and easy way to predict impending parturition in mares Conductivity of mammary gland secretions is a sensitive and specific predictor of parturition in mares shows how mammary secretion conductivity, measured using a handheld device, can aid the prediction of imminent foaling in mares.

Reproductive tumours in mares are infrequent but include genital squamous cell carcinomas (SCCs). Epithelial-mesenchymal transition in an EcPV2-positive vulvar squamous cell carcinoma of a mare suggests that equine papilloma viruses (EcPVs) may play a role in the aetiology of genital SCCs in mares.

“This important collection adds significant weight to our understanding of equine fertility to support the most successful outcomes,” said Professor Tom Stout, Associate Editor of the EVJ. “It’s an invaluable source of information for clinicians to bring them up to speed with the latest development, selection and validation of diagnostics and therapeutic tactics.”

The virtual issue can be found here and will be free to view until the end of August.