Research advances in equine reproductive biology and health over the past 50 years have contributed to improved fertility and breeding efficiency in horses. The development of endocrine and ultrasonography diagnostics aids the clinician in determining potential fertility of mares and stallions. Follicular dysfunction and ovulation failure can be diagnosed with high precision in mares, embryo and fetal development can be monitored from 10 days after ovulation throughout gestation, and fetal/placental abnormalities diagnosed to identify high-risk pregnancies. In stallions, biochemical and genetic sperm abnormalities have been identified that previously remained undetected based on traditional breeding soundness examinations. Furthermore, testicular abnormalities and blood flow can be determined with high accuracy by Doppler flow ultrasonography.
The current trend in horse breeding is characterized by a decline in mares that are bred and a new focus on well-being of established pregnancies as well as enhanced genetic selection with regards to health and future performance of the foal. In addition, great advances in the area of assisted reproductive technologies offer numerous possibilities that were not available in the past.
Monitoring Healthy Pregnancy
Biomarkers for high-risk pregnancies and efficient treatments to prevent late pregnancy losses are being investigated. As new potential markers for unhealthy conditions are discovered, effective treatment options need to be developed. A similar trend is seen with regards to endometritis as well as the development of new diagnostics for dormant bacteria and uterine biofilms. While new therapeutic regimens for endometritis often include non-antibiotic alternatives or supplements, most treatment options for high-risk pregnant mares include long-term use of broad-spectrum systemic antibiotics without the possibility of performing bacterial sensitivity tests. There is a global awareness of consequences when antibiotics are overused in human as well as veterinary medicine—and equine reproduction is not an exception. An overuse of antibiotics can lead to bacterial resistance and contribute to one of the most urgent public health issues today. New trends in equine reproduction reflect these challenges to veterinary medicine, public health and the global equine breeding industry, and additional nonantibiotic treatment options need to be developed.
Improved Genetic Selection
The art of breeding horses is based on genetic selection. Horsemen evaluate visible characteristics (phenotypes), such as conformation, coat color, speed, endurance as well as family history (blood lines) and make breeding decisions based on these evaluations. As equine breeding has shifted from quantity to a greater focus on quality, genetics and genomic research in reproduction has the potential to improve the accuracy of breeding selection criteria. Research leading to sequencing of the equine genome has resulted in the identification of a growing number of genes that have been linked to positive characteristics (speed, coat color, etc.) as well as heritable diseases. These scientific advances are currently used to test potential carriers before making breeding decisions. If allowed by the breed registry, pre-implantation genetic testing of embryos provides a more effective method to select for (or against) certain genetics. Recent and ongoing research allows us to recover embryos nonsurgically from the uterus a week after conception and test the conceptus for the presence or absence of specific genes. The embryo can be cryopreserved while molecular genetic testing is performed in the laboratory and if test results are favorable, it can be transferred into a recipient mare or even into its biological mother at a later time. The potential benefit of this technology to horse breeding is obvious. For example, unless genetic carriers of a debilitating disease (and potentially also carriers of some desirable genes) are completely removed from breeding, selection of healthy embryos based on pre-implantation genetic testing is the only way to eradicate the disease. Because of its practical benefit to horse breeders, the technology of pre-implantation genetic testing in horses can be expected to be further developed and gain popularity as research identifies additional genes that influence a variety of characteristic of horses, and tests become available. A related area of expected research intensity in the near future is investigations on the importance of environmental influence on gene expression during fetal development (epigenetics).
Assisted reproduction is not suitable to all breeders. However, current knowledge and expected future research advances in this area offer exciting possibilities for selective high-end breeding operations. Various assisted reproduction techniques carry some important advantages with regard to management of both fertile and subfertile stallions, allowing for treatment of the semen. For example, frozen and preserved semen can be shipped anywhere in the world, making valuable genetics available globally, and preserving genetics from stallions long after their death.
Researchers in Australia have recently developed a semen extender that allows for storage of equine semen for an extended period at ambient temperature. This is without doubt a major breakthrough that can simplify breeding with shipped semen and reduce some hurdles associated with timing of the shipment in relation to expected ovulation. Research advances on semen biology has also resulted in improved management of stallions with specific and selective fertility problems.
Stallions that produce ejaculates with poor sperm quality and viability can often be managed by centrifugation of their ejaculates and resuspension of the sperm in an extender with supportive nutrients. Furthermore, the quality of an ejaculate can be enhanced through gradient centrifugation that can separate “good” and “bad” sperm before insemination. Further research to enhance our understanding of relevant sperm biology and advanced diagnostics to identify preferable sperm populations in an ejaculate is expected to create a need for robust and efficient technologies to select superior sperm within an ejaculate.
Research that makes it possible to separate x- and y- bearing sperm through flow cytometry (based on the fact that x-bearing sperm contains more DNA), has been commercialized and is currently offered by specialized laboratories. The method is very accurate, but efficiency is still quite low and results in a low number of sex-sorted sperm. This hurdle has been a limiting factor for the method and more research is needed to improve the technique. Recent research exploring the use of nanotechnology to target specific sperm DNA sequences has the potential to develop a new efficient technology to select sperm for sexsorting, and maybe more importantly—to select sperm based on other genetics that can be used as a replacement or complement to pre-implantation genetic testing of embryos. Preliminary results from studies using nanotechnology for sex-sorting in donkeys and other species appear to be promising.
Logically, technologies that involve sperm selection in the laboratory will always result in limited number of sperm that can be used for breeding. However, recent scientific and clinical advances make it possible to use a single sperm from an infertile stallion and inject it into an egg that has been retrieved by ultrasound guided follicular aspiration from the ovary, culture the conceptus in a petri dish under controlled conditions and transfer the embryo into a recipient mare or back into the same oocyte donor mare at a later time. Intracytoplasmic sperm injection (ICSI) has become increasingly popular and a suitable method to use genetically valuable semen of poor quality or with restricted access because of disease or death of the stallion. Research to improve the efficiency of this technology would greatly benefit breeding operations that allow assisted reproduction.
Few, if any, breed registries accept foals resulting from somatic cell cloning of an existing individual. The technology is nevertheless offered for horses and has become popular among some horse owners that don’t need to register their foals. Although cloned horses have been used successfully as athletes, the greatest benefit of the technology may be the possibility to produce an intact stallion as a genetic copy of an existing gelding with a successful athletic career. It should be kept in mind that even if the genetic make-up of the cloned copy is identical to the original horse, the phenotype (looks and performance) may be different because of the influence of the environment that the embryo and fetus were exposed to in the test tube and uterus of the surrogate mother (epigenetics). The best prospective for the cloned intact stallion may therefore, be for breeding purposes. In other words—an expensive way to put testicles back on a gelding.
CONTACT: Mats H.T. Troedsson, DVM, PhD, DACT, DECAR–859/218-1133–firstname.lastname@example.org–Maxwell H. Gluck Equine Research Center, University of Kentucky, Lexington, Ky.
This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd’s, London.