These days, genetic testing poses few challenges for horse owners. They simply pluck a few hairs, place them in an envelope, mail the sample to the laboratory or registering body for testing, and koo koo kachoo, the results appear in their hot little hands in no time. The entire process, even interpretation, frequently requires little input on the part of their veterinarian.
Genetic testing, however, continues to develop and evolve, allowing us to identify genes that impact not only registration and breeding but also performance. This is where genetic testing is becoming a powerful and invaluable tool for equine practitioners.
Beyond the 3- and 5-panel tests required for registering Arabians and Quarter Horses, respectively, DNA and parentage testing for registering Thoroughbreds with the Jockey Club, and coat color/pattern genes of interest to breeders, learn how equine veterinarians can benefit from embracing the new age of equine genetic testing.
The Benefits of Genetic Testing in Horses, Scenario No. 1
An owner calls you out to examine her newly acquired eventer for poor performance. Like many similar cases, there is no overt lameness identified on examination or radiographs and no glaringly obvious health issues. She scopes clean, and doesn’t have a history of tying-up. The diet seems appropriate, and the horse is the picture of health, simply not performing as well as the owner had hoped.
Without a diagnosis and the owner staring at you, perhaps its time to think about the horse’s genes. A good candidate is the myostatin (MSTN) gene. Emmeline Hill, PhD, previously described the so-called “speed gene.” Hill’s data suggest that a single polymorphism at the g.66493737C/T location of MSTN can predict racing success at certain distances. Horses with two cytosine bases at that location are purportedly more suited to shorter races, whereas horses with two thymine bases have stamina and are better suited for longer races.
Genetic variations in the MSTN gene are reported in other breeds, as well. For example, a group of researchers collected DNA from 364 Quarter Horses, including 296 racing and 68 cutting horses. The key finding in that study was that racing Quarter Horses were almost exclusively homozygous for the C allele on the MSTN gene. Given the short length of Quarter Horse races, this finding is in-line with Hill’s data in Thoroughbreds.
Interestingly, that research group also found that both racing and cutting Quarter Horses with owner/trainer concerns regarding poor performance had a copy of the A allele (instead of C) for the DMRT3 gene—referred to as the “gait allele” that permits lateral gaits and favorable affect speed at the trot. Those findings have been confirmed in a number of breeds, including the following: Finnhorses, Nordic trotters, and Icelandic horses, to name only a few.
Together, these data indicate that a horse’s genetic makeup can make a big impact on performance, suggesting that not all cases of poor performance can be explained solely by physical ailments. Although not commercially available yet, tests for horses with a propensity for jumping, endurance, or other athletic endeavors would be warmly welcomed to better match a horse’s genetic make-up with their athletic/competitive career.
The Benefits of Genetic Testing in Horses, Scenario No. 2
During a prepurchase examination, you notice radiographic evidence of a well-managed, but clearly evident, club foot. The owner worries this will affect long-term soundness, whereas your concern is the impact of the club foot on the deep digital flexor tendon. And what is the health status of the other major tendons and ligaments? Should the owner purchase this horse?
There is no genetic test to help answer the club foot/DDFT question quite yet, but research in tendon and ligament injuries is certainly headed in that direction. For example, a group of British researchers identified two key genes involved in superficial digital flexor (SDF) tendon injuries in National Hunt Thoroughbreds: tenascin-C (TNC) and collagen type 5, a1 (COL1A1). Specifically, they found racehorses heterozygous for the TNC polymorphism (BIEC2-696469) were less likely to have SDF tendinopathy compared with racehorses homozygous for the wild-type allele. Further, racehorses homozygous for the novel COL5A1 variant allele were almost 3 times more likely to have SDF tendinopathy than racehorses homozygous for the wild-type allele. The study authors concluded, “In future genetic markers may be used to identify horse at risk of SDF tendinopathy.”
The Benefits of Genetic Testing in Horses, Scenario No. 3
One of your clients is looking into buying a yearling. You carefully review the filly’s radiographs in the repository and have the opportunity to scope her. She has good breeding and appears to be fit and healthy, but what if she had genes putting her at risk for a respiratory issue such as roaring or inflammatory airway disease? Would this impact your recommendation?
We know that roaring (paralysis of the arytenoid cartilage in the equine larynx due to degeneration of the left recurrent laryngeal nerve) is an important and common contributor to poor performance. We also know that offspring of roaring stallions are at risk of roaring themselves, suggesting a genetic component. Recent research confirms that long-held hypothesis, identifying a region on equine chromosome 3 that is believed to contribute to roaring.
Just as roaring is a cause of poor performance, inflammatory airway disease (IAD), a common affliction of young, athletic horses, is also problematic. Approximately 10-15 percent of racehorses and an unknown percentage of other athletic horses, including endurance horses, suffer from IAD. Bronchoalveolar lavage (BAL) can be used to help achieve a diagnosis; however, the procedure is invasive and requires sedation. One research group suggested that IAD has a genetic component, and that developing a genetic test for the condition would expedite and facilitate diagnosis.
Future Development of Genetic Testing
There are currently several companies and laboratories eager to accept DNA samples and have them analyzed for various conditions and coat colors/patterns. Many, however, rely on dated technology that allows only single tests to be run at a time. Plus, they often charge owners per test.
Samantha Brooks, PhD, principal investigator at the Brooks Equine Genetics Lab and an assistant professor in equine physiology at the University of Florida is a scientific advisor for Etalon Diagnostics, Inc. This company, according to Brooks, uses a novel and unique scientific platform that allows scientists to scan an entire genome to simultaneously search hundreds of genes for a single price. The panel currently includes genes for coat color/pattern, behavior, performance (athletic and breeding) and health. The company also banks DNA samples and uses that information to “discover” new genes. This was how the gene for impaired acrosomal reaction in stallions (contributing to infertility) was discovered and subsequently made commercially available.
As the technology continues to evolve, there is no question that owners will begin looking beyond basic genetic testing and begin asking your opinion on newer tests and genetic panels to help purchase horses for a specific purpose and help maintain their soundness and overall health.
