Thoroughbred “Performance Genes” Further Examined in Study

by: Stacey Oke, DVM, MSc
April 28 2011, Article # 18168

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The genetics behind the athletic performance of Thoroughbred racehorses has been a popular area of research in the past few years. A large-scale study was carried out recently in Japan, the results of which corroborate those of previous studies: Identifying specific “performance genes” on equine chromosome 18 could identify the ideal distance for a Thoroughbred to race. Some industry veterinarians, however, suggest that this information should still be evaluated and used with caution.

Researchers on four separate studies have previously identified a region of DNA on chromosome 18 (which the researchers referred to as ECA18) that they believe could be associated with racing performance. One of the research teams found a specific mutation in the myostatin gene (MSTN) located on ECA18. Researchers also previously evaluated the myostatin mutation and determined that it contributes to muscle hypertrophy (enlargement) and might be useful in predicting whether a horse is better suited for short-, mid-, or long-distance races.

To further assess the ECA18 genes’ influence on athletic performance, Teruaki Tozaki, PhD, from the Laboratory of Racing Chemistry’s Department of Molecular Genetics in Tochigi, Japan, recently analyzed genetic material from blood samples collected from Thoroughbred racehorses registered with the Japan Racing Association and born between 1993 to 2000 in Japan.

Tozaki’s team identified four specific genetic mutations (single variations in the DNA) called single nucleotide polymorphisms (SNPs) on ECA18 within or near the MSTN gene using standard polymerase chain reaction (PCR) and genetic sequencing techniques. Then they compared genetic sequences of these SNPs in 1,710 horses with performance records including most successful race distance, win-race distance (i.e., the average distance of races won), performance rank, and lifetime earnings.

Tozaki noted, “SNPs on ECA18 in both male and female Thoroughbred racehorses were clearly associated with performance, most notably optimal race distance.”

According to Tozaki, these results indicate that there is at least one gene or a combination of genes or SNPs in this region of ECA18 that likely influences a Thoroughbred’s optimum race distance.

“Regulation of the MSTN gene appears to affect racing performance and … these SNPs could be genetic diagnostic markers for racing performance indicators, as we previously suspected,” Tozaki explained.

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Presently, Tozaki’s team is focusing on the physiologic characteristics of Thoroughbreds with the MSTN mutation and how they relate to racing performance. For example, they’re looking at the differences in muscling among young horses not in training, those in training, and horses actively racing.

Although this study is included in a large group of related studies, a recently released statement from the American Association of Equine Practitioners (AAEP) indicates that industry veterinarians are still interpreting the results with caution: “The interpretation of the tests results may not be simple, and the predictive value of the tests may not be high. The AAEP supports the use of evidence-based tests for medical and performance related genetics if the methodology and results have been published in peer-reviewed journals and withstood appropriate scientific scrutiny.”

The study, “A cohort study of racing performance in Japanese Thoroughbred racehorses using genome information on ECA18,” is scheduled to be published in an upcoming edition of the journal Animal Genetics. The abstract is available online.

by Michael Scott, DVM; MVSc; Diplomate, American College of Veterinary Surgeons. Dr. Michael Scott is an equine surgical specialist practicing at Moore and Company Veterinary Services in Calgary, Alberta. He has been a lifelong rider, competitor, and horse enthusiast. His practice interests include equine sports medicine, lameness diagnosis and treatment, and equine surgery.

In order to have a better understanding of this problem, it is important to have some knowledge of the anatomy of the hock joint. The hock is the analogue of the human ankle, and is composed of 8 bones and 4 separate joints (Fig. 1). The upper joint, or tibiotarsal joint, is where the movement occurs during flexion and extension. The three lower joints (proximal intertarsal, distal intertarsal, and tarsometatarsal joint) move very little, and are often referred to as low motion joints.

It is in the two lower joints, the distal intertarsal and tarsometatarsal joints, where the horse most often becomes painful. This pain is due to varying degrees of inflammation and damage to joint structures, ranging from mild synovitis (inflammation of the synovial lining of the joint) or arthritis (joint inflammation) to advanced degenerative joint disease where there is extensive cartilage damage and bone re-modeling. The term “bone spavin” has long been used to describe the condition of degenerative joint disease affecting the lower hock joints.

Diagnosing lameness problems in this area may be a simple process or a challenging puzzle. In the simplest scenario the horse is obviously lame, becomes worse after the leg is flexed, and has obvious bone changes visible on radiographs and possibly even on the physical examination. The simple scenario doesn’t seem to be as common as the more diagnostically challenging case, where the unsoundness may only be apparent when the horse performs certain maneuvers. In such cases, communication between rider and veterinarian is important. Often the first signs of a developing problem are detected by the astute rider as a change in the way a horse feels, the way it stops or spins in a certain direction, or its willingness to perform certain maneuvers.

When a performance problem is suspected, there are several tools and techniques a veterinarian can use to look for unsoundness in the hock. After obtaining details about the history of the problem, a lameness examination is performed. Diagnostic tests such as anesthetic nerve blocks, radiography, and possibly nuclear scintigraphy may be used. Details about lameness diagnosis procedures are beyond the scope of this article, but an important point should be made regarding radiographs. In some cases, particularly in young horses with early stages of hock arthritis, no abnormalities will be seen on the radiographs. In these cases, it is assumed that the problem has not progressed enough to result in changes in the bones (which are the only tissues visible on radiographs).

Once arthritis of the lower hock joints has been diagnosed or is strongly suspected, decisions can be made regarding treatment. Options include rest, evaluation and possibly changes in shoeing, nutritional supplements and systemic medications, acupuncture, hock injections, and possibly surgery. The simpler and less invasive treatments and management options should be considered before joint injections are used.

Probably the most effective and often used treatment for arthritis and pain in the lower hock joints is joint injection. Several different medications are used. Corticosteroids, often referred to as “steroids” or “cortisone”, are actually a large family of drugs with potent anti-inflammatory activities of varying strengths and durations. Trimacinolone is one corticosteroid which has a medium duration of action and is widely used for joint injection. Methylprednisolone acetate (Depro-Medrol) is a potent, long acting corticosteroid most often used for injection in the lower hock joints. Another medication that can be used is hyaluronic acid (Hyonate or Legend). Hyaluronic acid is found naturally in the cartilage and synovial fluid within the joint, where it has important functions. Administered intravenously or into the joint, hyaluronic acid acts in numerous ways, including reducing inflammation, improving the quality of synovial fluid, and in cartilage maintenance. In the lower hock joints in particular, methylprednisolone acetate alone or in conjunction with hyaluronic acid is most often used. The duration of action of this treatment is variable and depends on many factors including the severity of disease and the intensity of work the horse is performing. In some cases benefit may only be seen for as little as a few weeks, whereas in other cases the effect may last for a year or more.

The horse owner and competitive rider will eventually face the question of “to inject or not to inject”. As a veterinarian, I often see clients struggle with this question and express concern about what they might be doing to their horse. This reaction is justified, as there are certainly some negative implications associated with joint injections. If we take a step back and ask the question “why has this horse become sore?”, the answer is often “because it is being asked to perform strenuous work”. If medication is injected into the joint, its primary effect is to reduce inflammation in the joint and therefore reduce pain. It will also have some beneficial effects in reducing the joint degeneration that occurs with chronic arthritis. However, if the horse continues to perform at the same level, the stresses placed upon the joint may continue to cause “wear and tear” type injury, leading to an eventual return of joint inflammation and pain. A scenario of repetitive joint injections of increasing frequency being needed in order to keep a horse going may develop, leading to concern about the long term health of the joint.

The important point to remember when discussing injection of the lower hock joints is that they are “low motion” joints. If the arthritic condition is affecting a “high motion” joint such as the coffin joint or the fetlock, ongoing joint degeneration can lead to permanent lameness. When the arthritic condition is affecting the lower hock joints, this is not necessarily the case. These joints are not as functionally important to the movement of the horse as high motion joints are. In some cases of advanced degenerative joint disease, one or both of the lower hock joint may actually fuse, which is a desirable result as it does not have a great impact on the function of the leg but it does result in relief from the pain, as there is no longer a joint to be painful. In fact, in some cases surgical techniques have been used to cause accelerated fusion of these joints.

So although there are reasons to be concerned about the risks and potential negative effects of joint injection in any situation, in the case of the lower hock joints the advantages of this treatment seem to outweigh the disadvantages, particularly if the objective of treatment is to reduce the discomfort that horse is experiencing and allow it to continue to perform.

Texas lawmakers are pondering legislation that would define the role of nonveterinarian equine dentists in that state.

Current state law does not prohibit nonveterinarian dentists from floating horses’ teeth. But defining just how much work nonveterinarian dentists can perform on horses has been an issue in Texas since 2006 when the Veterinary Medical Examiners Board changed its rules to prohibit lay equine dentists from performing procedures, including teeth floating. Last year, a Texas District Court judge ruled that the Texas Board of Veterinary Medical Examiners violated state law when it failed to follow state mandated procedures for making such policy changes.

The proposed legislation, HB 3637, would amend the Texas Agriculture Code to define an equine dental technician as a person who performs teeth floating on horses without the use of sedatives, tranquilizers, or general anesthetic drugs. It would also establish a registration system for nonveterinarian tooth floaters working in the state.

Loris Jones, spokesperson for the Texas Board of Veterinary Medical Examiners, said the group could offer no official opinion on the bill.

“We can’t comment,” she said. “We’re a resource. They (members of the Legislature) make the rules, we do what we’re told.”

No one from the Texas Veterinary Medical Association was available for comment.

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Lay dentist Carl Mitz said both nonveterinarian equine dentists and horse owners would benefit from the bill’s passage.

“It would mean I could keep my job of 20-plus years and that horse owners would continue to have the choice about who would float their horses’ teeth,” he said.

HB 3637 remains pending in the Agriculture Committee of the Texas House of Representatives