Exercise-Induced Inflammation and Injury in Racehorses

Horohov believes if researchers can develop a method to identify at-risk horses based on inflammatory response, they could prevent more training and racing injuries.

Photo: Anne M. Eberhardt/The Horse

When 2010 Horse of the Year Zenyatta crossed the finish line for the last time, most race fans focused on the fact it was the first loss of her career. But what might be even more notable is that it was also her 20th start--unusual in an industry that has seen an overall decline in starts (down to 6.11 per horse per year in 2010, according to The Jockey Club).

One of the reasons researchers propose for this decline is the modern-day Thoroughbred's questionable durability. David Horohov, PhD, William Robert Mills Chair at the University of Kentucky's Gluck Equine Research Center, however, believes the breed is not necessarily becoming more fragile, but rather more susceptible to injury due to breeding strategies, training methods, and increased drug use. During the Veterinary Science Seminar "The Effect of Training and Nutritional Supplementation on Exercise-Induced Pro-Inflammatory Cytokine Gene Expression in Two Year Old Thoroughbreds," held March 20 in Lexington, Ky., he explained how we might ultimately be able to identify individuals at risk for injury and, thus, try to prevent those racing and training injuries from occurring in the first place.

The statistics in racehorse "wastage" due to injury are shocking: Less than 60% of 2-year-olds in training race, and less than 80% of those continue to race as 3-year-olds, said Horohov. Career-ending injuries are, for the most part, musculoskeletal and include bowed tendons, suspensory ligament injuries, fractures, splints, and osselots (traumatic arthritis of the fetlock joint), among others.

Horohov presented the theory that these injuries are often the result of mild to moderate damage occurring over time at a rate that exceeds the affected tissues' healing capabilities. He believes if researchers can develop a method to identify at-risk horses based on inflammatory response, they could prevent more injuries.

Inflammation (identifiable by swelling, pain, loss of function, and redness) occurs when tissue signals cells to respond to damage. Even a small amount of tissue damage can trigger this inflammatory response; thus, exercise itself can induce inflammation, Horohov explained. This exercise-induced tissue damage is part of the musculoskeletal healing and repair process as an individual adapts to exercise.

"While some inflammation is necessary for tissue repair, exaggerated inflammatory responses are likely associated with injury," he said.

In his recent study, Horohov and his colleagues evaluated exercise-induced inflammation in young racehorses in training. He hypothesized that as the horses become better conditioned, their inflammatory responses will decrease and they will eventually reach an anti-inflammatory state. He added a nutritional supplement (by Equine Nutraceuticals) containing antioxidants and anti-inflammatory components as top-dressing to half the study horses' daily rations to determine whether it also helped reduce inflammation.

The study involved 25 2-year-old racehorses in training under one Maryland trainer. In the randomized, double-blind experiment, 12 horses received a placebo top-dressing and 13 received the supplement. All horses trained on a grass track at increasing speed and duration over an eight week period. The researchers collected blood samples prior to exercise, immediately post-gallop, and two hours after exercise at the beginning of training and again at weeks 2, 6, 7, and 8. They then measured blood samples for lactic acid (which increases fatigue) and malondialdehyde (MDA, an indicator of oxidative damage to membranes) levels as well as pro-inflammatory cytokines (inflammatory mediators) and lymphokine-activated killer cell activity induced by exercise.

Upon reviewing the results, Horohov observed a time- and intensity-dependent increase in lactate immediately after exercise. "Horses are, in fact, responsive to different intensity exercise," he said. Though not statistically significant, he also observed an MDA increase. The supplement appeared to have minimal effect on the results.

The blood samples taken two-hours post-exercise also showed a time- and intensity-dependent increase--this time in inflammatory cytokine expression. Horohov noted that inflammatory response was less in horses given the supplement.

"As training continued, we saw a decrease in pre-exercise inflammatory cytokines (indicating adaption to exercise)," he explained. "The most profound effect was seen in the supplemented horses."

Horohov concluded that exercise results in characteristic changes in cytokine gene expression in acute and late response. He noted that racehorse owners and trainers could potentially use this information to identify successful adaption to exercise. The supplement appeared to enhance the horses' adaptation to exercise, perhaps by reducing inflammation and oxidative damage.

Of note, Horohov said three of the 25 study horses failed to finish the study due to injury. All three completed the third exercise test and developed lameness days later, after subsequent training. These horses all showed a significant increase in cytokine levels two hours post-exercise in comparison to the healthy horses. He noted that this information could be used to monitor horses in training for changes that could indicate impending injury, necessitating a break from training.

Horohov now intends to conduct another study of 100 2-year-olds in training to determine the relationship between cytokine gene expression and occurrence of specific training-related injuries. His ultimate goal is to use this technique to identify horses at risk of injury during training and on the racecourse.

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About the Author

Alexandra Beckstett, The Horse Managing Editor

Alexandra Beckstett, Managing Editor of The Horse and a native of Houston, Texas, is a lifelong horse owner who has shown successfully on the national hunter/jumper circuit and dabbled in hunter breeding. After graduating from Duke University, she joined Blood-Horse Publications as Assistant Editor of its book division, Eclipse Press, before joining The Horse.

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