A better method for detecting heart problems in horses

The accumulation of scar tissue within the horse's heart muscle can diminish performance and lead to a potentially fatal cardiac episode.

Researchers in Australia are using tissue analysis and high-tech computer scanning techniques to detect scarring of the heart that can diminish performance and lead to potentially fatal cardiac problems.

Myocardial fibrosis is the accumulation of scar tissue within the heart muscle. Racehorses are susceptible to this condition because during athletic exertion “exceptionally high pressures are generated in the heart, and this can cause inflammation of the heart muscle wall. Following inflammation, the heart muscle wall might then heal with scar tissue,” explains Laura Nath, BVSc, of the University of Adelaide.

Three racehorses head toward the wire
Cardiac scar tissue has been linked to the development of irregular heart rhythms that can lead to diminished performance and even sudden, fatal cardiac arrest in racehorses.

When the amount of scar tissue becomes excessive, it can interfere with cardiac function. Cardiac scar tissue has been linked to the development of irregular heart rhythms, particularly during intense exercise, that can lead to diminished performance and even sudden, fatal cardiac arrest in racehorses. “Cardiac arrhythmia is an important cause of poor performance and sudden death in athletic horses,” says Nath. “Normally, the electrical impulse is conducted through the heart following a standard pathway. Cardiac fibrosis can interrupt normal cardiac electrical conduction and support the conduction of impulses through abnormal pathways.”

Nath, working in conjunction with researchers from the University of Melbourne, is using specialized cardiac muscle staining techniques along with computer analysis to quantify the amount of scarring within equine heart muscle tissue samples. In one preliminary study, the researchers compared heart muscle tissue from eight horses who died of sudden cardiac arrest with samples from horses euthanatized for orthopedic conditions and from untrained wild horses known as brumbies.

“We have found that horses that have died suddenly during racing or training from suspected sudden cardiac death have a higher incidence of heart muscle scarring in the free wall of their main heart chamber when compared to both the previously untrained brumbies and other racehorses that were euthanatized for serious orthopedic injuries,” says Nath.

These findings could help researchers quantify the level of cardiac fibrosis likely to trigger potentially fatal arrhythmias. “We are also trying to find out which particular parts of the heart muscle are most likely to be affected by fibrosis and inflammation,” says Nath.

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Although some support for this ongoing study comes from the Racing Victoria Equine Research Fund, these findings can benefit not just racehorses but all horses involved in strenuous activities, says Nath. Further funding is being provided by Agrifutures, a government-funded research group supporting agriculture in the country. “Cardiac arrhythmia is a concern for horses in all disciplines but particularly those exercising strenuously such as Thoroughbred and Standardbred racehorses, eventing horses and endurance horses,” Nath explains.

Reference: “Myocardial fibrosis in Thoroughbred racehorses, potential substrate for cardiac arrhythmia?” Australian and New Zealand College of Veterinary Scientists Annual Conference Proceedings, July 2019

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