Heart Irregularities in Thoroughbred Racehorses

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By Mark Andrews 

Atrial fibrillation is the most commonly recognized disturbance of heart rhythm in athletic horses. It is an important cause of poor performance and has implications for safety of horse and rider. 

In atrial fibrillation (AF) the heart beats with a haphazard, “irregularly irregular” rhythm. The condition may be termed “paroxysmal” (when it recovers spontaneously within 72 hrs) or “persistent” (which continues if not treated).  

Laura C. Nath of the School of Animal & Veterinary Sciences, University of Adelaide, South Australia, and colleagues studied records of horses racing in Hong Kong over a ten-year period up to July 2017. They looked at the incidence of AF in poorly performing horses, and whether having had a previous episode of AF increased the likelihood of a horse being affected.

They report that, from a total of 96,135 race starts, atrial fibrillation was identified in 4.9 percent of horses, with an overall incidence of 2.7 episodes per 1000 starts. Many horses were retired after the first episode of AF. 

Having had a previous episode of AF increased the risk of AF, and recurrence was more likely in horses that had been treated previously for persistent AF than in horses that previously had paroxysmal AF. 

The researchers comment: “We identified a high rate of recurrence in Thoroughbred racehorses after both paroxysmal and persistent episodes of AF. This substantial AF burden could arise from underlying microstructural myocardial lesions and electrical remodelling.” 

They concluded: “Although horses can have long and successful careers after AF, the arrhythmia should not be considered benign, and the suitability of horses to continue their racing careers should be assessed on an individual basis.” 

The full, open access, report of their study, titled Incidence, recurrence, and outcome of postrace atrial fibrillation in Thoroughbred horses was reported in the Journal of Veterinary Internal Medicine. 

Related: The Equine Heart

Published with the kind permission of Mark Andrews, Equine Science Update. 

Photo: Shutterstock/Gabriel12