Study reveals insight into cardiac arrhythmia in thoroughbred racehorses


Post-mortem assessed underlying triggers and substrate

Approximately 50% of healthy racing thoroughbreds suffer from cardiac arrhythmia (irregular heartbeat) when exercising despite being bred for athletic performance.1 To demonstrate the underlying triggers and substrate for cardiac arrhythmia in racing horses, at the 2023 American College of Veterinary Internal Medicine (ACVIM) forum in Philadelphia, Pennsylvania, Laura C. Nath, BVSc, CertEM, FANZCVS, PhD candidate, with the University of Adelaide in Kew, Victoria, Australia, presented the e-poster “Histopathological evaluation of myocardial hypertrophy and fibrosis in thoroughbred racehorses.”2

Creative Digital Art /

Creative Digital Art /

The hypothesis of the study was that thoroughbred racehorses will have myocardial fibrosis, increased myocyte diameter, and a greater density of fibroblasts in relation to untrained horses.2

The retrospective research looked at thoroughbred racehorses, including 15 that experienced sudden cardiac death (SCD); 18 that died from other fatal injuries (OFI); compared to 10 healthy untrained (UT) wild horses.

In a post-mortem, cardiac tissues were sampled with the OFI and UT groups age matched to the SCD group. Two atrial and 3 ventricular sites were stained with Sirius red to look for presence of myocardial fibrosis and triple antibody stain (WGA, vimentin, and GS-IB4) looked for myocyte diameter and fibroblast density. To make measurements, a blinded operator utilizing automated analysis with commercial software was used. The results were tested for normality using the Shapiro-Wilk test and differences evaluated with 1-way ANOVA with various comparisons.2

It was revealed that the SCD group had a greater fibrosis proportion compared to the UT group at 4/5 examined sites.2 The UT group had reduced myocyte diameter at 5/5 sites compared to both SCD or OFI groups. There were no notable differences between SCD and OFI groups for either fibrosis proportion or myocyte diameter and there were no differences between any group for fibroblast density.

In conclusion, thoroughbred racehorses often suffer from histological myocardial hypertrophy. Myocardial fibrosis is significant in the SCD group, and this could be a substrate for arrhythmia. The lack of increased fibroblast density may infer underlying chronic fibrotic remodeling.2


  1. Histopathological evaluation of myocardial hypertrophy and fibrosis in thoroughbred racehorses. Nath LC. Presented at: American College of Veterinary Internal Medicine (ACVIM) forum; Philadelphia, Pennsylvania. June 14-16, 2023.
  2. Arrhythmia in racehorses. University of Minnesota. October 26, 2021. Accessed August 2, 20223.
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