Several distinct risk factors for sudden cardiac death in racehorses have been identified by Australian researchers, but pinpointing horses at the greatest risk of dying won’t necessarily be easy.
Understanding the causes of racehorse deaths is an important step in addressing this welfare concern, Laura Nath and her fellow researchers wrote in the journal Animals.
Previous studies have shown that, during Thoroughbred racing, catastrophic musculoskeletal injuries requiring euthanasia account for about 75% of fatalities, with sudden athletic death occurring in the remaining quarter of cases.
In cases of sudden athletic death, a post-mortem examination is needed to determine the cause. Around half of them are attributed to a fatal cardiac arrhythmia, which is termed sudden cardiac death.
The study team noted that, in humans, a fatal cardiac arrhythmia is often associated with underlying cardiovascular disease and is the most common mechanism of cardiac arrest and death during sports.
Inherited or congenital cardiac diseases present almost exclusively in younger athletes and encompass most sudden cardiac deaths in this group.
The underlying factors associated with sudden cardiac death in horses are not well established in horses, they said, although myocardial inflammation and fibrosis, chamber dilation and trabecular and myocardial hypertrophy – abnormal muscular thickening of the heart muscle – have been described.
In order to investigate risk factors for sudden cardiac death, researchers from the University of Adelaide and the University of Melbourne reviewed post-mortem reports from horses who died on racetracks in Melbourne. They grouped the horses into sudden cardiac death and all other fatal injuries.
In all, 57 horses with sudden cardiac death and 188 control horses with other fatal injury causes were identified for analysis. All were Thoroughbred, with a median age of 4.2. They comprised 69 females, 121 geldings and 55 stallions.
Post-mortems were performed at a single institution by one of 19 qualified veterinary pathologists.
In the 57 horses with a final classification of sudden cardiac death, 26 received that diagnosis without any other significant lesions, three also had bleeding into the lungs, two also showed pulmonary oedema and congestion, two had pulmonary oedema, and one had bleeding into the lungs, pulmonary oedema and congestion.
Of the six horses with myocardial lesions, three had a final diagnosis of cardiomyopathy, two had myocarditis and one had left ventricular concentric hypertrophy.
Of the remaining 17 horses in which sudden cardiac death could not reasonably be excluded, 11 horses had severe lung bleeding; two had mild lung bleeding; three had lung bleeding and pulmonary oedema and congestion; and one had pulmonary oedema alone.
The great majority of the 188 control horses who died had suffered leg fractures, although 10 had a tendon or ligament injury, and two suffered an internal haemorrhage.
“We found that horses with sudden cardiac death were more likely to die during training than during racing,” they reported – a finding that suggests exercise intensity is not critically important in precipitating sudden cardiac death in horses. Affected horses tended to have fewer lifetime starts and were less likely to be stallions.
Sudden cardiac death also tended to occur earlier in the careers of affected horses, at a median age of 3.6, when compared with other fatalities, with a median of 4.5 years.
Discussing their findings, the researchers said a diagnosis of sudden cardiac death is restricted to horses with a diagnosis of likely fatal cardiac arrhythmia.
“Often, the diagnosis of likely fatal cardiac arrhythmia is reached in the absence of definitive evidence and following the exclusion of other known causes.”
The incidence of sudden death in horses is estimated to be 200 times the rate in human athletes. Sudden athletic death accounts for 1 to 3 deaths per 10,000 starts in Thoroughbred racehorses and comprises about 13 to 25% of Thoroughbred flat racing fatalities.
“Previous studies have investigated risk factors for sudden athletic death. Ours is the first study to include horses in training when investigating risk factors for sudden cardiac death.” It occurred in association with training in three-quarters of the horses in the study.
“This finding highlights that many cases of sudden cardiac death could be overlooked if focusing only on race-day fatalities.”
In comparison, around half of fatalities from other causes in the study were sustained during training.
The authors noted that bleeding in the lungs, known as exercise-induced pulmonary haemorrhage, occurs in most racing Thoroughbreds and was commonly identified in horses in this study classified with sudden cardiac death. Lung bleeding, pulmonary congestion and oedema were also observed in some horses in the study who were euthanised because of an orthopaedic injury.
“This further supports the concept that pulmonary lesions might not be causative and can be observed in many horses dying suddenly or euthanized subsequent to exercise,” they said.
The study team acknowledged that the identification of horses at risk of sudden cardiac death before a clinical event remains challenging.
“Our study identified that the majority of these events occur during training and this raises the possibility of improved monitoring for cardiac disorders of horses as they commence and continue through training.
“Emerging technologies allow for enhanced monitoring of horses during exercise, with device capabilities including heart rate, ECG, velocity, stride length and stride frequency.
“The sensitivity of these devices for identification of abnormalities of cardiac rhythm might be enhanced through the application of heart rate variability algorithms.”
More research is needed to determine the capability and application of such devices for screening horses, they said.
They said their findings amount to important new information and support the potential role of inherited or congenital cardiac disorders in the development of sudden cardiac death in horses.
The study team comprised Nath, Adrian Elliott and Samantha Franklin, with the University of Adelaide; Andrew Stent, with the University of Melbourne; and Andre La Gerche, with the Baker Heart and Diabetes Institute in Melbourne.
Nath, L.; Stent, A.; Elliott, A.; La Gerche, A.; Franklin, S. Risk Factors for Exercise-Associated Sudden Cardiac Death in Thoroughbred Racehorses. Animals 2022, 12, 1297. https://doi.org/10.3390/ani12101297