The most likely explanation for about half of the sudden deaths in apparently healthy racehorses is heart rhythm irregularities, a researcher believes.
Sudden death in athletic horses is rare but distressing. By definition it is seen in horses that, moments before, had appeared perfectly fit and healthy.
The absence of any warning signs complicates the task of identifying causes.
Catriona Lyle recently led a multicentre study in a bid to gain a better understanding of the problem and find ways to reduce the risk of sudden death occurring.
The work, undertaken at the Royal (Dick) School of Veterinary Studies in Edinburgh, was made possible by Britain’s Horserace Betting Levy Board, which is funding a three-year scholarship.
Lyle co-ordinated a collaborative study drawing information from racecourses in North America, Australia, Japan and Hong Kong, the latest issue of Equine Science Update reports.
Post-mortem data from 284 cases across a 20-year period were studied. In Britain, post-mortem examinations are not always carried out in cases of sudden death, so gaining access to international records was essential.
“The study has shown that the cause of death can be quite variable,” Lyle said. They ranged from severe lung bleeding to a pelvic fracture that causes massive bleeding into the abdomen.
“But in approximately half the cases I studied, the pathologist was uncertain as to the cause of death. The most likely explanation for death in these situations is cardiac rhythm irregularities, but this is very difficult to prove.”
Following her analysis of the international data, Lyle then looked at cases of sudden death in British racehorses.
She found that sudden death of horses during races was very rare. Over seven years, in 705,914 race starts, there were 201 sudden deaths on British race tracks. The same syndrome is known to occur in eventing, showjumping and hunting, but statistics have not been established in these sports.
In the British study, she found that increasing age is a risk factor, with steeplechases posing more of a risk than flat races, and racing during summer was associated with a greater risk of sudden death.
However, this should be put in the context that, on average, steeplechasers were older than hurdlers or flat horses, she noted. Horses that had raced within the last 60 days were less likely to be affected.
The chief veterinary officer of the British Horseracing Authority, Jenny Hall, welcomed the findings.
“This was an extremely useful project. We are continuing to build on Catriona’s research with an ongoing investigation currently running at Britain’s northern racetracks.
“Sudden death is very distressing and we hope that owners will understand that allowing a full investigation into every racecourse death will help us reduce this risk.”
Lyle CH, Uzal FA, McGorum BC, Aida H, Blissitt KJ, Case JT, Charles JA, Gardner I, Horodagoda N, Kusano K, Lam K, Pack JD, Parkin TD, Slocombe RF, Stewart BD, Boden LA (2011) Sudden death in racing Thoroughbred horses: an international multicentre study of post mortem findings. Equine Vet J 43 324-331.
Lyle C, Blissitt K, Kennedy N, McGorum BC, Newton R, Parkin T, Stirk A, Boden L (2012) Risk factors for race-associated sudden death in Thoroughbred racehorses in the UK (2000-2007). Equine Vet J 44 459-465. DOI: 10.1111/j.2042-3306.2011.00496.x