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Study Explores Causes of Sudden Death in Racing HorsesOctober 2008


He doesn't get to the racetrack very often, but Ontario Veterinary College clinician Luis Arroyo can imagine the shock that grips grandstand patrons when a racehorse drops dead in mid-stride. Perhaps more distressing than those occasional sudden deaths is the inability to pinpoint what has felled the animals, he says.

In a paper published last month in Veterinary Pathology, Arroyo and co-authors examine a possible cause of these unexplained deaths in standardbred and thoroughbred racehorses. He wrote the paper with Prof. Laurent Viel, Clinical Studies; Prof. Tony Hayes, Pathobiology; Josepha DeLay, an Animal Health Laboratory (AHL) pathologist; McMaster University forensic pathologist Dr. Chitra Rao; and Bruce Duncan, a veterinarian with the Ontario Racing Commission (ORC) in Toronto.

About 150 horses die on racetracks in Ontario each year. About half of the cases result from fractures. In about one-quarter of the remaining cases, says Arroyo, experts can't explain what causes the animals to die.

A clue may lie in arterial calcification, or hardening of the pulmonary artery, in young racing horses. “It's as hard as a rock,” says Arroyo, who identified that hardening during post-mortems of horses sent to OVC under the ORC's death registry program. The program requires any horse that dies within two weeks of training or racing to be examined using a standard forensic protocol.

His tissue slides show how the artery walls fill with fibrous collagen and lose the elasticity that normally enables this large blood vessel to pump blood from the heart to the lungs, where it's enriched with oxygen.

Almost 85 per cent of racing horses show this hardening, mostly in their pulmonary arteries. In humans, vascular stiffness often occurs in patients with renal damage or diabetes. Neither disease is involved in racing horses, says Arroyo.

The researchers suspect that artery calcification partly explains some of these mysterious deaths. Along with U of G engineering professor John Runciman, they've studied artery biomechanics and found changes in the behaviour and structure of artery walls that may make the vessels susceptible to hardening.

Nobody knows what risk factors are involved, but Arroyo says the lifestyle of these racing horses may play a role.

The researchers looked at tissues from more than 100 racing horses that died between early 2006 and early 2007.

About 1,000 horses have come to the AHL for post-mortem in the past six years; Arroyo has examined about one-third of those cases.

“I was curious about the unknown cause of death in these cases. Are we missing something?” He completed this study while doing his PhD in pathobiology. A D.V.Sc. graduate of Guelph, he will join the faculty of the Department of Clinical Studies in January.

Funding for this study came from the ORC.