Sometimes racehorses collapse on the track for reasons that are not immediately apparent; these exercise-related sudden death cases are uncommon and "poorly understood," according to an international team of researchers that recently completed a retrospective study on the subject. They noted that veterinarians are only able to determine the actual cause of death in about half of these cases that undergo postmortem examinations.

"In the study sudden death was defined as an acute death in a closely observed and previously apparently health animal, in the absence of a traumatic event (e.g., falling at a jump)," explained Catriona Lyle, BVM&S, the Horserace Betting Levy Board senior clinical scholar in equine internal medicine and cardiology at the University of Edinburgh. "In racing Thoroughbreds, sudden death is rare and our understanding of causes of sudden death is limited."

Lyle worked on this study with a team of researchers from the United States, Great Britain, Japan, Australia, and Hong Kong). She explained, "The motivation for this study arose from a Dorothy Havemeyer Foundation-funded workshop where it was recognized that there was a need for an improved understanding of the causes of sudden death in racehorses."

Team members collected post-mortem data from 268 Thoroughbred racehorses that suffered sudden death during or within one hour after exercise. These animals were from six racing jurisdictions around the world. According to Lyle, "causes of death were identified in only 53% of the cases