SUDDEN DEATH SYNDROME
Cardiovascular equine sudden death syndrome Contemporary horseracing news has identified a small medical trend in which assumedly disease-free racehorses are suddenly expiring before their time. Several investigations indicate that the trainers of such horses are not culpable in these tragedies sustained by their animal charges. If so, then it is necessary to examine these cases in a new light, such that the physiological “machines” that racehorses are provide some answers as to the source of this Sudden Death Syndrome (SDS).
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WORDS: THOMAS MAHONEY PHOTOS: HORSEPHOTOS, FRANK SORGE/GALOPPFOTO
ETER Physick-Sheard, BvSc, MSc and fellow of the Royal College of Veterinary Surgeons of the Ontario Veterinary College at the University of Guelph in Ontario, Canada, remarks, “Horses have been evolving ever since first used by man [7-10,000 years ago]. Embarkment on the domestication and selective ‘typing’ of horses [3-5,000 years] has been followed by subsequent use of intense physical selection pressure [for as little as 200 years in some breeds]. However, these manipulations have been sufficient to produce the competition horses we now have. “The cardiovascular (CV) system of the horse is remarkably similar to that of other mammals with a few minor, but important, differences related to elements of their large muscle contractility, skeletal properties, and vessel flow dynamics” that result from maintaining a proportionately larger heart, says Nicholas Robinson, PhD, assistant professor of veterinary pathology at the College of Veterinary Medicine at the University of Minnesota. Robinson continues, “Equine athletes have been developed to go fast; the CV systems of racehorses [Thoroughbreds and Standardbreds, as well as Quarter Horses, Arabians, and Appaloosas], as a group, have genetically striven to keep up.” Physick-Sheard opines that such rapid evolution in body type has not necessarily been accompanied by corresponding changes in functional CV capacity and robustness. “Due to the type of repetitive work racehorses are asked to perform in today’s racing world they are subjected to physical stresses that don’t really have an
equivalent in their evolutionary history.” So, it has been observed that, under certain circumstances, this cardiovascular fitness can break down. In research by Catriona Lyle and her colleagues, SDS has generally been defined as an acute collapse and death of a closely observed and apparently healthy racehorse during or immediately following exercise, i.e. racing or training. A host of Thoroughbred risk factors, quotes this research group, may include higher age; increased race distance; a greater number of starts in the past 60 days; racing in high heat; and being engaged in hurdling, steeplechase, or National Hunt. The roster of prominent racehorses that have succumbed to this condition is legion: Admire Rakti, ventricular arrhythmia; Irrefutable, cardiopulmonary failure; Mr. Nickerson, cardiac condition; Swale, collapse, heart abnormality. Jumpers include Tiptoeaway, collapse, cardiac condition; Do Rightly, heart attack; Earthstopper, collapse, post-race; and Hickstead, aortic rupture. Physick-Sheard comments, “Each horse displays a certain CV reserve capacity to do extra work [when training or racing] until a ceiling or maximal level is hit.” It may be that SDS is an adverse consequence of attempts to surpass this point. Subsequently, the dividing line between “sickness” and health may be a fine and murky one. In these cases, we are lead to question what, despite expensive and extensive post-mortem investigation including toxicology screens, precipitates SDS. Indeed, in about half the occurrences documented, no definitive reason for this syndrome is detected by pathologists in necropsy. Sudden death, a calamitous event, is an
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