The Horse:Your Guide To Equine Health Care provides monthly equine health care information to horse owners, breeders, veterinarians, barn/farm managers, trainer/riding instructors, and others involved in the hands-on care of the horse.
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45 October 2015 THE HORSE TheHorse.com unexplained. Interestingly, aortic rupture, which the public considers to be a com- mon cause of sudden death, occurred in only 1% of the cases in the study. Another problem that contributes to the difficulty in diagnosing sudden death's cause is the variation in post- mortem techniques and lesion descriptions during necropsy. Standards for dissecting and examining horses post-mortem would be helpful going forward, says Lyle. Another diagnostic challenge is that sudden death can have more than one cause. "Due to the limited knowledge of the pathophysiological background for these fatal events, it's difficult to identify risk factors, whether this could be pre- existing conditions like congenital or ac- quired disorders," says Buhl. "Among the horses that suffer sudden death, 20-68% of cases do not have structural lesions sufficient to account for death on nec- ropsy. Cardiac and/or pulmonary failure are found to be the most common causes diagnosed on post-mortem examinations, but also trauma, neurologic lesions, and acute hemorrhage are reported." Doug Corey, DVM, of Associated Vet- erinary Medical Center, in Walla Walla, Washington, who has worked with the Professional Rodeo Cowboys Association, also adds blood vascular issues (aneu- risms), stroke, stress, and abnormalities in the horse's internal systems to the list of possible causes. He says cases are very rare in the rodeo industry, and he has only seen one, which he believes was due to a heart condition; however, no autopsy was performed. In 35 years as an official event veterinarian in Western disciplines, Jerry Black, DVM, of Colorado State Uni- versity, says he has never seen a case. Elsbeth Swain, DVM, Dipl. ACVIM, also of Colorado State University, says there is little warning at maximal exercise before a horse collapses. "The veterinar- ian present may be able to intervene with supportive care after a horse acutely collapses, but depending on the cause, all efforts may be futile." She has a few recommendations for ways horse owners might avoid a tragedy: "The most important thing for prevention is to have your horse evaluated by a vet- erinarian at least once per year, especially prior to big events. Assessing the physical examination, diet, and conditioning of the equine athlete may aid in prevention. It is also important to ensure that supple- ments or medications will not negatively affect the horse in competition." Understanding the Equine Heart During Buhl's 15 years of studying the equine heart, her research results have correlated race performance to heart size: the larger the heart, the better. Her studies have also shown that training can induce significant cardiac hypertrophy, or thickening of the heart muscle. Stud- ies in humans have shown that this can increase an athlete's risk of collapsing and even dying during exercise. She is also studying atrial fibrillation, an irregular and often rapid heart rate that can cause poor blood flow, in racehorses. "The syn- ergy between equine and human research is strong and visible, and we hope our re- search will gain new insight into diseases that are of importance for both horse and man," says Buhl. Meanwhile, Physick-Sheard and Mc- Gurrin have been at the forefront of defin- ing horses' normal heart rhythm. They've shown that horses' heart sounds, rhythms, and rate vary greatly. Heart rate ranges from 16 beats per minute during rest to more than 200 beats per minute during intense exercise. At the end of an intense exertion, it's normal for the horse to experience irregular heartbeats. In most cases the heart rate eventually returns to normal. Yet, sometimes this rhythm disturbance can be fatal. "The frequency with which these seem to occur without consequence, and in horses that continue to race successfully, has been a significant observation," says Physick-Sheard. His latest research includes looking at biomarkers in a blood test to detect heart muscle damage in horses, which he says could be used as a screening tool for at- risk horses. Research in the Field The eventing world has also been on the forefront of sudden death research. The United States Eventing Association's (USEA) Cardiopulmonary Research Group (CPRG) is comprised of veterinar- ians, physicians, and EMTs dedicated to applying scientific principles to the study of death or collapse of event horses. Cath- erine Kohn, VMD, professor emeritus in the Department of Veterinary Clinical Sci- ences at The Ohio State University and co- chairperson of the CPRG, says the group has been working to perfect a system that reliably records an electrocardiogram (ECG, a measure of the heart's electrical efficiency) of a horse as it competes on cross-country and analyze that data. The CPRG has a special interest in the heart, as cardiac and lung issues have been the leading cause of collapse and sudden death in human athletes. In their first study they performed echocardio- grams (ultrasound examinations of the heart's structure) of 20 horses before and immediately after the cross-country phase at the 2009 Plantation Fields Horse Trials, in Kennett Square, Pennsylvania. They Researchers studying sudden death have been evaluating athletic horses' heart rates and rhythms, blood biomarkers, and lung fuid. PAULA DA SILVA Only 53% of sudden death cases in racehorses had a definitive cause