The “X factor” is that indefinable something that makes for star quality. It’s all about heart, and in horse health, it can separate champions from chaff. Heart disease is the third most common cause of poor performance in horses.
Keeping great hearts ticking at their best is important. In an equine sports medicine overview, Common Causes of Poor Performance in a Horse, the Cummings School of Veterinary Medicine cites: “Within the animal kingdom, horses are elite athletes because of their unique physiology. Since they perform at such a high level, even the smallest change in health can knock down performance. Often, subtle effects on health that affect performance are challenging to detect, and sometimes require special diagnostics. It is important to investigate a decline in performance right away since it will only get worse with further athletic activities.”
According to Dr. Herb Maisenbacher, clinical assistant professor of cardiology at the University of Central Florida (UCF) College of Veterinary Medicine (CVM), heart disease is the third most common cause of poor performance in horses, after musculoskeletal and respiratory challenges.
Yet equine heart health remains relatively uncharted territory. “There are not a lot of studies on the prevalence of heart disease in horses,” Dr. Maisenbacher says. “The incidence of clinically significant heart disease seems to be lower than other animals, such as cats and dogs.” However, when compared to other large animals, heart disease becomes a greater concern because horses are performance animals.
“Heart disease can greatly affect the ability of a horse to race or jump, and may even lead to death,” says Dr. Johanna Elfeinbein, UCF CVM large animal internal medicine resident. “If a horse with heart disease collapses or suddenly dies when being ridden or pulling a cart, the handler may be seriously injured. Other large animals, such as cattle, develop heart disease, but are not asked to exercise and generally are not ridden, so there is less concern over injury to handlers.”
THE X FACTOR
According to UCF, the most common heart problem is an arrhythmia (irregular heart beat) called atrial fibrillation. This may occur due to underlying disease of the heart valves or muscle, or may occur in otherwise healthy horses, simply because large equine hearts are not unusual.
H.M. Gunn’s Journal of Anatomy (1989) says the adult horse heart generally weighs between 4 1/2 and 9-pounds. Very large or athletic horses, and Thoroughbreds, can have hearts that weigh more. Pony hearts tend to weigh between 1 1/3 to 2 1/2 -pounds.
BUILDING STRONG MUSCLE
Like ours, a horse’s heart has four chambers. Smaller-volume chambers at the top of the heart, known as the right and left atria, receive blood returned from either the body (right atrium) or lungs (left atrium) while cells within the heart conduct the electrical activity that coordinates muscles to contract for optimum blood pumping.
When the atria contract, each delivers blood to the larger volume ventricle below. The right side of the heart takes un-oxygenated blood from the body and pumps it to the lungs (allowing red blood cells to uptake oxygen). Then oxygenated blood returns to the left side of the heart, where the left ventricle pumps it into the aorta and through the body.
“The larger the heart is, the more volume of blood it can pump,” says Todd C. Holbrook, DVM, author of The Equine Heart. “It is generally accepted that the ability of the horse’s muscle mass to consume oxygen exceeds the ability of the heart and lungs to provide oxygenated blood. Thus, conditions that result in improved cardiac output positively impact performance.”
In looking at atrial fibrillation (the most common arrhythmia causing poor performance) Dr. Holbrook says, “During this condition, the electrical activity of the top chambers of the heart (atria) is uncoordinated. This results in reduced pumping of blood to the ventricles, as well as an irregular ventricular rhythm. The resulting reduced cardiac output can cause poor performance.”
LARGER THAN AVERAGE
Thoroughbreds are more likely to have extremely large hearts, because of a genetic condition known as the “X factor,” passed down via dam lines tracing to the undefeated 18th century British racing champion, Eclipse. Upon his necropsy in 1789, Eclipse was found to have a heart weighing twice (14 pounds) that of a normal horse and breeders believe this trait continues to be passed on through his daughters.
Pedigree research has verified that the dam line of Triple Crown champion, Secretariat, traced to a daughter of Eclipse. At the time of his death, the veterinarian who performed the necropsy, Dr. Thomas Swerczek, head pathologist at the University of Kentucky, did not weigh Secretariat’s heart, but did state, “We stood in stunned silence. We couldn’t believe it. His heart was perfect. There were no problems. It was just this huge engine.”
When Swerczek later performed the 1993 necropsy on Secretariat’s rival, Sham, he recorded the stallion’s heart weighed 18 pounds. Based on that measurement, and having necropsied both horses, he estimated Secretariat's had weighed 22 pounds, or about 2.5 times that of the average horse.
Understanding your horse’s heart at rest and during exercise is an important step in keeping it ticking soundly. According to Lesley Young, MRCVS, “A horse’s resting heart rate is between 28 and 36 beats a minute, while the maximum is between 225 and 240 a minute. The huge range of heart rate is the most important mechanism for increasing cardiac output during exercise, particularly as work intensity increases. Just as muscles for movement become stronger with training, so does heart muscle. Research has shown that the amount of heart muscle, and the width and area of the left heart chamber, increase dramatically with training.”
Neither heat nor stress seems to have a large impact on the horse heart, according to UCF veterinarians. “Work or training can cause some enlargement of the heart, so that it may pump blood more effectively,” says Dr. Maisenbacher. What is more likely is, “If a horse has an underlying heart disease, stress or work can negatively affect it, as the heart cannot function efficiently and congestive heart failure, or even death, may occur.”
WHEN THE TICK GOES TOCK
Symptoms of heart disease include poor performance, increased respiratory rate or effort, foamy nasal discharge, poor recovery from exercise, ventral edema (swelling of the bottom of the chest), collapse, or sudden death. “These may happen at any age,” says Dr. Elfeinbein, “depending on the disease.”
With age, horses can develop heart valve degeneration, usually affecting the aortic or mitral valves on the left side of the heart. The severity of valve degeneration tends to progress with time. While heart murmurs in horses, especially athletes, are a common result of turbulent blood flow, Dr. Holbrook adds that, “Murmurs associated with the valves between the atria and ventricles can result in poor performance, especially on the left (mitral valve) side of the heart. A thorough cardiac examination, including ultrasound of the heart, is warranted in any horse that has a significant heart murmur.”
The most common type of heart disease that horses are born with is a ventricular septal defect (VSD) or a hole between the two main pumping chambers (the left and right ventricles) of the heart.
“Horses with congenital heart disease (one that a horse is born with) typically show signs at a young age,” says Dr. Elfeinbein. “Horses with acquired (developed over time) heart disease, such as atrial fibrillation, valve disease, or disease of the heart muscle, are more common in adult or older horses.” According to the U.K.-published guide, Problems in Equine Cardiology, some congenital abnormalities appear more often in certain breeds. Standardbred horses have an increased incidence of congenital heart disease (CHD) compared with Thoroughbreds; complex CHD has been reported most frequently in Arabians, although overall, CHD is uncommon in horses (compared with other domestic species).
APPROACHING HEALTH FROM THE INSIDE OUT
Nutritionists at Omega Fields note that equine cardiac study is relatively limited, but that research, conducted in 2002 at the University of Minnesota College of Veterinary Medicine, has demonstrated how horses are susceptible not only to the geriatric concerns of degenerative valve disease and congestive heart failure, but also pericarditis, arrhythmia, and bacterial endocarditis.
Omega-3 fatty acids from plant sources, such as flax seeds, can support heart health. Stabilized ground flax for horses is rich in lignans, antioxidants shown to benefit the metabolic system, and flax seeds provide an Omega-3 fatty acid called alpha-linolenic acid (ALA), which a horse's body converts to eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).
“Although there are no specific equine studies on the effect of Omega-3 on equine heart disease, preliminary laboratory and human clinical scientific research points to ALA (Omega-3) as an effective stroke reducing agent,” says Omega Fields. “Research is also learning that ALA appears to protect the heart against arrhythmia, a decrease of the electrical stability of the heart. ALA inhibits atherosclerosis, an inflammatory condition. ALA may also be important in lowering blood triglyceride levels and because of this, it is believed to lower the risk of heart disease. It also reduces the chances of blood clots forming in the vessels and may protect against sudden cardiac death.”
Research on Omega-3 supplementation by Arenus® and Novus Nutrition, LLC looked at how mare nutrition affects foal growth and development, and found that DHA may have important implications for cardiovascular health in developing foals. When fed to pregnant mares, high quality Omega-3 fatty acids have been shown to optimize fetal heart development, as well as that of the brain and eyes. After foaling, Omega-3 fatty acids continue to transfer to the foal through its dam’s milk, so DHA originating from all-vegetarian sources, like micro-algae, is a good choice for pregnant mares since (unlike fish oil) it will be free of potential oceanic pollutants or toxins.
DON’T MISS A BEAT
Probably the most important advice to horse owners about heart disease is that their horses should have yearly examinations by a veterinarian. “Evidence of heart disease, such as murmurs or irregular heartbeats, can be determined on physical exam,” say UCF veterinarians.
“These findings may lead to early detection, so that treatment can be started, and injury to the horse or handlers prevented. If heart disease is suspected from abnormal exam findings or poor performance, a complete cardiac evaluation including an electrocardiogram (ECG or EKG) and echocardiogram (ultrasound of the heart) would be recommended.”
Horses, as the Cummings School notes, are elite athletes among the animal kingdom, thanks in no small part to the powerful muscle that beats within their chest. When you care for your horse’s heart like a champion, you are most likely to earn a whole-hearted performance in return.
L.A. Pomeroy of Northampton, Massachusetts, has been an equestrian photojournalist, award-winning publicist, and member of American Horse Publications since 1992, working with the U.S. Equestrian Team, Atlanta Committee for the Olympic Games, Equisearch.com, and Intercollegiate Horse Shows Association, as well as heading development and marketing for zoological institutions in New England. She enjoys trail riding in her native Catskill Mountains of upstate New York, collecting/researching equestrian art and collectibles, and making life better for the animals that share this planet with us.