In North America, heart failure is a major cause of death for humans. It’s the number two cause in Canada, and number one in the United States.

In horses, though, it’s quite rare. Veterinarian and Ontario Veterinary College Professor Emeritus Dr. Peter Physick-Sheard said, “It’s not something the average horse owner will ever need to worry about – the heart is actually one of the least problem-prone organs in a horse’s body. Equine cardiologists tend to be rather lonely. However, even though sudden death from heart problems is not a common event, we need to understand and deal with it.”

Death isn’t the only issue to worry about. Colorado State University professor and veterinarian Dr. Janice Bright added, “Despite its relatively low prevalence, heart disease may seriously impair performance and longevity of affected horses.”

The Cardio-Vascular System

All the organs in the horse’s body work together, but Dr. Physick-Sheard says the heart in particular can’t really be considered on its own. The cardio-vascular system includes the heart and all the blood vessels. “The heart has its specific function – it is the pump at the centre of the system,” he explained. “But it is at the capillaries, where the blood vessels interact with the tissues of the body that the rubber hits the road. The blood flowing into those capillaries brings nutrients, including oxygen, to those tissues and removes wastes that can be toxic, keeping everything happy and healthy.”

When horses are working at maximum effort, the muscle tissues make high demands on those blood vessels, because they need more nutrients and have lots of wastes to be removed. The capillaries expand and the blood flow increases dramatically.

“And here is where the heart comes in,” said Dr. Physick-Sheard. “In response to those changes, the heart beats faster and harder to maintain the pressure, meeting the needs of the capillaries and the tissues.” The heart is not in charge, it simply responds to the drop in blood pressure caused by the dilated capillaries.

He added that the heart is also able to anticipate an increased workload. If you approach your horse carrying a saddle, his heart rate will go up and muscles will tense. His body is getting ready for the demands you are about to place on him.

Congenital Heart Problems

Some heart problems are congenital, meaning the foal is born with a heart that is not functioning well. Dr. Bright said: “Although only a few congenital heart defects have been proven to be heritable in certain breeds (for example, glycogen branching enzyme deficiency in Quarter Horse foals and ventricular septal defects in Welsh Mountain ponies), congenital heart diseases are very likely to be inherited. Most will cause a characteristic, pathologic murmur and possibly pathologic arrhythmia as well.”

Dr. Physick-Sheard mentioned two other potentially inherited tendencies to develop heart problems: arterial disease in Friesian horses and atrial fibrillation in Standardbreds.

In foals, not only is the veterinarian likely to hear a loud heart murmur when examining the horse, the foal may be stunted and fail to gain weight well, may be weak and prone to collapse, may have some swelling in the lower legs and abdomen and may have pulsating, distended neck veins, according to Dr. Bright.

“This is one reason to have a veterinarian check all newborn foals,” said Dr. Physick-Sheard. “However, unless they are severe, many heart murmurs will disappear or at least improve significantly within two or three weeks.” Some congenital heart problems are not associated with obvious signs.

Foals with more severe problems are usually euthanized, but with more minor problems this may not be needed. Dr. Physick-Sheard pointed out that horses can survive with many of the abnormalities, although performance may be affected. In some cases, they are able to compete and do well. “I have seen racehorses who turned out to have holes in their hearts,” he said.

A serious heart rhythm irregularity could cause a drop in blood pressure enough to make the horse collapse. This is called cardiac syncope, said Dr. Physick-Sheard. “Basically, the horse faints,” he said. While that doesn’t sound too serious, the collapsing horse can be injured, cause rider injury, and the arrhythmia that caused the blood pressure to drop may be fatal in itself.

Often the horse has had some incidents of “wobbliness” previously. This is most likely to happen when the horse is doing very intense work. Dr. Physick-Sheard said several researchers are currently studying this with the goal of identifying causes and potential treatments.

Acquired Heart Problems

There are some less common causes of heart problems. The medication monensin sodium is widely used in chicken feed and feed for beef and dairy cattle. It promotes growth and also kills the coccidiosis parasite in chickens. But for horses, even a small amount of the feed can cause serious problems.

Monensin affects how ions are transported in the various cell membranes. Sodium and potassium ions are essential for normal contraction of the heart muscle, so monensin can cause the heart to contract abnormally and eventually lead to cardiovascular collapse. The horse might appear to have colic at first, but will eventually develop a rapid heart rate – often double or triple his normal rate – and show laboured breathing even at rest.

Researchers don’t know why horses are much more sensitive to this drug, compared to other farm animals. It is never used in feed sold for horses, but there have been cases where it was accidentally mixed in at the manufacturing plant. In other situations, horses living on a farm with other animals have discovered some chicken or cattle feed that has been spilled or left in an area where the horse can access it, and then consumed it.

Many horses will die fairly quickly after eating these feeds, but if they have not consumed too much, your veterinarian may try treating the horse with Vitamin E and selenium. However, these treatments don’t repair damage that was already done. The horse may appear to have recovered, but his heart is still weakened, and he is at risk of developing congestive heart failure if he is stressed in some way, such as participating in competitive sports.

There’s another type of feed that can pose a risk to your horse’s heart, said Dr. Physick-Sheard – hay that’s been infested with blister beetles. These tiny beetles feed on alfalfa flowers and can easily end up in alfalfa hay during the harvesting process. The beetles are small – less than an inch long – and often they get crushed or cut into even smaller pieces by the machinery used in baling. That makes them even harder to detect. (But any bales of alfalfa hay should be inspected before feeding.)

The problem with blister beetles is that they contain a chemical called canthardin, which is very dangerous for horses. Just touching the beetle will cause blisters on the horse’s skin (hence the name). But eating them is worse. The canthardin can cause ulcers inside the horse’s mouth and stomach, but can also get into the circulatory system and affect the heart. In some cases, this can cause death within hours.

There is no specific treatment for canthardin ingestion, but veterinarians will try to reduce the amount that the horse absorbs by giving it activated charcoal, mineral oil and intravenous fluids. The effectiveness of this treatment, and the risk to the horse, depends on how much the horse has consumed.

Various infectious diseases and other toxins, such as influenza and botulism, can also weaken the heart.

Sudden Cardiac Death

Horses don’t have heart attacks like people do, said Dr. Physick-Sheard, and they don’t have coronary artery disease, which is the forerunner to a heart attack for most humans.

Dr. Bright explained that horses can die from sudden cardiac death, which is often described in the media as a “heart attack.” Strictly speaking though, it’s not. She said, “Sudden cardiac death in horses occurs usually during exercise as the result of a rupture of the aorta, the largest artery in the body. When rupture occurs, the horse very quickly bleeds out into the chest cavity.” These horses, she added, are likely to have an aneurysm of the aorta, or a prior weakening of the blood vessel wall, which predisposes it to rupture when the horse’s blood pressure increases during exercise. She mentioned cardiac rhythm disorders as another cause of sudden collapse or sudden death in horses.

A 2015 study, involving researchers from Spain, Kentucky and Florida, reviewed 137 cases of horses that had died suddenly of aortic rupture. They found that there was evidence of degeneration of the fibres in the wall of the aorta in these horses that seemed to predispose the wall to rupture during intense exercise. The main risk factors they detected were previous hypertension (high blood pressure), previous vascular problems (such as aneurysm) and an enlarged heart.

The researchers pointed out that while in a few cases the horses may show a decline in performance or intolerance for exercise, in most cases the death is unexpected and sudden.

Heart Smarts

Dr. Physick-Sheard said good general management is really the best way to keep your horse’s heart healthy. “That includes progressive and consistent training, with no sudden increases in exercise level,” he noted.

Many people don’t know how to condition a horse to gradually strengthen his cardiovascular system and muscles, he added. “It’s, unfortunately, common for people to ask too much of the horse, and then if there are any problems with the heart it can be over-stressed.”

By working with your trainer to plan a gradual program of conditioning, you can increase your horse’s fitness and competitive abilities without excessive stress. Pay attention to your horse’s responses, advised Dr. Physick-Sheard. If he is having difficulty with what you are asking him to do, drop back to the previous level for a while, and then move up gradually again.

Ensuring good nutrition and hydration will also protect the heart from the strains of vigorous exercise, he added. Dehydration puts considerable stress on the heart, and horses lose a significant amount of fluid and essential electrolytes when working hard in hot weather.

Dr. Physick-Sheard said it’s important for horse owners to be familiar with the normal sounds and rhythms of their horse’s heart. “All horse’s hearts make sounds, and they can sound like geese honking, like bedsprings squeaking, like sandpaper scratching or other noises. Two-year-old Thoroughbreds often have what is called the ‘two-year-old squeak.’ You want to be familiar with your own horse and what his heart normally sounds like, both at rest and right after a workout.”

Additionally, the horse’s heartbeat is likely to show a wide variation in rhythm. “That’s just part of being a horse. Every heart drops beats and adds beats while at rest, and every horse will have its heart rate go up and down significantly after a workout. There’s no need to get worried about this kind of variability.” He added that while there are huge variations between horses, each individual horse tends to follow its own pattern, so if you are familiar with how your horse’s heart reacts, you’ll be more likely to catch any changes.

Most of the more significant problems also involve a change in the horse’s behaviour. If you see a change in your horse’s heart rate pattern that is accompanied by the horse having less energy than usual, or becoming tired quickly, or not performing well, then Dr. Physick-Sheard recommended consulting with your veterinarian.

“Horses normally have lots of cardiovascular reserve,” he added. “Heart disease tends to be quite advanced before you really notice the issue.”

Despite the range of conditions that can affect a horse’s heart, these more serious problems are really quite rare. Dr. Bright pointed out, “Although the popularity of horse racing has increased awareness of acute collapse and death in horses, the overall death rate during track racing is reported as only one to three out of 10,000 starts. Of these, about half are believed to result from heart or vascular disorders.”

In other words, Dr. Physick-Sheard said, sudden, unexpected death from a heart problem is not something most riders need to be worried about. And the best things you can do to prevent the development of heart issues are simply basic management: know your horse, and provide good nutrition, hydration and gradual conditioning.