Common Heart Issues in Horses

A vet uses a stethoscope to check for heart issues in a horse
Photo by JNix/Shutterstock

Horses, like humans, can experience a variety of heart disorders or defects, ranging from inconsequential to life-threatening issues. Though some diagnostic tools are the same between species, such as the use of stethoscopes in physical exams, echocardiography (ultrasound) and electrocardiography (ECG), some tools used in human medicine are simply not adapted for equine heart health. This makes it even more critical that a qualified veterinarian help determine the cause and potential ramifications of the issue.

Signs of Heart Issues in Horses

Not every horse that has a heart issue presents with the same clinical signs. Some horses may become short of breath quickly or take longer to cool out after exercising. Others may become weak to the point of collapsing or develop fluid accumulation in their legs or abdomen.

“The vast majority of heart problems are incidental findings picked up during routine physical examinations or during a pre-purchase examination; in most cases, if clinical signs of a heart problem are observed, the cardiac disease is advanced,” says J. Barry David, DVM, Dipl. ACVIM, of Hagyard Equine Medical Institute in Lexington, Ky. David is a board certified specialist in internal medicine who sees many horses with suspected cardiac issues each year.

An illustration of a horse's cardiovascular system, used to explain heart issues in horses
The horse depends on his cardiovascular system, powered by the heart muscle, for speed and stamina. Photo by SciePro/Shutterstock

If a heart condition is suspected, it’s important to contact a veterinarian who can complete a through physical exam. A heart issue can affect the horse’s performance and his life expectancy, as well as potentially affecting the safety of his rider, driver or handler.

The most common issues David finds when examining horses for potential cardiac issues include heart murmurs, arrhythmias and congenital abnormalities. Each of these conditions has a range of possible implications based on their severity.

Heart Murmurs

Among the most common heart issues in horses, a heart murmur is suspected when a veterinarian hears an extra sound while listening to a horse’s heartbeat. Instead of the two-beat “lub-dub” made by a healthy heart, he or she may hear a whooshing or swishing sound in between these beats. This sound is made by blood flowing through the heart at abnormal times. Horses can be born with murmurs, or they can develop later in life.

There are multiple types of murmurs a horse can have, but the most common are mitral valve murmurs, tricuspid murmurs and aortic valve murmurs. In each of these cases, the murmur is caused by a leak in the valves that separate the chambers of the heart, allowing blood to seep into a chamber before it should.

Graphic of circulation through the heart
Mitral valve murmurs, tricuspid murmurs and aortic valve murmurs are caused by a leak in the valves that separate the chambers of the heart, allowing blood to seep into a chamber before it should (human heart shown). Photo by Olga Bolbot/Shutterstock

Though heart murmurs can have multiple causes, they are often related to infection, degeneration or congenital defect. The exact location of the murmur is determined by where it is heard the loudest, both physically in the heart and in the phase of the heartbeat.

A mitral valve murmur is detected when there is a leak at the valve that separates the left ventricle from the atrium. This type of murmur is often caused by degeneration related to old age or an infection in the heart valve. It can range from mild to severe; the diagnosis will affect how the horse is managed.

David sees tricuspid and aortic valve murmurs frequently as well. Tricuspid murmurs occur when the blood flows backward from the right ventricle to the right atrium. Aortic valve murmurs are when the blood flows backward through the aortic valve. An aortic murmur is the most common valve-related problem found in older horses. Although it is a degenerative disease, it tends to progress slowly.

To determine how severe a murmur is, the treating veterinarian will combine the findings of a physical exam, performance history and exercise testing. The murmurs are then assigned a grade between 1 and 6 based on severity. Heart murmurs frequently become worse with advanced age, David notes.

An equestrian rides her horse on the beach
Some types of heart murmurs allow a horse to perform at a high level for a long period of time. Photo by Cool_photo/Shutterstock

Owners of horses with moderate to severe murmurs are often encouraged to buy a stethoscope and learn what their horse’s heart rate and rhythm sound like so they can monitor it regularly for changes. A vet may also recommend that the affected horse be examined yearly for this heart issue.


“Arrhythmia” is a general term for an abnormal rate, rhythm, or conduction (movement of electrical impulse) in a horse’s heartbeat. Arrhythmias are a common heart issue in horses, but their cause can vary widely.

Though arrhythmias can be brought on by cardiac disease, other causes are often to blame, like fever, colic, electrolyte imbalance, congenital defect or toxemia. While some arrhythmias are not constant, those associated with a heart murmur may become permanent over time, says David.

To detect an arrhythmia, the vet will use a stethoscope to determine if the heart is beating fast or slow, or if there are long pauses between beats, an irregular pattern or extra sounds.

There are multiple types of arrhythmias, and proper diagnosis is crucial. Once the type of arrhythmia is determined, the significance, if any, can be defined and the vet can work to determine the cause.

A cowboy rides his horse on a trail
There are a few arrhythmias where a horse may be ridden lightly and safely for years. Photo by Vanessa van Rensburg/Shutterstock

Many of the horses David sees have atrial fibrillation, a type of arrhythmia. Atrial fibrillation (also called “A-fib”) is an abnormal rhythm of the heart. In A-fib, the electrical signals that control heart rate and contraction become disorganized, causing an erratic and rapid heartbeat. Horses can live for years with A-fib, but their performance may be affected.

There two main treatment options for atrial fibrillation: oral quinidine administration, which can have side effects like low blood pressure and gastrointestinal problems, and transvenous electrical cardioversion (TVEC), where the horse’s heart is shocked back to a normal rhythm. TVEC is performed at veterinary hospitals with the capability to place catheters to the heart via cardiac ultrasound.

Congenital Defects

Congenital defects of the heart are those the horse is born with. These heart defects can be caused be a variety of issues, such as the environment in which the horse’s dam was housed, medications the mare was taking, genetic defects, poor maternal nutrition and other factors, although it is often hard to pinpoint exactly why a defect occurs.

Similar to murmurs, congenital defects can range in severity and potential health outcome; horses may show no ill effects or have significant health issues related to the defect. It’s important to remember that although congenital defects may occur, they are rare.

Like with people, one of the most common defects found in horses is a ventricular septal defect. This condition occurs when the wall that separates the two lower chambers of the heart (called the ventricular septum) does not close completely, allowing blood to flow backward and causing the heart to work harder.

Other congenital defects seen in horses include patent ductus arteriosus, a hole in the vessel that connects the pulmonary artery to the aorta, and tetralogy of fallot, a combination of defects that prevent enough oxygen from getting into the bloodstream.

Ongoing Management of Heart Issues

Determining the initial cause behind a horse’s heart issue can be difficult, and often the root issue is never determined. In addition to being born with the issues, illness from toxins in some plants and cattle feeds can permanently damage heart muscle, and exposure can be fatal, says David.

“Bacterial infection of a heart valve may lead to permanent scarring and a permanent heart murmur,” he explains. “Some murmurs are considered to be the result of age and ‘wear and tear,’ but the majority of the time we can’t determine what initially causes heart problems.”

With regards to cures, David notes that unfortunately, none exist at this time in equine medicine. “[However,] some treatments may improve heart function and may increase the horse’s longevity,” he says.

Additionally, not all cardiac conditions require the horse to become a pasture pet.

“Many horses may perform at a high level for a long period of time with several different types of [heart] murmurs,” David adds. “Certain conduction disturbances [arrhythmias] make a horse dangerous to ride, but there are a few arrhythmias where a horse may be ridden lightly, safely, for years. When exercise intolerance is moderate to severe due to a cardiac condition, a horse should not be ridden.”

Having a horse diagnosed with a heart issue may be intimidating, but it’s important to remember you’re not alone. Involving a veterinarian to tailor a management plan that’s in the best interest of the horse is key, as is keeping ongoing tabs of his heart health yourself.

This article about horse heart issues appeared in the May 2022 issue of Horse Illustrated magazine. Click here to subscribe!


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