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Heart Disease in Horses

November 2018 by Dr Katharyn Mitchell

From mild problems that are well tolerated and allow the horse to perform normally, to diseases causing poor performance or, in severe cases, resulting in collapse or even death, heart disease in horses is more common than people realise. In this article, Dr Katharyn Mitchell, a senior clinician in Equine Cardiology at the University of Zurich, Switzerland, explains the most common heart conditions and how they will affect the horse.

The horse has an extraordinary heart, capable of pumping 30 to 40 litres per minute at rest and up to 250 litres per minute around the body during peak exercise. However, like all things, this magnificent piece of engineering can fail. 

Heart disease can cause poor performance since performance is directly related to the ability of the heart to pump blood loaded with oxygen around the body. If the pumping ability is reduced, then the body does not get enough oxygen and poor performance occurs.

Horses and heart disease 

Horses can get heart disease and it is more common than most realise. Depending on the age and breed of your horse, it has been estimated that up to 80% of horses may have a heart murmur heard on physical examination, especially older performance and racing horses. Nevertheless, it is not just older horses that can have problems, some young and seemingly ‘healthy’ horses occasionally present with quite severe and performance-limiting heart problems. 

Irregular heart rhythms, while less common, can also affect horses, in some cases causing exercise intolerance and poor performance. Horses can also be born with congenital heart defects, similar to people, cats and dogs. It is also possible for horses to suffer from infections that affect the heart muscle, heart valves or the sack around the heart called the pericardium. Other uncommon causes of heart disease can be caused by toxicities (such as monensin, a feed additive commonly found in cattle feed) or deficiencies such as white muscle disease (Vitamin E/Selenium deficiency). 

Most of the time, heart disease is an incidental finding that is not causing the horse a problem, but in some cases, the heart disease can be associated with a reduction in performance, collapse or even sudden death. Less commonly than in people, horses can develop heart failure, which can cause lethargy, weight loss, peripheral oedema, coughing or difficulty breathing.

Types of heart disease

Heart murmurs 

The most common type of heart disease found in horses is a heart murmur. Heart murmurs are caused by turbulent blood flow inside the heart. Most of the time, these murmurs are considered ‘pathological’ in that they are caused by blood leaking through a heart valve that is not working properly, so that blood is flowing in the wrong direction. In some cases, especially in younger horses, ‘physiological’ murmurs of blood flow can be detected. These murmurs are typically soft/quiet and not associated with any valvular leaking. 

There are four heart valves present in the heart, two on the right side and two on the left side. When the valves leak, it is called valvular regurgitation or insufficiency. The most common leaking valve depends on the breed and age of the horse. For example, Standardbreds frequently have tricuspid regurgitation, while Warmbloods more commonly have mitral and/or aortic regurgitation. 

The severity of a cardiac murmur is graded out of six. A grade 1/6 murmur is very quiet and difficult to detect, whilst a grade 6/6 murmur barely requires a stethoscope and can be felt with your hand on the side of the horse’s chest. Most pathological murmurs are grade 3 and above, whilst most physiological murmurs are grades 1-2. It is also essential to listen carefully to both sides of the chest, as some murmurs may only be heard from the right side. 

The loudness of the murmur does not always correspond with the severity of the heart disease, so additional testing may be necessary to correctly understand what is going on. However, most horses with mild valvular regurgitations can tolerate them quite well and go on to live healthy lives with normal athletic performance. 

There are situations where the valvular regurgitation is progressive and enlargement of the heart can occur. In these situations, the horse may be at risk of developing an irregular heart rhythm or suffering from poor performance. With a severe heart murmur, a horse could eventually develop heart failure or fatal rhythm disturbances.

Heart rhythm disturbances (arrhythmias) 

Serious irregular heart rhythms are not detected as commonly as murmurs, but when they occur, they can cause significant problems for the horse. Sometimes, they can develop without any other underlying heart disease, but often they appear secondary to heart enlargement, inflammation or infection. Arrhythmias can be present at rest and disappear with exercise, but in more concerning cases, they can develop during exercise where they may cause poor performance, collapse or even sudden death. Arrhythmias can be hard to detect, especially when exercising and an exercising electrocardiogram (ECG) may be recommended to help identify them.

Atrial fibrillation is the best known pathological arrhythmia affecting horses. It is most frequently recognised in racehorses, when it can cause a substantial reduction in performance. When atrial fibrillation occurs in high level dressage horses, show-jumpers and eventers, poor performance is sometimes reported at higher exercise intensities. 

Atrial fibrillation is characterised by an irregularly irregular heartbeat. It can occur spontaneously in horses with normal hearts, but it can also occur if the heart has become enlarged secondary to another problem. Historically, horses with atrial fibrillation were not considered ‘unsafe’ to ride. However, in the past few years, we have become aware that some horses with atrial fibrillation can develop pathological ventricular arrhythmias during exercise, which may lead to collapse or even sudden death. For this reason, we recommend all horses with atrial fibrillation be assessed during exercise, and treatment considered for those that show poor performance or develop nasty arrhythmias. 

Congenital heart defects 

Horses can be born with heart defects, similar to people and other animals. The most common is a ventricular septal defect (VSD), which is a hole in the heart between the left and right large chambers (ventricles). 

Some breeds predispositions have been reported including Welsh Mountain ponies, Standardbreds and Arabians. Horses with congenital heart diseases will often have heart murmurs detected shortly after birth that do not resolve. Depending on the severity of the abnormality, the horse may go on to live a fairly normal life, but some horses can show ill thrift, failure to grow, reduced performance or even heart failure. Any murmur detected at birth that does not resolve within a few days as expected should be evaluated further.

Heart disease versus heart failure 

Heart disease simply describes having an abnormality involving the heart, while heart failure is when the heart stops working effectively to pump blood around the body. 

Many horses will have some version of heart disease (like a heart murmur, for example), but only a very small number go on to develop heart failure. 

The heart has a number of compensatory mechanisms that allow it to continue functioning with quite severe disease, but when these mechanisms have been exhausted, decompensation will occur and signs of heart failure will develop. Depending on which side of the heart has failed, the signs may be different. 

Left-sided heart failure results in fluid building up in the lungs, whereas right-sided heart failure causes fluid to build up in the extremities, liver and abdomen. Early signs of heart failure include weight loss, lethargy and a reduction in exercise tolerance. 

Heart disease and poor performance 

Some heart diseases can cause poor performance. Performance is directly related to the ability of the heart to pump blood loaded with oxygen around the body. If the pumping ability is reduced, then the body does not get enough oxygen and poor performance occurs.

Most horses with mild valve disease and normal sized hearts will have normal athletic performance, but horses with pathological arrhythmias or severe valve disease and enlarged hearts can certainly show exercise intolerance. 

How can I tell if my horse’s poor performance is caused by heart disease? 

Firstly, your horse will need a thorough evaluation by a veterinarian. Many problems can contribute to the poor performance of a horse, and often, we do not find just one thing wrong, but rather, several things that are all playing a role. 

If a murmur is detected, a heart ultrasound examination (echocardiography) is indicated to check for chamber enlargement and to confirm the source of the murmur. An ECG can be performed at rest if an arrhythmia is detected, but even more useful is an exercising ECG, with the horse either being lunged, ridden or exercised on a treadmill. Ideally, the degree of exercise should be similar to the level where poor performance is reported (over fences if a showjumper, galloping at high speed for an eventer). 

During exercise, the heart rate and rhythm can be evaluated. Horses with heart disease can have abnormally high heart rates for the intensity of exercise. When combined with modern GPS devices, the speed and distance can be calculated and compared to heart rate. Arrhythmias that develop during exercise can also be detected. 

In some cases, blood samples can be taken during or immediately after exercise to measure lactate, which is produced by the body during exercise. Horses with heart disease causing poor performance can have a greater increase of lactate than normal horses. 

Can heart disease cause collapse or sudden death? 

Fortunately, in most horses, heart disease is mild and does not put the horse at risk of collapse or sudden death. Occasionally, however, a horse may develop a life-threatening heart problem during exercise and will do so with little warning.

The two life-threatening heart-related diseases that we recognise most frequently are ventricular arrhythmias and rupture of a large blood vessel (usually the aorta, infrequently the main pulmonary artery). 

With ventricular arrhythmias, a horse may have an increased risk if there is ventricular enlargement or atrial fibrillation. For this reason, we recommend horses with grade 3/6 or louder murmurs or atrial fibrillation have a veterinary assessment to determine if the horse is safe to ride or not. 

When horses rupture a large vessel, there is not often prior warning and little can be done to prevent this, unless severely enlarged great vessels have been previously detected on a heart ultrasound examination, in which case the affected horse should be retired. 

If my horse has heart disease, is it safe to ride? 

This is an important question, especially for horses ridden by children or at high speeds, over obstacles or in dangerous terrain. 

Following a thorough veterinary evaluation, possibly after echocardiography and an exercising ECG, an assessment can be made of the degree of heart disease and the risks involved in riding the horse. 

In the large majority of cases, the heart disease will be mild and the risk will be no greater than when riding a normal horse of a similar age. But, for some horses, the heart disease will be identified as moderate or severe and the horse might be considered at an increased risk to ride. Like with most things, however, we cannot guarantee safety, and heart disease can progress and change over time. For this reason, we often recommend regular assessments of these horses to determine if disease progression has occurred and an increased risk is present. 

Horse riding on its own is an inherently risky pastime, and we can only do our best to manage and reduce this risk where possible.

Heart facts:

  • The average adult horse heart weighs around 3.6 kilograms. The size of the heart doesn’t always match the size of the horse.
  • The heart of the legendary racehorse Phar Lap (pictured on right) which is on display at the National Museum of Australia, weighs 6.35 kilograms - approximately 1.5 times bigger than average.
  • Another Thoroughbred’s heart kept since 1927 at the Institute of Veterinary Anatomy of the University of Giessen in Germany weighs in at 8.9 kilograms!
  • The heart grows until the horse is four years of age, although it can increase slightly in size with proper exercise and training.
  • Just like in people, exercise, fear, pain, stress and excitement can increase a horse’s heart rate.
  • A study showed that horses can sense the rider’s heart beat. When the rider’s heart rate went up, so did the horse’s!

Heart work:

  • A typical adult horse pumps about one litre of blood out of each side of the heart per beat. This is called the stroke volume. With a resting heart rate of 30-40 beats a minute, this means the total blood pumped in one minute is 30 to 40 litres.
  • During maximal exercise, the normal horse’s maximum heart rate is similar to humans (approx 220 beats/min). A conditioned horse's maximal exercise cardiac output might reach 250 litres per minute - that’s six to eight times more than resting!
  • Horses with arrhythmias can have instantaneous heart rates of 250-350 but this can not be tolerated for more than a few seconds before the horse will show signs of weakness/collapse.
  • In resting horses, a heart rate over 60 can be a sign of a serious problem.