Heart disease in horses encompasses a broad range of conditions that affect the heart’s structure, function, or electrical activity, potentially compromising the horse’s health and performance. The heart is responsible for pumping oxygen-rich blood throughout the body, making it essential for a horse’s overall vitality.

Equine cardiac disease can be categorized into two primary forms: congenital and acquired. Congenital heart disease refers to defects present at birth, which can range from minor abnormalities to severe, life-threatening conditions.

Acquired heart disease develops later in life due to factors such as aging, infections, or trauma. Both forms can have significant impacts, from a subtle decline in performance to life-threatening cardiac events.

While not all heart conditions are fatal, they can seriously impair a horse’s athletic abilities and, in some cases, make them unsafe to ride. Understanding the different types of heart disease and their causes is essential for effective management and treatment, helping horse owners and veterinarians safeguard equine health.

Heart Disease in Horses

The heart is the organ responsible for pumping blood through the horse’s body. It is located on the right side of the horse’s chest, sitting between the second and sixth rib. [1][2]

A horse’s heart is proportionally large compared to other mammals. It typically accounts for about 1% of its body weight. For an average horse weighing 500 kg (1,100 lbs), this means the heart weighs approximately 5 kg (11 lbs). [1][3]

The horse’s heart is remarkably efficient at pumping blood, which contributes to their exceptional athletic abilities. During exercise, the heart’s cardiac output — the total volume of blood pumped by the heart per minute — can increase by as much as 7 to 8 times compared to resting levels. [2]

This ability allows horses to sustain intense physical activity and deliver oxygen-rich blood to their muscles, making them highly capable athletes.

Heart disease in horses encompasses a range of conditions that affect the heart’s ability to function properly.

Heart disease in horses can be classified into two primary types: [2]

  • Congenital heart disease: Horses with congenital heart disease are born with structural or functional heart defects. Some congenital issues are mild and do not not significantly affect a horse’s life, while others are severe and require immediate medical intervention.
  • Acquired heart disease: Horses with acquired heart disease develop the condition later in life.

While congenital heart defects are present from birth, acquired heart disease can develop due to: [2]

  • Degenerative changes: As horses age, the heart, like any other organ, can deteriorate, leading to issues such as valvular disease.
  • Infections: Bacterial or viral infections can damage heart tissue, leading to inflammation or scarring.
  • Poisoning: Exposure to certain chemicals or toxins can damage the heart muscle or its electrical impulses.
  • Inflammatory conditions: Inflammation, also called myocarditis, can results from a variety of conditions like infection or toxins.

Not all cases of heart disease in horses are life-threatening, but even mild heart conditions can significantly impair a horse’s performance. In addition, some congenital and acquired heart conditions can worsen over time.

The severity of heart disease in horses varies widely. While some conditions can be managed with careful monitoring and treatment, others are rapidly life-limiting.

Horses with mild heart disease may show no visible symptoms and live relatively normal lives. However, as heart disease progresses, it can compromise their athletic abilities and, in some cases, their life, as the risk of sudden, potentially fatal, cardiac events increases.

Cardiac Anatomy & Circulation

To understand the cardiac pathologies that can affect horses, it’s helpful to have a basic knowledge of equine cardiac anatomy and physiology.

Located in the horse’s chest cavity, between the lungs and protected by the ribs, the heart is responsible for pumping blood throughout the body. Blood circulation is essential for all bodily functions, as blood supplies oxygen and nutrients to tissues and removes waste products. [1][3]

Circulation

The horse’s heart is divided into four chambers: [1][3]

  • Two atria: The upper chambers (right and left) receive blood and pump it into their adjoining ventricles.
  • Two ventricles: The lower chambers (right and left) pump blood out of the heart.

The right atrium receives deoxygenated blood from the body, passing it to the right ventricle, which then pumps it to the lungs via the pulmonary arteries for oxygenation. [2]

Oxygenated blood returns to the left atrium, where it is pumped into the left ventricle. From there it’s pumped out through the aorta (a major blood vessel in the chest) to the rest of the body. [2]

Of all the heart chambers, the left ventricle has the thickest walls, as it requires significant strength to propel blood through the systemic circulation. [2]

The Beating Heart

The average resting heart rate for a horse is around 28-40 beats per minute, but it can increase to 200 beats per minute during strenuous activity. A heart beat is divided into two separate phases: diastole and systole. [2] Diastole refers to the part of a heart beat where the chambers are pumping out, and systole refers to the refill time.

The heart’s pumping action is coordinated by electrical impulses that travel through specialized myocardial cells. When the muscle contracts, the heart is in diastole; when the muscle relaxes, it is in systole. [2]

These impulses ensure the heart muscles contract and relax in a synchronized way. Disruptions to these electrical signals can lead to arrhythmias (heart beat irregularities). [2]

Equine Heart Conditions

The equine heart can develop a number of abnormalities that impact a horse’s health and performance. Cardiac abnormalities can be categorized into three main types:

  • Structural abnormalities: Involve physical defects or malformations, such as heart valve defects or chamber enlargement, which can alter blood flow. These abnormalities can be both congenital or acquired.
  • Functional abnormalities: Affect the heart’s ability to pump efficiently, leading to conditions such as heart failure.
  • Electrical abnormalities: The heart’s rhythm is controlled by electrical signals, and any disruption in these signals can lead to arrhythmias, which are irregular heartbeats.

Heart disease in horses can result from a variety of abnormalities in the cardiovascular system. These conditions may impair the heart’s ability to function properly, affecting overall health and performance.

Forms of equine heart disease include:

  • Valvular disease
  • Myocardial disease
  • Arrythmia
  • Vascular disease

Another feature of cardiac anatomy is the pericardium – a membrane of tissue that covers the entire organ. Some types of heart disease are associated with changes in this structure, which put external pressure on the heart muscle. This can lead to overall issues with circulation. [4]

Valvular Disease

Valvular disease occurs when the heart valves fail to open or close properly. The heart’s atria and ventricles are separated from eachother by valves, as are the ventricles and blood vessels. There are four valves present in the equine heart: [2]

  • Mitral: between the left atrium and left ventricle
  • Tricuspid: between the right atrium and right ventricle
  • Pulmonary: between the right ventricle and pulmonary artery
  • Aortic: between the left ventricle and aorta

These valves work to ensure blood flows in the right direction and prevent backflow. [2]

When any of these valves malfunction, either by not opening fully (stenosis) or not closing properly (regurgitation), blood flow becomes compromised. Valvular disease can lead to heart failure. [5][6]

Myocardial Disease

Myocardial disease occurs when there is an abnormality in the heart muscle itself. The myocardium, the thick muscular layer of the heart, contracts and relaxes, thereby pumping blood through the body. [5][6]

Myocardial disease refers to cases where the heart muscle is too weak to pump blood effectively (systolic dysfunction) or when it is unable to relax properly between beats (diastolic dysfunction). [5][6]

Myocardial disease in horses is most commonly associated with viral or bacterial infections and ingestion of cardiotoxic plants (e.g., Oleander, Yew, White snakeroot). [5][6]

Arrhythmia

Arrhythmia is defined as irregularity in the heart’s rhythm. Depending on the type, arrhythmia causes the heart to beat too slowly (bradycardia), too fast (tachycardia), or in an erratic pattern (fibrillation). [5][6]

Arrhythmias disrupt the normal electrical signals that regulate the heartbeat, impairing the heart’s ability to maintain a consistent rhythm. Arrhythmias can cause collapse, poor performance, or sudden cardiac arrest. [5][6]

Vascular Disease

Vascular disease involves abnormalities in the blood vessels that impede circulation. If the blood vessels are too narrow, blocked, or rigid, the heart has to work harder to pump blood through them, leading to increased strain on the cardiovascular system. [5][6]

Vascular disease can increase the risk of hypertension (high blood pressure), organ damage, and heart failure. Vascular disease is not common in horses. [5][6]

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Effects of Heart Disease

Heart disease can lead to a number serious consequences for horses, these include: [5][7]

  • Exercise intolerance and poor performance: When the heart is not able to pump blood effectively, the body’s ability to deliver oxygen to the muscles is reduced. This leads to fatigue, poor stamina, and a decreased ability to perform physically. Depending on the severity of the condition, horses may show signs of poor performance even with very light exercise.
  • Syncope: Commonly referred to as “fainting,” loss of consciousness related to low blood pressure or disruption in cardiac electrical activity is one possible outcome of heart disease.
  • Heart failure: As heart disease progresses, it can lead to heart failure. Heart failure occurs when the heart can no longer meet the demands of the body, either because it cannot fill with blood (diastolic dysfunction) or because it cannot pump blood effectively (systolic dysfunction).

Signs of Heart Disease

Signs of heart disease in horses often develop gradually, with subtle indications observed by owners. Symptoms to watch out for include: [5]

  • Decreased performance
  • Labored breathing, especially after exercise
  • Fatigue
  • Exercise intolerance

Diagnosis

It’s important to seek veterinary guidance at the first sign of cardiovascular disease in horses. Many cardiac conditions do not have clinical signs until they have progressed into more severe stages. [8] Early intervention provides the best chance of a positive outcome.

Diagnosing heart disease in horses requires a multifaceted approach. Diagnostic tools used by veterinarians include:

  • Auscultation (listening to the heartbeat)
  • Electrocardiography
  • Echocardiography
  • Blood work
  • Stress testing

Auscultation

This aspect of diagnosis is part of a standard physical exam. The veterinarian uses a stethoscope to listen for abnormalities, including irregular rhythms, murmurs, or muffled heartbeats, which can indicate underlying cardiac issues. The location, timing, and intensity of the murmur can help the veterinarian determine the severity of heart disease. [9][10]

Auscultation is performed on both sides of the chest for a minimum of 30 seconds to accurately calculate heart rate. [9][10]

Electrocardiogram

Electrocardiogram (ECG or EKG) is used to assess the electrical activity of the heart. This test can detect arrhythmias, conduction abnormalities, and other electrical disturbances that may signal heart disease. [10][11]

While resting ECGs are commonly used, they may not always reveal heart problems that only manifest during exercise, making it important to consider exercise testing as part of the diagnostic process. [10][11]

Echocardiography

Echocardiography, or ultrasound of the heart, is commonly used for evaluating heart structure and function. This non-invasive test uses high-frequency sound waves to create images, allowing veterinarians to evaluate the size, shape, and activity of the heart. [10][11]

Echocardiograms are particularly useful for diagnosing valvular diseases, congenital heart defects, and myocardial disorders. [11]

Blood Tests

Blood tests are another crucial element in diagnosing heart disease, as they can help identify potential infections or particular markers that indicate damage to the heart muscle. Additionally, a complete blood count and blood chemistry can provide valuable information about the horse’s overall health, helping to rule out other conditions. [5]

Stress Testing

Exercise testing, also known as “stress testing,” is often used to assess a horse’s cardiovascular function during physical activity, as a number of heart symptoms only manifest when the horse is under physical stress.

Similar to stress testing in human medicine, this test is performed by having the horse exercise at different intensities with an Electrocardiogram (ECG) machine monitoring heart output. Exercise testing is helpful in revealing exercise-induced arrhythmias or poor cardiac output. [11][12]

Treatment & Management

The management and treatment of heart disease in horses depends on the specific condition, the severity of symptoms, and any underlying factors contributing to the disease. Treatment approaches range from medications to improve heart function and manage symptoms, to more advanced medical interventions for severe cases.

Medical Treatment Options

The primary goals of medical treatment for horses affected by heart disease are: [13]

  • Protecting the heart muscle
  • Improving circulation
  • Regulating heart rate and rhythm
  • Reducing the risk of blood clots and heart failure

This may be achieved with one or a combination of surgery and medications (e.g, diuretics, vasodilators), depending on the specific case. [13]

Lifestyle & Environmental Management

Horses diagnosed with heart disease can benefit from a number of environmental and lifestyle changes, including: [13]

  • Regular monitoring: Horses diagnosed with heart disease need to be closely monitored so that any changes in behavior, performance, or physical condition are noticed in good time. Regular veterinary check-ups are crucial to assess the horse’s heart function and overall health.
  • Exercise regimen: Depending on the severity of the condition, veterinarians may recommend a modified exercise program. Light to moderate exercise might be beneficial to maintain muscle tone and cardiovascular health.

Always work with a veterinarian to make lifestyle changes if your horse is living with chronic heart disease. It’s important to balance rehabilitative management with supportive care to avoid over exertion of the heart.

Riding horses with certain forms of heart disease not only poses a serious health risk to the horse but also endangers the rider. A horse experiencing a sudden cardiac event, such as an arrhythmia or heart failure, during exercise can collapse unexpectedly, potentially leading to falls, injuries, or accidents for the rider.

This makes it essential to follow veterinary guidance and avoid riding horses with forms of heart disease that could compromise their safety and the rider’s well-being.

Some horses with heart disease benefit from dietary changes to support cardiovascular health. Nutrition plays a vital role in supporting heart health. Ensuring proper hydration is also crucial, as dehydration can exacerbate heart issues and hinder interpretation of diagnostic tests. [14]

If your horse has heart disease and you think their diet may need adjustment, a qualified equine nutritionist can work with you to come up with an individualized plan that is aligned with your veterinarian’s recommendations.

Summary

Heart disease is an important health issue in horses that can significantly impact their performance, safety, and quality of life.

  • Signs of heart disease include poor performance, fatigue, and labored breathing during mild exercise
  • Some forms of heart disease can be managed, but the prognosis for a return to athletic performance varies
  • Management may include a combination of medical, surgical, and lifestyle interventions
  • Horses with chronic heart disease require ongoing monitoring and management; many forms of heart disease are degenerative

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References

  1. Buergelt. C. D. and Buergelt. C. D., Equine Cardiovascular Pathology: An Overview. Animal Health Research Reviews. 2003. View Summary
  2. Bonagura. J. D., Overview of Equine Cardiac Disease. Veterinary Clinics of North America: Equine Practice. 2019. View Summary
  3. Hinchcliff. K. W. et al., Eds., Equine Exercise Physiology: The Science of Exercise in the Athletic Horse. Saunders Elsevier, Edinburgh. 2008.
  4. Blissitt, K., Brown, C., Heart: Pericarditis. Vetlexicon.
  5. Cunningham. S. M. and Roderick. K. V., Heart Disease and Heart Failure in Horses. Merck Veterinary Manual. 2019.
  6. Cardiology: Medical Conditions . Cornell University College of Veterinary Medicine. 2024.
  7. Thal, D., Syncope (Fainting). Horse Side Vet Guide. 2018.
  8. Eades. S., Equine Cardiology Cases (Proceedings). DVM 360. 2009.
  9. Jago. R. and Keen. J., Identification of Common Equine Cardiac Murmurs. In Practice. 2017.
  10. Marr. C. M. and Bowen. I. M., Eds., Cardiology of the Horse. 2nd ed. Saunders, Edinburgh ; New York. 2010.
  11. Durando. M. M., Clinical Techniques for Diagnosing Cardiovascular Abnormalities in Performance Horses. Clinical Techniques in Equine Practice. 2003.
  12. Exercise Testing in Horses. South Eastern Equine Hospital.
  13. Cunningham. S. M. and Roderick. K. V., Treatment of Cardiovascular Disease in Horses. Merck Veterinary Manual. 2019.
  14. Underwood, C. et al., Echocardiographic Changes in Heart Size in Hypohydrated Horses. J Intern Vet Med. 2010.