The Heart of Horse Health

Heart attacks in the human sense are actually rare in horses.

I just finished reading the novel “Secretariat,” which is an outstanding book, regardless of the breed you’re partial to. I was amazed at how many great racehorses had reportedly died of “heart attacks” while racing or training. In the 35 years I’ve been in practice, I honestly can’t recall seeing a horse die of a heart attack, although I hear about them often. Granted, I’m not a racetrack veterinarian, but I’ve taken care of plenty of performance horses that work pretty hard.

The term “heart attack” has incorrectly evolved into referring to sudden death associated with heart disease. In human medicine, the term refers to myocardial infarction (coronary heart disease) caused by clogged blood vessels that reduce blood flow to the heart. The result is damage to or death of the heart muscle that can cause death. Because horses are herbivores and do not suffer from coronary artery disease like people, heart attacks rarely occur.

Racehorses that die unexpectedly due to sudden death syndrome (or Swale syndrome) are frequently said to have died of a heart attack, but heart lesions are very seldom found during post-mortem examination. Swale syndrome is named after Swale (TB), who won the 1984 Kentucky Derby, lost the Preakness and went on to win the Belmont.

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Several weeks after the Belmont, following a fast mile gallop, Swale reared up and fell dead, apparently from a heart attack. A very careful necropsy was performed to rule out foul play. The pathologists determined there was no damage to the heart, and so attributing his death to a heart attack was inaccurate. However, the term “hear

t attack” is still commonly used by some when a horse dies suddenly during exercise.

Although heart attacks are very rare in horses, they can suffer from heart disease that can develop quickly (acute heart disease) or slowly over time (chronic heart disease).

Acute equine heart disease typically results from direct insult to the heart muscles or the electrical signaling system that controls the heartbeat. Examples are arrhythmias (irregular heart beat), drug- or toxin- induced disruption of electrical signals, snakebite, nutritional deficiencies, and bacterial or viral disease of the heart muscle or valves. Because the heart is incapable of regenerating new heart muscle, any acute insult can lead to death.

One good example of acute heart disease would be sudden death following ingestion of ornamental English or Japanese yew bushes that contain taxine, which causes slowing of the heart muscle, followed by death. Another example is accidental ingestion of cattle rations containing monensin, which causes damage to the horse’s heart muscles and often death within 24 to 36 hours.

In contrast, chronic heart disease develops slowly and is usually due to conditions that require the heart to increase in size (cardiomegaly) to provide adequate blood to the body. Common lesions leading to chronic heart disease include birth defects of the heart chambers, valves or blood vessels; cancer; previous insult to the heart valves due to viral or bacterial infection; and disease in other organs, especially the lungs, that alter blood flow into and out of the heart.

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As the heart weakens, it eventually becomes overworked and is unable to keep up with the horse’s oxygen needs, which results in heart failure and death. Older horses often suffer from chronic heart disease with clinical symptoms that include swelling of the legs and decreased exercise tolerance. A good example of chronic heart disease is dysfunction of heart valves leading to blood leak-back and decreased blood flow. In an effort to compensate, the heart enlarges and may eventually fail.

Equine heart disease can have many causes and shows variable symptoms. Although horses do not suffer from traditional heart attacks, they do develop heart disease that can impact their performance and longevity.

Dr. Thomas R. Lenz is a trustee of the American Horse Council, serves on AQHA’s research committee and is a past president of the American Association of Equine Practitioners.