The Heart of Horse Health

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

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

Race horse.
Horse can suffer from heart disease that can develop quickly or over time. Journal photo.

By Dr. Thomas R. Lenz in The American Quarter Horse Journal

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.


6 thoughts on “The Heart of Horse Health”

  1. This is an excellent article by Dr. Lenz. I appreciate the fact that he brought clarity to a very important topic and in a manner that the equine layperson can completely understand.

  2. wow, very interesting. I’ve heard about a lot of great racehorses having enlarged hearts, I understand now how it happens. Great article informing performance horse owners about heart conditions. maybe post one on heart care of your horse.

  3. I had a Smokey Duster Two son in the 1980’s that I had bought at their ranch in Nebraska. He was a big fine yearling. I breed all my mares the following year to him and after breeding season we began to break him to the saddle. It was the last of June and I bought a really nice Big Step filly. My husband and I talked about breeding her, but decided it was too late in the season. My husband left out for work that morning about 4:30AM. I walked through the barn to check on everyone. He was bucking and playing in the stall. I went back to bed and woke up about 7:00AM, dressed and went to the barn to feed. I found him stone dead. There has been no rolling or thrashing. It appeared he just fell over dead. I had an autopsy done by vet and he said that the whole left side of his heart had blown out. He said it was cogential heart failure. He never had any signs.

  4. Hello, I am compelled to write to you about white muscle disease that is caused by selenium deficiency. My sister & I both feed orhard grass hay from northern California. We were not aware until last year that the hay is completely selenium deficient. My sister had two horses die (yes from heart attacks)-white muscle disease. This is quality hay, now we have it tested. The range for selenium blood test is .08 to .5. She had 20 horses many of which were below .08 and some were non detectable. They all had selenium shots. My horses were low, but not in the dangerous zone as I supplimented with Purina Stradegy GX. Most Vets don’t think this is a serious thing, but most people don’t have their hay tested. If I can save just one horse by spreading the word to horse owners to have their horses tested, it would be wonderful. This was a heart breaker for us. My sister, Mary Ann Savory wrote a story about this if you would like me to forward it to you, please let me know. If you would like to check on this please contact Dr.Tina Kemper DVM & Dr. Ken Feigner DVM at San Luis Rey Equine Hospital 760 726-4566 Bonsall, CA

  5. I’m struck by the phrase that heart failure can be “drug- or toxin- induced disruption of electrical signals”, following on from the observation that a considerable number of racehorses die from so-called heart attacks.

    The writer is careful not to state it directly, but it’s clear that there’s a connection between heavy medication of racehorses and heart failure.

    Thank you for this article!

  6. Thanks for the article, I was curious myself on how the term “heart attack” end up in sudden death on athletic horses. I never understood how a horse could develop a condition where their valves get clogged causing insufficient blood flow in result, death. I’ve read some of race horses calapsing during a race. I was guessing it may have been some type of drug they have been giving that could cause some type of arrhythmia. In people there is fat burners that causes this type of affect. Maybe I was guessing the same for race horses. I’m sure there is other causes but rare causes like one stated, a defiency in nutrients . Thanks for the article, I will be looking into this for the safety of my horses. Just curious on what are the symptoms if any other than swollen legs or lack of energy, or these are most commonly seen on heart disease in horses?

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