Those Cold-Blooded Horses

Published: October 26, 2012
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A few weekends ago, I went to a Renaissance Festival. There was an abundance of turkey legs, maidens with ample bosoms, mead, and overuse of the old-timey adjective "olde."  But there was also jousting. Well, I should say, "jousting."

Despite the corniness of the epic battle between the red knight and the black knight, a large crowd collected in the bleachers around the arena for this event. As we cheered and booed for our respective knights, we weren’t really watching the humans; we were watching the horses.

A large majority of horses used for today’s “jousting” are some variation of a draft breed. This is probably the most accurate representation of the medieval era at the entire fair since such noble draft breeds as the Clydesdale and Shire originated from Great Britain. Such breeds, or ancestors of what we understand to be those breeds, back in the Dark Ages were required to be large and heavy-boned to carry a full sized man in full armor. The calm demeanor which is standard for such breeds was likely helpful in battle and competition and the gorgeous feathering down the legs was likely beneficial for extra protection. Such breeds are referred to as "cold blooded breeds" due to this steady, calm disposition. This is in contrast to "hot blooded breeds," such as the normally high-strung Arabian and Thoroughbred.

If you’re familiar with sport horses, you might be wondering where the term "warm blood" came from. This term actually developed as horse breeders began to cross "hot bloods," normally a Thoroughbred, with a "cold blood" draft horse to produce an animal with sturdy bone structure and a somewhat toned-down disposition, while still maintaining the endurance that hot bloods are known for.

From a veterinary standpoint, some of my draft horse patients are my favorite. I’m in awe of their beauty, their size, and their patience; they truly earn their unofficial title of "gentle giants." With hooves the size of dinner plates, I’m thankful that most draft breeds have fairly healthy feet. There is, however, a well-characterized draft horse health issue, and that is something called equine polysaccharide storage myopathy (EPSM).

EPSM is a metabolic disease that is characterized by rhabdomyolysis, which means the breakdown of muscle tissue. A genetic condition, EPSM causes a buildup of unmetabolized carbohydrate in the form of glycogen in muscles. This excess glycogen in the muscles becomes toxic and causes damage to the muscle cells. Additionally, because glycogen can’t be broken down into smaller, usable portions of carbohydrate for the cells, the muscles begin to break down and are unable to function efficiently.

Clinical signs of this disease are usually noticed once the horse begins training under saddle. An affected horse may have an "attack" once or twice a year or as often as every time the horse is exercised; there is a range of severity seen from individual to individual.

Mild clinical signs may include a “camped-out” stance and small muscle fasciculations. Others may experience signs so severe that they result in recumbency (excessive lying down) from muscle pain and weakness. As muscle cells break down, they release a cellular component called myoglobin. Myoglobin, although required for muscle cells, becomes toxic to the body, primarily the kidneys, if released into the blood stream. Severely affected horses can die from renal failure for this reason.

As this is a genetic disease, there is no cure, only management. Once diagnosed, strict dietary restriction of carbohydrates along with a defined exercise regime are the best ways to manage this condition. Horses with EPSM should be kept on pasture, not stalled, and introduced slowly into work with careful monitoring.

This condition can be seen in Quarter Horse-type breeds as well; any breed with heavy muscling in its genes may be at risk. However, draft breeds seem to the worst affected.

I’ve never had a patient with EPSM. And luckily, this condition is a recessive trait and informed breeding choices can help prevent it in the future.

Dr. Ann O’Brien

Image: Gypsy Horse by jcopes / via Pixtus