Neuromuscular Disease Can Lead to Performance Issues

Neuromuscular diseases, which affect the function of muscles and the way they interact with the nervous system, can lead to performance issues, said Kelsey A. Hart, DVM, PhD, Dipl. ACVIM, internal medicine clinician and graduate fellow at the University of Georgia's department of large animal medicine, at the recent American Veterinary Medical Association meeting in Atlanta, Ga.

The problem can be subtle. "The horse is just reluctant to work or quits while working," said Hart. "Or, you might have a horse that is truly lame or is generally stiff, but the owner cannot pinpoint the (cause for the) lameness."

The most common equine muscle disorder that affects performance is exertional rhabdomyolysis (tying-up, ER). Whether the problem ends the horse's career depends on the type of ER he has.

Sporadic ER
Sporadic ER can affect any breed horse at any age and usually is due to overexertion for the horse's level of fitness. Recent viral infections, dietary factors, or low blood levels of vitamin E and selenium could be related to the disease, but no one knows for sure.

Horses with sporadic ER are stiff and reluctant to move, and their muscles are tight and painful. Sometimes owners might think the horse has colic, Hart said.

Treatment for sporadic ER includes stall rest, muscle relaxants, non-steroidal anti-inflammatory drugs (NSAIDs), and IV fluids. "The prognosis for these guys is good," Hart said. "If you can manage the horse and protect his kidneys (from harmful substances released from damaged muscle), they usually heal just fine and go back to doing their job."

Chronic ER
A horse with chronic ER has frequent episodes of tying-up due to an inherited defect in the muscle function. Polysaccharide storage myopathy (PSSM) is one type of chronic ER. Horses with PSSM--usually Quarter Horses, draft horses, and Warmbloods--cannot efficiently metabolize the glucose that provides energy to the muscle cells.

"If the horse is normal until it starts work, but then it starts tying-up pretty quickly after it moves into heavier work, that might be a sign that it has a muscle disease," Hart said. "The tying-up or episodes of stiffness are associated with exercise that isn't all that tough, exercise you wouldn't think would be related to tying-up."

She recommended a low-carbohydrate diet, such as those made for horses with equine metabolic syndrome, but advised owners to make sure the diet has a higher fat content for energy.

Chronic ER might limit the horse's athletic potential, because he can't train hard. "This can be a career-ending diagnosis for some horses," Hart said.

Recurrent ER
Recurrent ER is another type of chronic ER seen in Thoroughbreds, Standardbreds, and Arabs and is believed to be due to an inability to regulate calcium, which helps muscles contract.

Mares in active race training, horses with nervous or excitable temperaments, horses with concurrent lameness, and horses on high concentrate diets are more likely to have episodes.

"The animals usually start presenting when they enter active race training, which is not surprising given that exercise precipitates it," Hart said.

Management requires a low-key environment, to help the horse "chill out." Providing the horse with a companion, such as a goat, can be helpful. "It is important to have a routine, daily exercise, and turnout," she said.

"If they have frequent episodes they are usually unable to train and if they can't train they can't race. So, often it affects their racing career. But many can still be athletes if they have a less demanding career," Hart said.

About the Author

Marie Rosenthal, MS

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