• External rhabdomyolysis, which is also called tying up, is a disorder of muscle metabolism that results in muscle cramps in horses.
• Various treatments and preventive measures are available.

External rhabdomyolysis (ER), which is also called tying up, is a disorder of muscle metabolism that results in muscle cramps in horses. In severe cases, an affected horse’s urine appears red from the release of myoglobin (an oxygen-transporting protein) by damaged muscle and muscle enzyme levels are greatly increased. Most cases of ER occur in horses that have not been in work for several days and are then exercised strenuously. Typically, an affected horse develops a stiff gait with excessive sweating and rapid breathing during or shortly after exercise. The horse’s muscles are hard and painful to the touch.

Cause

This condition may occur sporadically; however, some horses are prone to developing ER. Causes include overexertion beyond the horse’s training level, electrolyte imbalances, a diet with a high level of grain, underlying illness, and vitamin E or selenium deficiency.

Treatment

Treatment typically includes stall rest, antiinflammatory medications, sedatives, muscle relaxants, and, sometimes, intravenous fluid therapy, especially in severe cases. The horse is fed a hay diet for a few days and gradually turned out in a small paddock and/or briefly hand-walked until a veterinarian determines that the muscle enzyme concentration has returned to normal. Most horses with ER respond to a few weeks of rest, a hay diet, and a gradual increase in exercise or training. Grain should be fed at the minimum amount needed. If additional calories are necessary, vegetable oil or rice bran can be added to the feed. Electrolyte supplementation of at least 1 oz of sodium chloride (NaCl) is important. Vitamin E and selenium supplementation may also be needed. After an episode of ER, muscle tissue can be repaired within 4 to 8 weeks.

Prevention

ER can often be prevented by supplementing the feed daily with an electrolyte supplement that includes at least 1 oz of NaCl and by ensuring that the dietary carbohydrate level is not too high. Affected horses should be kept on a consistent exercise program.