Muscle disease in performance horses commonly is referred to as tying-up, exertional rhabdomyolysis, azoturia, or Monday morning disease. Horses experiencing tying-up show a stiff gait; reluctance to move; firm, painful muscle cramps; profuse sweating; increased heart rate; and increased respiratory rate. Horses that tie up also have moderate to marked elevations of certain muscle proteins in their blood, indicating muscle cell damage.

For years, all horses that tied up following exercise were thought to suffer from the same disease. However, treatment and prevention protocols that worked on some horses did not help others. As a result, confusion and controversy developed regarding the cause and treatment of tying-up. Researchers have only recently begun to classify the different diseases that result in the common signs of tying-up.

Initial classification of tying-up is now based on the frequency of mus-cle cramps and damage following exercise. Horses which tie up only a few times in their life are classified as "sporadic," while horses that tie up on a repeated basis are termed "chronic."

Sporadic Tying-Up

Many horses experience some muscle soreness and strain associated with exercise. Muscle strains are much more severe and are typically accompanied by elevated muscle proteins (aspartate aminotransferase and creatine kinase) in serum and myoglobinuria (coffee-colored urine). This darkening of the urine is a result of the kidneys filtering myoglobin (a muscle protein) from blood, which is an indication of severe muscle damage.

The most frequent causes of sporadic tying-up are exercise that exceeds a horse's underlying state of training, electrolyte imbalances, hyperthermia (overheating), and strenuous exercise while suffering from a respiratory disease.

Sporadic tying-up should be considered a veterinary emergency if horses are sweating profusely, are reluctant to move, or have dark urine. Veterinarians might administer medications to relieve anxiety and muscle pain and correct dehydration. Further management includes stall rest followed by hand walking and turnout once initial muscle stiffness is gone. Grain intake is drastically reduced or eliminated, since these horses are on a reduced exercise program. The amount of time the horse must remain out of training has not been established by science. Following an episode of tying-up, training should be resumed gradually and consistently to prevent further muscle damage.

Prevention of further tying-up episodes is often a result of common sense: Don't overexert unfit horses; do fortify the diet with salt on a daily basis; use electrolytes prior to heavy sweat loss; and don't overfeed carbohydrates (grain). Feed grain concentrates fortified with fat and necessary antioxidant vitamins and minerals to provide energy while supplying the building blocks to protect muscle tissue. The combination of these strategies can prevent another bout of tying-up. In areas where the soil is deficient in selenium, adding that and vitamin E can also improve muscle health.

Chronic Tying-Up

Many breeds of horses have been reported to have chronic bouts of tying-up, including Quarter Horses, Thoroughbreds, Standardbreds, Paints, Morgans, Arabians, and various breeds of draft and warmblood horses. It is the study of these chronically tied-up animals that has advanced our knowledge of the causes, treatments, and preventions of the syndrome.

There have been many proposed causes for tying-up that are unsubstantiated. Recently, muscle biopsy techniques have identified two causes of chronic tying-up--a disorder in carbohydrate storage (polysaccharide storage myopathy, PSSM) in Quarter Horses, warmbloods, and drafts; and a muscle contraction disorder (recurrent exertional rhabdomyolysis, RER) in Thoroughbreds.

Preliminary genetic research and breeding trials point to these separate conditions as being inherited. Both of these causes of tying-up appear to be at least partially responsive to treatment with decreased starch and increased fat in the diet, gradually increasing daily training, and daily turnout. Commercial diets have also been developed for horses with these diseases.

About the Author

Stephanie Valberg, DVM, PhD, Dipl. ACVIM, ACVSMR

Stephanie Valberg, DVM, PhD, Dipl. ACVIM, ACVSMR, is the Mary Anne McPhail Dressage Chair in Equine Sports Medicine and a Professor of Large Animal Clinical Sciences at Michigan State University. She is a leading researcher on the subject of tying-up and the genetic basis for equine neuromuscular disorders.

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