Q. I just bought a Thoroughbred filly which was in race training, but retired without running because of recurrent bouts of ‘tying-up.’ She had been turned out for six months before I got her, and never showed any problems while on the farm. What can you tell me about this disease?

A. Tying-up is a syndrome or description of a horse with muscle damage that has many different causes. It probably is one of the most misunderstood and controversial syndromes in the athletic horse. Since there are several causes, some of which appear to be inherited, there is no single cure. Typical signs of tying-up include a horse which becomes stiff, sweats, and is reluctant to move. Researchers have learned a great deal about tying-up—or exertional rhabdomyolysis—in recent years. Unfortunately, the information has shown that some of the most common beliefs about tying-up have been proven wrong by scientific study. Thus, what was considered by some early researchers to be a problem that had one basic cause, e.g. lactic acid, is actually a broad-scale syndrome that will require continued research on a variety of fronts before every aspect is understood.

In other words, tying-up is not one disease, but several different diseases that have similar signs and different causes. Therefore, the management of a Thoroughbred that suffers from tying-up would differ from the management of a Quarter Horse that is tying-up, would differ from the management of a backyard pleasure horse that has the same symptoms.

Some horses are healthy athletes that tie-up sporadically likely due to exercise in excess of their training level, electrolyte depletion, or dietary imbalances. They respond well to rest, a gradual return to a graduated training regime, and balancing the diet. Other horses will suffer from chronic episodes of tying-up that can be debilitating. Our research suggests that there might be several inherited reasons for chronic tying-up.

Certain lines of Thoroughbreds seem more susceptible to one form of tying-up that has a basis in abnormal regulation of muscle contraction. Muscle contractions are initiated by propagation of electrical impulses along the outer cell membrane and along membranous connections extending into the cell, which then stimulate the release of calcium from intracellular membranous storage sites. Muscle relaxation requires energy-dependent pumping of calcium back into storage sites.

Our research indicates that a common cause of tying-up in Thoroughbreds is an inherited abnormality in the way calcium is regulated by membrane systems in the skeletal muscle. The narrow genetic origin of Thoroughbreds and the common lineage of the pedigrees of horses with tying-up would support the possibility of an inherited trait. The disease might lie dormant unless specific factors trigger the calcium regulatory system to malfunction. Triggering events include stress, excitement, lameness, high grain diets, and exercise at submaximal speeds.

Young fillies are most commonly affected and usually are the most nervous and high strung. Prevention of further episodes of tying-up in susceptible horses should include standardized daily routines and an environment that minimizes stress and excitement. The diet should be adjusted to include a balanced vitamin and mineral supplement, high-quality hay, and a minimum of carbohydrates (such as grain and sweet feed). Feeding less than five pounds of sweet feed and adding additional calories in the form of fat decrease muscle damage with exercise. Daily exercise is essential, either in the form of turnout, longing, or riding.

Another form of tying-up that affects Quarter Horses and related breeds, as well as warmbloods and draft breeds, is polysaccharide storage myopathy (PSSM). With this disorder, horses have an increased sensitivity to insulin resulting in increased storage of sugar (glycogen) in the muscle. This imbalance in sugar storage and accumulated precursors apparently disturb energy metabolism, and affected horses develop muscle cramping with light exercise. Affected horses are often calm, sedate horses that tie-up after a lay-up, especially when fed grain.

Treatment of polysaccharide storage myopathy involves supplying them with an alternative source of energy rather than sugar. Eliminating grain and sweet feed completely and feeding fats such as rice bran stabilizes blood sugar and decreases sugar storage in muscle. It is essential that horses with PSSM be turned out as much as possible and exercised often, even if only for 10 minutes every day. Horses with mild to moderate clinical signs might be able to return to full athletic performance with careful dietary and management changes, which include regular daily exercise without extended periods of inactivity.

Breeding Quarter Horses with polysaccharide storage myopathy has resulted in offspring with the same disorder. More research is needed in this area.

If your horse ties up, here are suggestions of what to do: 1) Stop exercising the horse and move it to a box stall. Do not force the horse to walk. 2) Call your veterinarian. 3) Blanket the horse if the weather is cool. 4) Determine if the horse is dehydrated due to excessive sweating. 5) Provide fluids with small, frequent sips of water to hot horses, and provide free access to water once the horse has cooled out. 6) Relieve anxiety and pain. Drugs such as acepromazine might be prescribed by your veterinarian. 7) Remove grain and feed; provide only hay until signs subside. 8) Small paddock turnout is good once the horse walks freely, usually in 12-24 hours. If the problem recurs, have the horse evaluated for a specific cause of recurrent exertional rhabdomyolysis.

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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