*Dr. Joe D. Pagan is president and founder of Kentucky Equine Research Inc., which, through consultation and research, aims to bridge the gap that may exist between basic research and horse production.
TYING-up is a generic term commonly used to describe muscle disease in performance horses. Other terms often used interchangeably with tying-up include exertional rhabdomyolysis, azoturia and Monday morning sickness.
Clinical signs of tying-up include a stiff gait, reluctance to move, firm and cramping muscles, profuse sweating, accelerated heart rate and increased respiratory rate.
In addition to clinical signs, horses that tie-up will have moderate to marked elevations in blood serum levels of myoglobin, creatine kinase (CK), lactate dehydrogenase and aspartate aminotransferase. Such elevations indicate muscle cell damage and are considered a diagnostic tool to detect tying-up.
For years, 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 other horses. As a result, confusion and controversy developed regarding the cause and treatment of tying-up.
Researchers have only recently begun to classify and study the many different disease conditions that result in the common symptoms of tying-up. Initial classification of tying-up is now based on the frequency of the disease symptoms following exercise. Horses that tie-up only a few times in their lifetime are classified as "sporadic," while those horses that tie-up on a repeated basis are termed "chronic."
The following is a brief discussion of both sporadic and chronic tying-up and steps that can be implemented to help prevent the disease. Much of the research involving tying-up and treatment/prevention protocols has been conducted by Dr. Stephanie Valberg's group at the University of Minnesota's College of Veterinary Medicine.
Many horses experience some muscle soreness and strain associated with exercise. These horses often are mistakenly thought to be suffering from a specific muscle disease.
Inadequate warm-up, pre-existing lameness, exercise to the point of fatigue and insufficient training can result in muscle soreness and injury. A similar scenario in people might be overexertion from strenuous physical activity in a person who is conditioned to sit at a desk most of the day.
In horses that actually tie-up, muscle soreness is much more severe and is typically accompanied by elevated blood serum muscle proteins and myoglobinuria (coffee-colored urine). This darkening of the urine is a result of the kidneys filtering myoglobin (a muscle protein) from the blood, which is an indication of severe muscle damage.
The most frequent causes of sporadic tying-up are exercise that exceeds a horse's level of training, electrolyte imbalances, hyperthermia 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 may administer medicine to relieve anxiety and muscle pain. In addition, corrections in hydration are made to account for fluid losses and myoglobinuria that may impair kidney function.
Further treatment strategies include stall rest, followed by hand walking and turnout once the initial muscle stiffness has resolved. Grain intake should be drastically reduced or eliminated because these horses are likely to be on a reduced-exercise program.
The amount of time the horse must remain out of training has not been firmly established. However, any training regimen following an episode of tying-up should be resumed gradually and consistently to prevent further muscle damage.
Sound management practices and training may help prevent tying-up. Learning not to overexert unfit horses and remembering to fortify the diet with salt on a daily basis, as well as electrolytes prior to heavy sweat loss, are logical strategies. Adjusting the amount of grain fed to satisfy the energy needs of the horse is also tremendously important.
In other words, one does not want to overfeed carbohydrates (grains) to horses as this may be a potential cause of tying-up. Feeding grain concentrates fortified with fat and necessary antioxidant vitamins and minerals will provide energy while supplying the building blocks to protect muscle tissue. The combination of these strategies will often prevent healthy horses from having another bout of tying-up.
When horses have repeated episodes of tying-up, the disease is considered chronic. Many different horse breeds 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.
The proposed causes of chronic tying-up include electrolyte imbalances, hormonal imbalances, hypothyroidism, muscle ischemia with lactic acidosis and a deficiency of vitamin E and/or selenium.
While chronic episodes of tying-up are frustrating and painful for both horses and their owners, studying these chronic cases has advanced the knowledge of the causes, treatment and prevention of the disease.
Recently, two specific causes of chronic tying-up have been identified in the horse, including a muscle contraction disorder called recurrent exertional rhabdomyolysis (RER) and a disorder in carbohydrate storage and utilization called polysaccharide storage myopathy (PSSM).
RER. RER is common in nervous fillies of Arabian, Standardbred and Thoroughbred breeding. These individuals often develop the condition when they are excited or stressed and/or when a period of stall rest preceding exercise has occurred. Preliminary genetic research and breeding trials point to this condition as an inherited trait in Thoroughbred horses.
The exact cause of RER in horses has challenged scientists for several years. RER in racing horses was believed to be similar to lactic acidosis. However, recent research has shown muscle lactate concentrations to be low -- not high -- in these horses when tying-up occurs.
The most recent thinking is that RER is an abnormality in the way muscle contraction is regulated in the horse. Muscle biopsies from horses with RER have revealed an increased sensitivity to contraction when exposed to various chemicals compared to normal horses. The altered contraction and relaxation of muscle suggests that abnormal intracellular calcium regulation is the cause of RER.
Treatment of horses experiencing RER requires veterinary assistance. Management designed to make the horse comfortable and prevent further stress and muscle damage is warranted. The use of sedatives prior to exercise to calm a nervous horse is also a common practice. Other drugs designed to regulate intracellular sodium and calcium are now being prescribed for horses.
Daily exercise for horses diagnosed with RER is essential. Beginning approximately 24 hours after an RER episode, horses should be hand walked or turned out on a daily basis. Prolonged stall rest seems to be counterproductive and may predispose the horse to further episodes of RER once training resumes. A gradual return to full training can begin once serum muscle protein CK has returned to normal.
Preventing further episodes of RER is difficult. Controlling the environment of these horses is essential. A low-stress environment with a well-established daily routine seems to help. Reducing the amount of carbohydrate (grain) and increasing the amounts of fat (vegetable oil and rice bran) and fiber in the diet of horses with RER reduce excitability and enhance tractability.
Research at the University of Minnesota, in conjunction with Kentucky Equine Research, also suggests that replacing the grain in the diet with a low-starch, high-fat feed will significantly decrease the amount of muscle damage in RER horses.
In a feeding trial, Thoroughbred horses with RER were exercised on a treadmill for five days a week while they consumed hay and a variety of energy supplements.
When the daily caloric intake of a high-starch ration was kept low, the horses had lower post-exercise serum CK than when this feed was increased to a level commonly fed to racehorses. In contrast, if extra calories were provided from a low-starch, high-fat feed (such as Re-leve), no increase in post-exercise serum CK activity occurred. Since these studies were conducted, Re-leve has been used to successfully manage RER in hundreds of Thoroughbred racehorses around the world.
PSSM. PSSM is a glycogen (muscle sugar) storage disorder that is characterized by the accumulation of an abnormal polysaccharide in muscle. Horses with PSSM are able to quickly clear sugar from their blood and store 1.5-4.0 times the normal amount of muscle glycogen.
The accumulation of an excessive amount of muscle glycogen is not due to the inability of these horses to utilize muscle glycogen for energy production but, instead, appears to be from the creation of more glycogen, as well as an abnormal form of muscle glycogen. The horse may utilize the abnormal filamentous polysaccharide at a much slower rate, thus allowing it to accumulate in the muscle.
PSSM has been identified in Quarter Horses, Paints, Appaloosas, draft horses, warmbloods and a few Thoroughbreds. Horses with PSSM are different from horses with RER in that they have a calm, instead of nervous, demeanor. Horses with PSSM typically have a history of tying-up problems associated with the onset of training while the animal is still relatively unfit.
Horses with PSSM exhibit classic tying-up symptoms, including the long-term elevation of muscle enzymes in serum. A confirmed diagnosis is based on an examination of muscle biopsies with the distinctive feature of abnormal glycogen storage.
Treating horses with PSSM following a tying-up episode involves many of the same veterinary procedures as with other causes of tying-up. Treatment protocols also attempt to minimize the occurrence of future episodes through dietary manipulation.
Horses with PSSM should be fed diets low in sugar (grain) and high in fiber and fat. It is important to completely eliminate grain or sweet feed from the diet of horses with PSSM.
A number of studies at the University of Minnesota have confirmed that PSSM can be successfully managed with Re-leve. Daily activity, riding or longeing, along with pasture turnout, is essential for minimizing the occurrence of PSSM tying-up episodes. Confining horses in box stalls for more than 12 hours per day appears to increase the incidence of tying-up.
Research on factors involved in the cause and treatment of tying-up continues to be conducted. With careful attention to feeding and exercise programs, many horses with this condition can be managed to allow an active and useful life.