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 disease. Horses experiencing a bout of tying-up show signs of stiff gait, reluctance to move, firm painful cramping muscles, profuse sweating, increased heart rate and increased respiratory rate. In addition to clinical signs, horse that tie-up will have moderate to marked elevations in serum level of muscle enzymes. Elevation in muscle enzymes indicates muscle cell damage and is considered a diagnostic tool to detect tying-up.

For years, all horses that tied-up following exercise were thought to suffer from the same disease. However, treatment and prevention protocols that seemed to work 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 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 typing-up and steps that can be implemented to prevent future episodes of the disease. Much of the research into understanding tying-up and treatment/prevention protocols has been performed at the college of veterinary medicine at the University of Minnesota.

Sporadic Tying-Up

Many horses experience some muscle strain and soreness associated with exercise. These horses often are mistakenly thought to be suffering from 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 human scenario might be overexertion from strenuous physical activity in a person conditioned to sit behind a desk. In horses that actually tie-up, muscle soreness is much more severe and is typically accompanied with elevated serum muscle enzymes and myoglobinuria (coffee-coloured urine). This darkening of the urine is a result of the kidneys filtering myoglobin (a muscle protein) from blood, an indication of severe muscle damage. The most frequent causes of sporadic ‘tying-up’ are exercise that exceeds a horse’s underlying state of fitness, electrolyte depletion, hypothermia, and strenuous exercise while suffering from a respiratory disease.

Sporadic tying-up should be considered a veterinary emergency if horses are sweating profusely, reluctant to move or have dark urine. Veterinarians may administer treatment to relieve anxiety and muscle pain. In addition, fluids and electrolytes are often given to account for fluid losses and myoglobinuria that may damage kidney function. Further treatment strategies include stall rest followed by hand walking and turn out once initial muscle stiffness is gone. Grain intake is drastically reduced or eliminated since these horses are likely to be on a reduced exercise program, and may need to be reduced once the horse resumes full work. The amount of time the horse must remain out of training has not been firmly established by science, but monitoring of serum muscle protein levels may be useful. However, any training regime following an episode of tying-up should be resumed gradually and consistently to prevent further muscle damage.

Prevention of further tying-up episodes is often a result of common sense. 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 over feed carbohydrate (grain) to horses, as this may be a potential cause of tying-up. Feeding grain concentrates fortified with fat and necessary antioxidant vitamins and minerals (for example NRM Free-Up) 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. In areas where the soil is deficient in selenium or when the feed does not supply the full requirements of vitamin E and selenium, an additional selenium and vitamin E supplement will improve muscle health.

Chronic Tying-Up

When horses have repeated episodes of tying-up, the disease is considered chronic. Many different breeds of horses have been reported to have chronic bouts of tying-up. Fillies and mares are more susceptible and nervous horses are thought to have a higher risk of chronic tying-up. The proposed causes of chronic tying-up include electrolyte imbalances, hormonal imbalances, lactic acidosis, vitamin E and /or selenium deficiency, hypothyroidism, and muscle ischemia. While chronic episodes of tying-up are frustrating and painful for both the horses and their owners, it is the study of these chronically tied-up animals that had advanced our knowledge of the causes, treatment and prevention of the disease.

Recently, two specific causes of tying-up have been identified in the horse. These causes include a muscle contraction disorder (Recurrent Exertional Rhabdomyolysis, RER) and a disorder in carbohydrate storage and utilisation (Polysaccharide Storage Myopathy, PSSM). Both of these causes of tying-up are being investigated extensively and appear to be at least partially responsive to changes in diet, training and daily turnout.

Summary

In summary, tying-up is a generic term used to describe the symptoms of many different muscle diseases. Tying-up can be career threatening for horses and extremely frustrating for horse owners. Recent research looking into tying-up has classified horses on the frequency of tying-up episodes. Horses that show signs of tying-up one or two times in a lifetime are considered “sporadic” cases. Potential causes of these episodes can be many, but most frequently exercise that exceeds the horses’ underlying state of training, electrolyte depletion, hypothermia and respiratory disease are key triggering factors. Most of these horses can be successfully returned to competition with common sense management strategies that focus on proper training and dietary management. Horses that show repeated signs of tying-up are classified as “chronic” cases. Recently, two specific causes of chronic tying-up have been identified in the horse. These causes include a muscle contraction disorder (RER) and a disorder in carbohydrate storage and utilisation (PSSM). Both of these causes of tying-up are being investigated extensively and appear to be at least partially responsive to changes in diet, training and daily turnout.

Above is an excerpt from an article written by Stephen Duren and Stephanie Valberg

Stephen Duren, PH.D. & Dr Peter Huntington (B.V.Sc)
Kentucky Equine Research

Stephanie Valberg DVM PhD
Associate Professor, University of Minnesota, St Paul