Tying-Up the Equine

by Jeanne van der Veen, M.S., P.A.S.

Many factors and interactions often come into play when dealing with equine disorders or diseases. The syndrome known as “tying-up” is no exception. Tying up involves muscle tension or cramping to the point that large muscle groups seize up and remain in a state of contraction. Common names for conditions associated with tying-up include exertional rhabdomyolysis (ER), azoturia, and monday morning sickness. Once thought to be the result of a single condition, tying-up is now known to be caused by a number of specific disorders that may involve genetics, training management and/or nutrition.

Affected horses suffer from mild to severe muscle cramping and soreness, usually during exercise or just after working out. Due to pain over the shoulders, back and hind limbs, symptoms generally include a stiff gait, reluctance to move, an unusual stance, or a complete inability to stand. In cases of severe pain, horses may also exhibit profuse sweating, an accelerated heart rate and increased respiratory rate. The end result of tying-up is damage to the muscle cells. Depending on the extent of the muscle damage, severely affected horses may pass a dark, red-brown colored urine due to muscle proteins leaking into the blood and subsequently being excreted in the urine. Diagnosis of tying-up can be confirmed by elevated levels of muscle cell proteins or enzymes in the horses’ blood serum or, in some cases, muscle biopsy is required.

The syndrome of tying-up should not be confused with mild muscle soreness or injury resulting from an inadequate warm-up, improper level of training for desired exercise, or over-exertion to the point of fatigue. This can be common in horses that are not conditioned or exercised on a daily basis.

From research conducted at the University of Minnesota, episodes of tying-up are currently being classified into one of two categories: Sporadic Exertional Rhabdomyolysis, for horses that tie-up on rare occasion and Chronic Exertional Rhabdomyolysis, for horses that tie-up repeatedly.

Sporadic Exertional Rhabdomyolysis

Causes of sporadic tying-up generally involve a level of exercise that exceeds the horse’s level of conditioning or training. This often leads to excessive levels of lactic acid building up in the muscle and blood which in turn affects muscle cell energy metabolism and subsequent contraction. Excessive grain intake or carbohydrate overload in relation to actual requirements can also lead to muscular lactic acidosis and tying up. Electrolyte imbalances, particularly in endurance horses, have also been implicated as a cause of sporadic tying-up. Other potential causes include dehydration, mineral and vitamin deficiencies, and intense exercise in sick horses, particularly with respiratory infections.

It is important that horses susceptible to sporadic tying-up are exercised regularly, properly conditioned, and not confined to a stall for more than eight hours at a time. Horses should be fed salt on a daily basis along with fresh, clean water and a properly fortified diet including vitamin E and selenium. Grain should be fed according to the level of exercise and the horse’s body condition.

Chronic Exertional Rhabdomyolysis

Although hormonal imbalances, lactic acidosis in muscle, deficiencies of vitamin E and/or selenium, and electrolyte imbalances have all been implicated as causes of chronic tying-up, recent research leads to genetic variations or defects in the structure or metabolism of muscle cells as the underlying cause. Research at the University of Minnesota has defined two specific causes of tying-up that are associated with genetics and abnormalities in muscle mechanics or carbohydrate metabolism.

The first is recurrent exertional rhabdomyolysis (RER). In RER, there appears to be a defect in the mechanism for muscle contraction in which the muscle is supersensitive to certain chemicals or stimuli and responds to this stimulation by remaining in a state of contraction. Research indicates this condition may be linked to abnormal intracellular mineral regulation, particularly calcium. Thoroughbreds, Standardbreds, and Arabians tend to be more susceptible to this condition particularly when they are excited or stressed. RER also tends to be very common in nervous young fillies, especially on the race track and may be genetically linked.

RER is best managed by keeping affected horses calm, minimizing stress and providing a daily exercise routine. Long periods of stall rest are not recommended. Feeding a reduced starch and sugar diet tends to help reduce excitability in nervous horses, while feeding a higher fat and fermentable fiber diet will help to maintain body weight. Proper mineral and vitamin nutrition is equally important to minimize oxidative stress and promote proper muscle function.

The second defined cause of tying-up is known as polysaccharide storage myopathy (PSSM). Horses with PSSM appear to have a flaw in their ability to metabolize glucose. Glucose is rapidly cleared from the bloodstream, possibly due to an increase in insulin sensitivity, resulting in excessive glycogen synthesis and storage in skeletal muscle. PSSM is characterized and confirmed through muscle biopsy by the accumulation of excessive levels of glycogen and abnormal polysaccharide complexes in skeletal muscle. How this defect affects muscle cells and tying up is not completely understood at this time.

PSSM can affect any horse but is more common in Quarter Horses and related breeds, Warmbloods, some thoroughbreds and draft horses and appears to be hereditary. PSSM can develop at a young age, typically during the start of training. Horses with PSSM tend to have a calm disposition compared to horses with RER. In many draft breeds, the glycogen storage disorder is known as equine polysaccharide storage myopathy (EPSM or EPSSM). Although the signs and symptoms of EPSM vary from PSSM, they have similar metabolic characteristics and the management of the two disorders is similar.

To minimize excessive glycogen storage in PSSM and EPSM horses, starch and sugars (typically found in grain and sweet feed) should be significantly reduced or eliminated from the diet. Fat and fermentable fiber should be used to supply energy and maintain body weight and condition. A good quality grass hay along with proper balance of minerals and vitamins is also important. When horses are fed a high level of oil or fat, additional Vitamin E and selenium should be fed to minimize oxidative stress and deficiencies often seen with feeding high fat diets. PSSM horses should be exercised daily and turned out as often as possible.

Tying-up is the resulting condition from a variety of different disorders that can be influenced by many factors including genetics, management and nutrition. Research into the causes, prevention and treatment of the disorders and conditions that cause horses to ty-up is active and ongoing. Providing affected horses with daily exercise, sound management techniques and dietary changes that have a low starch and sugar content will allow many horses to continue to lead healthy, productive lives.

© March, 2006. Blue Seal Feeds, Inc.

© Blue Seal Feeds, Inc. – March, 2006