11/21/2022
Muscle Disorders in Horses
Myopathies are diseases that primarily cause damage to muscles. They may be present at birth (congenital) or occur due to nutritional imbalances, injury, or ingestion of a poisonous substance. Myositis is an inflammatory reaction in muscle. Common causes include infections, parasitic diseases, and immune-mediated conditions.
Muscle disorders can appear with a variety of signs ranging from muscle stiffness and pain to muscle atrophy, weakness, exercise intolerance, and muscle fasciculations (twitching). The most common signs are muscle pain, stiffness, and reluctance to move due to rhabdomyolysis, which may or may not be related to exercise.
Muscle weakness or damage can also occur as a sign of many different disorders (such as nerve trauma or influenza). Only conditions in which myopathy or myositis are primarily involved are discussed in this section.
Nutritional Myopathies of Horses
Disorders related to vitamin E, selenium (an element required in small amounts for normal nutrition), and fat metabolism can all affect the muscles, leading to inflammation and degeneration.
Selenium or Vitamin E Deficiency
Degeneration of muscle is sometimes associated with a deficiency of selenium or vitamin E. The condition may cause rapid, unexpected death in adult horses. Other affected horses may show a staggering gait, difficulty swallowing, swelling of the cheek and tongue muscles, difficulty breathing, and a rapid heartbeat. Physical changes involve the skeletal muscles and the heart muscle. Blood tests are used to confirm a selenium or vitamin E deficiency and muscle damage (indicated by increases of muscle-associated enzymes). Treatment includes providing selenium and vitamin E.
In foals, this nutritional myopathy may be seen at birth or shortly thereafter and may be accompanied by inflammation of fatty tissue or “yellow fat disease.” Stiffness and pain are noticeable when feeling for the firm, fat masses below the skin, and severely affected foals may be unable to suckle. Treatment involves addressing the underlying dietary deficiency. Vitamin E supplementation appears to be effective.
Exertional Myopathies in Horses
Exertional myopathies in horses involve muscle fatigue, pain, or cramping associated with exercise. Most exercise-associated myopathies result in death of skeletal muscle, which is called exertional rhabdomyolysis. Once considered a single disease, it is now understood that there are several conditions that appear similar but have different causes.
Signs are usually seen shortly after the horse begins to exercise. Excessive sweating, increased breathing rate, rapid heartbeat, reluctance or refusal to move, and firm, painful hindquarters are common. The severity varies between horses and even between episodes in the same horse. Severe episodes may involve muscle damage with kidney failure and reluctance to stand. A diagnosis of exertional rhabdomyolysis is confirmed using blood tests.
Exertional rhabdomyolysis can be sporadic, with single or very infrequent episodes occurring with exercise. It can also be chronic, with repeated episodes, often occurring after mild exertion. Horses of any breed may develop sporadic exertional rhabdomyolysis.
Sporadic Exertional Rhabdomyolysis (Tying Up)
The most common cause of sporadic exertional rhabdomyolysis is exercise that exceeds the horse’s physical condition (level of training, health, etc.). Respiratory disease and deficiencies of sodium, calcium, vitamin E, or selenium in the diet may also play a role. Affected horses generally have no previous history of the condition.
Signs include muscle cramping and stiffness following exercise. Blood tests will reveal elevated levels of certain enzymes. As soon as the condition is diagnosed, exercise should stop and the horse should be moved to a stall with comfortable bedding and access to fresh water. Treatment should aim to relieve anxiety and muscle pain and to correct dehydration and metabolic imbalances. Appropriate tranquilizers or pain relievers may be prescribed to relieve pain and inflammation. Most horses are relatively free of pain within 18 to 24 hours.
Severe rhabdomyolysis can lead to kidney problems. In severely affected animals, regular blood and urine tests are advised to assess the kidney damage. Treatments that induce urination (diuretics) are not recommended except in certain cases (such as in horses receiving intravenous fluid treatment). Your veterinarian will monitor your horse’s kidney function and develop an appropriate treatment plan.
Horses should be kept on a hay diet and stall rest for a few days. The horse should continue to rest with regular access to a paddock until the blood muscle enzyme levels are normal. Because the cause is generally temporary, most horses recover with rest, a gradual return to normal training levels, and dietary changes. Horses ridden for endurance should be encouraged to drink electrolyte-supplemented water during an endurance ride. They should be watched especially closely in hot, humid weather for signs of dehydration or muscle cramping.
Chronic Exertional Rhabdomyolysis
Some horses have multiple episodes of rhabdomyolysis that occur after even light exercise. Four forms of chronic exertional rhabdomyolysis have been identified from samples of muscle tissue (biopsies)—two forms of polysaccharide storage myopathy, malignant hyperthermia, and recurrent exertional rhabdomyolysis.
Type 1 polysaccharide storage myopathy is frequently seen in Quarter horse-related breeds, Morgans, and draft horses but is also present in at least 20 other horse breeds. It is caused by a dominantly inherited gene mutation and can be diagnosed with genetic testing of blood or hair samples. Affected horses often develop episodes of rhabdomyolysis at a young age with little exercise. Rest for a few days before exercise is a common triggering event. Signs include a tucked-up abdomen, a camped-out stance, muscle twitching, sweating, abnormal gait, stiffness in hindlimbs, and reluctance to move. Pawing or colic signs may also occur. Draft horses may have more dramatic signs that include loss of muscle mass, progressive weakness, and lying down.
Type 2 polysaccharide storage myopathy occurs in light breeds such as Arabians, Morgans, Thoroughbreds, a variety of Warmbloods, and some Quarter horses. Diagnosis is based on a muscle biopsy showing specific abnormalities and is supported by negative test results to the genetic test for Type 1 disease. In Quarter horses
