Most commonly associated with Quarter horse bloodlines, EPSM or Equine Polysaccharide Storage Myopathy is a muscle condition or disease. There are documented occurrences of EPSM in other horses such as American Paint horses, Warm bloods, draft horses, quarter horse crosses, and draft crosses. Evidence shows that in Quarter horses, EPSM is a inheritable disease, but this remains unproven in many other breeds. A horse with Equine Polysaccharide Storage Myopathy has an excessive glycogen buildup or storage in the skeletal muscles due to improper digestion of sugars, starches and carbohydrates found in cereal grains such as barley, corn, and oats.
Often, there are very few or very subtle symptoms in the early stages and because of the recent discovery of EPSM, it is often not considered or diagnosed until fairly advanced. Some of the most obvious symptoms of horses with advanced EPSM include sweating, hesitancy to move, involuntary hock flexion, lack of coordination and strength in hindquarters, weakness and listlessness, trembling or shivers, and a stiff or rigid gait or gait abnormality, especially in the hind end. Some horses with EPSM are uncoordinated when backing or display poor performance, work intolerance and skeletal muscle tissue breakdown.
Veterinarians often misdiagnosed Equine Polysaccharide Storage Myopathy as colic, hock or back soreness. The most dependable clinical test for diagnosing EPSM is a muscle biopsy done on the sedated horse while it is standing. The vet injects a local anesthetic into the caudal rump muscle and then makes a small incision to obtain a small strip of muscle for the biopsy. This relatively simple procedure usually results in very little scarring but provides highly valuable information. Another diagnosis technique called the endocrine test, done by taking a blood sample after physically exercising the horse, is not as reliable.
Exercise and dietary change therapy are the primary treatments for Equine Polysaccharide Storage Myopathy once the veterinarian makes the proper diagnosis. Veterinarians recommend changing the diet by replacing the starches, sugars and carbohydrates found in such things as cereal grains, pellet horse feeds, and sweet feeds, with fat as the main energy source. The added fats diet includes fiber found in pasture or hay, amino acids from protein, and fatty acids from oils or fats. Their daily food allowance should include good quality hay or forage; vegetable oil such as canola, soy, or corn; selenium supplement if the area is not naturally selenium-sufficient; vitamin E; and normal mineral and salt supplements. Horses should always have access to a supply of fresh water. Exercise is very important as it helps rebuild the horses damaged muscles caused by EPSM, but the amount of exercise depends upon its comfort level from the severity of the disease.
When starting dietary therapy on horses with early stage Equine Polysaccharide Storage Myopathy, many cases show a one hundred percent improvement. Changing an affected horse's diet to a low carbohydrate, high fat diet often prevents, delays, or decreases EPSM signs and could be life-saving. Many horses show increased energy, improved muscling and can return to work. In advanced stages of this disease were the horse is severely affected, there is the possibility of death, often due to late diagnosis. Veterinarians recommend that owners should not breed affected horses as the colts may inherit Equine Polysaccharide Storage Myopathy.