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Medical Index
First Posted: Dec 28, 2010
Dec 28, 2010

Equine polysaccharide storage myopathy (EPSM) and Polysaccharide Storage Myopathy (PSSM)

EPSM or Equine Polysaccharide Storage Myopathy is a muscle disease most commonly associated with heavy horse breeds. Common heavy horse breeds in the United States include the Clydesdale, Shire, Belgian Draft or Belgian, Suffolk Punch and Percheron. EPSM may also occur in the American Quarter Horse and other light horse breeds or draft mules and horses that have draft lineage. The Quarter Horse community calls the condition PSSM (Polysaccharide Storage Myopathy). EPSM may, less commonly, be found in most other breeds including Standardbred, Arab and Thoroughbred.

Nature of the Condition

EPSM is a genetic predisposition to fail to digest grains properly in the horse. The result leads to damage to muscle tissue during exertion. It is estimated by DNA testing that approximately two-thirds of all draft horses have the predisposition for EPSM. A biopsy test can determine whether the horse has EPSM. An endocrine test after exercise may detect EPSM related muscle damage. EPSM can be fatal to affected horses.


While many heavy horses have the predisposition for EPSM, most do not show symptoms. What triggers the condition from predisposition to active threat is not known. It may be that unexpected changes in work or diet cause the horse to lock up or over conserve electrolytes, causing failure of normal neuronal controls. The active result, failure to properly digest starches and sugars from grains, leads to a general system effect. The muscle tissues showing effect or damage varies widely. EPSM is not commonly diagnosed from symptoms, but rather by ruling out other potential causes for symptoms.

The three primary techniques for confirming a diagnosis of EPSM are:

  • Try the added fats diet therapy for 4-6 months. If the horse improves, then EPSM is a likely suspect, and the horse is 4-6 months along the road to recovery.
  • Muscle biopsy. This is a dependable, clinical diagnosis.
  • Endocrine test, from a blood sample after exercise. This was the first diagnosis technique available, but is not completely reliable.

Beth Valentine, DVM, proposed the first successful therapy to halt and reverse the progression of EPSM, except in advanced cases. This is a diet change. Because EPSM is a genetic condition, there can be no 'cure' in the symptomatic horse, but many affected horses can be restored to full health and work status.

The approach considers that the horse gains no nutrition from grains, other than the calories. Since the starches and sugars in grains cause specific digestive problems, the approach is to replace starch and sugar calories with fat-based calories. The result is the 'added fats' diet. In basic outline the daily ration is divided into two feedings:

  • 1.2% to 2.1% bodyweight in good quality forage (hay)
  • 2 cups vegetable oil (corn, soy, canola) per 1,000 lb bodyweight (1 liter vegetable oil per 1,000 kg bodyweight)
  • 1-2 IU (International Units) Vitamin E per pound (2 to 5 IU/kg)
  • Selenium supplement
  • Plenty of fresh water
  • Exercise to the horse's comfort level
  • Usual salt and mineral supplements
  • Improvement of EPSM symptoms may take 4-6 months

Adding the fat calories (vegetable oil) to the diet is as important as removing the starches and sugars from grain sources. Depending on severity of the condition, the horse may have comfort restrictions in exercising or work, but some exercise is required to begin rebuilding damaged muscles. As the recovery proceeds, exercise may increase to best occupy the horse's attention and maintain recovery.

Molasses is added to many feeds and grains to control dust and to sweeten (sugar) the feed to entice appetite. The sugars and starches in molasses, however, are a problem for EPSM-symptomatic horses. The horse's appetite will often decline when molasses is withdrawn. This will pass as the horse relearns the taste of feed without molasses.

Some regions are naturally selenium-sufficient, and additional supplements should be added with care. A local veterinarian can advise about local selenium levels


Part of the hay ration may be taken as beet pulp shreds, alfalfa pellets, or a combination, to provide a carrier to pour the oil over for feeding.

The specified level of vegetable oil, 2 c./1,000 lb bodyweight, targets a dietary level of 24% calories from fat sources. Several commercial feed companies now provide feeds with reduced sugars and starches, and added fat sources. These feeds can reduce the amount of additional vegetable oil needed to meet the dietary goal ((2 c./1,000 lb)/day), or in some cases will meet the goal without additional fat calories. At least one company manufactures a dry fat supplement that can be used instead of liquid vegetable oil.

One strategy to introducing vegetable oil is to begin with 1/4 cup (125 g) vegetable oil on one feeding. Continue for several days, until the horse is eating both feedings well. Then add 1/4 c. to the other feeding for 2-3 days. Every 2-3 days increase by another 1/4 c. If at some point the horse stops accepting the increase, back up a step or two for 2-3 days and resume the progression, until the target amount for that horse is achieved. This approach is consistent with always making diet changes gradually, over at least 2-3 weeks.

Because the therapy links diet to weight of the horse, EPSM horses need to be separated at feeding time (usually 45 minutes or less), so that the horse gets fat calories proportional to body weight.

The therapy appears to be quite benign, with most horses benefiting, even non-EPSM horses. Other observed effects of the non-grain, fats based diet include improved recovery time from exercise (from trials before the Atlanta Olympics) and improved heat tolerance (reported by farmers, loggers). Horses that experience mood swings due to blood sugar excursions from starches and sugars in the diet often show a more even attitude on the added fats diet. Some horses with chronic colic have shown improvement, and some 'hard to keep' horses have been more thrifty (less feed needed to maintain condition) on the added fats diet.


Several researchers, including Beth Valentine while working at the Cornell University pathology labs, investigated deaths of heavy horses from muscle wasting. They found that the muscle wasting, an occasional cause of death in heavy horses, was cause by a peculiar type of starvation of the muscle. Positing the starches and sugars link, Dr. Beth proposed the vegetable oil/Vitamin E/selenium therapy, and the endocrine test to detect presence of the condition in living horses.

The recent (1990s) announcement of the condition in horses in use for decades or centuries met with a degree of skepticism, especially as the therapy was to remove grains from the horse's diet. Anecdotal and further research has confirmed the condition.

For More Information:

Tying-Up in Thoroughbreds: Narrowing the Genetic Search
Equine polysaccharide storage myopathy (EPSM)
Chronic Exertional Rhabdomyolysis

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