Muscular and Skeletal Illnesses in Horses

Equine Polysaccharide Storage Myopathy

Equine Polysaccharide Storage Myopathy (EPSM) is an illness that affects the skeletal and muscular systems in many of the stockier horse breeds. Among those breeds affected are the American Quarter and Paint horses, as well as Warm Bloods and any horse that is cross-bred with the aforementioned breeds. Generally, the heavier the horse is, the more serious the condition is. EPSM is also more likely to affect mares than male horses.



A horse with EPSM will usually avoid exercise, lay down frequently, and be reluctant to move altogether. It will have muscle pain in its gluteal, bicep or hind leg regions, which results in uncontrolled twitching or “attacks.” These “attacks” commonly occur shortly after the horse’s exercise routine has begun, but may occur at random as well. Some other common symptoms include:

  • Strange gait
  • Trouble keeping its balance
  • Stiffness in legs
  • Abnormal flexion in one or both back legs
  • Severe weight loss/muscle wasting
  • Thinness in the rump/thigh area
  • Heightened level of certain enzymes (i.e., Creatininekinase, Lactate dehydrogenase, Aspartetransaminase)


There has been substantial research done to discover the cause of EPSM, and it is now thought genetics may play a large part in the transmission of the illness. Some horses fail to produce glycogen properly in their muscles, allowing large amounts of polysaccharides to collect inside the muscles. In essence, the muscles have no fuel to perform.


In order to properly diagnose EPSM, a veterinarian will likely perform a muscle biopsy of the affected area on the horse.


The rear end of an animal


A type of animal feed that is high in fiber; may include hay or pasture crops


The term used to describe the movement of an animal


The process of removing tissue to examine it, usually for medical reasons.


The study of the laws of inheritance n living things; may also be referred to as breeding

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