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.

http://en.wikipedia.org/wiki/Equine_polysaccharide_storage_myopathy