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PSSM (Polysaccharide Storage Myopathy) is a common cause of exertional rhabdomyolysis, which is rapid breakdown of muscle with exercise, muscle soreness and weakness. The breeds that are most frequently affected by this condition are Quarter Horses, draft horses, and warmblood breeds. In Quarter Horses, it has been found to have a genetic basis; more genetic research is needed in draft and warmblood breeds to determine if a genetic basis is present.

The average age of clinical signs is 5 years old in Quarter Horses, and can range from 1 to 14 years old. The age of onset in warmbloods is between 8 and 11 years old. There is a very high prevalence of PSSM in draft breeds, especially Belgians, Percherons, and crosses of these breeds to light horses. It is estimated that 80% of draft horses may test positive based on muscle biopsy. There is no significant temperament, body type or gender predilection for PSSM. Some believe there is a seasonal pattern to the onset of clinical signs.

The most common trigger for clinical signs is less than 20 minutes of exercise at a walk and trot, particularly after several days of rest. The hindend is usually the most affected; back, abdomen and forelimbs may also be involved.


  • Firm, painful back and hindquarter muscles
  • Trembling, muscle fasciculations (muscle twitching)
  • Muscle wasting
  • Stiffness, weakness, difficulty rising
  • Reluctance to collect and engage hindquarters
  • Poor rounding over fences
  • Gait abnormalities, excessive limb flexion
  • Abnormal stretching or posturing
  • Unusual or excessive sweating
  • Reluctance, refusal or inability to move
  • Mild colic

Horses should only be tested for PSSM if they are showing overt clinical signs of the disease. After a thorough physical exam, bloodwork is generally the next step when evaluating a horse for PSSM. Muscle enzymes such as creatine kinase (CK) and asparartate transaminase (AST), will be elevated when muscle damage has occurred. The muscle damage can be due to PSSM, but an acute injury or strenuous activity can also elevate muscle enzymes.

A definitive diagnosis of PSSM can only be determined with a biopsy of a small (2×1 cm) section of locomotor muscle tissue. There is no need to haul the horse to a clinic or hospital for this procedure. Our field veterinarians surgically harvest the sample tissue at the farm–which minimizes the stress for both horse and owner. There is little disruption to the horse’s normal routine following the procedure.

Owners need to be aware that any horse diagnosed with PSSM will always have an underlying muscle soreness. The goal is to properly manage these horses to minimize clinical signs of the disease. A low starch, high fat and high fiber diet is recommended. An extended “warm up/stretching” period before strenuous exercise will also help these horses be more comfortable. And we all know:
A more comfortable horse is safer and more enjoyable!

Please feel free to contact Dr. Lewis at if you have any questions or concerns about whether your horse should be evaluated for Polysaccharide Storage Myopathy.