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Polysaccharide storage myopathy (PSSM) in horses

Quick facts

  • PSSM is a disorder that causes muscle cramping in horses from abnormal glycogen (sugar) storage in the muscles.

  • Sore muscles, muscle weakness and cramping are all signs of PSSM.

  • Type 1 PSSM is caused by a mutation in the GYS1 gene.

  • Feeding hay with less than 10 percent nonstructural carbohydrates (NSC) and providing regular exercise are good care practices for horses with PSSM.

What is PSSM?

Tying up refers to painful muscle cramping in horses. University of Minnesota veterinarians identified PSSM as a form of tying up. This disorder causes the horse’s body to abnormally store or store too much sugar (glycogen) in the muscles.

Signs of illness

  • Muscle break down and progressive weakness in draft horses.

  • Muscle soreness and gait abnormalities in warmbloods.

  • Tying up in quarter horses and related breeds.

Type 1 PSSM

Type 1 PSSM is caused by a mutation in the GYS1 gene and occurs in over 20 horse breeds. It’s likely an old genetic mutation that occurred prior to the foundation of most modern horse breeds. Thus, this mutation isn’t attributed to any given stallion or pedigree within a breed.

A horse only needs one copy of the GYS1 gene mutation to be affected by Type 1 PSSM. The GYS1 gene plays a role in glycogen production. A mutation in this gene causes the muscle cells to continually make glycogen.

As a horse exercises, they aren’t able to use this extra glycogen for energy. Consequently, after light work the horse may experience soreness and muscle cramping.

Type 2 PSSM

Type 2 PSSM refers to PSSM that occurs without genetic mutation. This PSSM is more common in warmbloods.

Insulin sensitivity

Insulin is a hormone that controls a horse’s blood sugar level. Thus, sugar from a horse’s diet can stimulate an insulin response. PSSM horses tend to be more sensitive to insulin. Insulin can further worsen PSSM because it causes the GYS1 gene to produce even more glycogen.




Genetic testing for PSSM 1 is available through the Veterinary Genetics Laboratory at the University of California Davis. Muscle biopsies are required to test for PSSM 2. You should consider a muscle biopsy for horses with muscle pain if they test negative for PSSM 1.

Contact your veterinarian for additional information about testing for PSSM.

Authors: Molly McCue, DVM,  Stephanie Valberg, DVM, and Nichol Schultz

Reviewed in 2023

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