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 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.
You can manage PSSM in your horse by providing an alternate energy source to sugar. Avoid feeding grains, sweet feeds and other feedstuffs high in sugar. Fat can be a great alternative. Rice bran or vegetable oils can stabilize blood sugar and provide energy.
In addition to grains, you should be aware of the sugar content in your PSSM horse’s forage. High sugar content in hay can trigger a horse’s insulin response. Select and feed hay that contains less than 10 percent NSC (sugar). Horses with PSSM may not be able to graze.
Work with a professional nutritionist to make sure your horse’s diet meets their needs.
Make sure to provide ample turn out and exercise for your PSSM horse. Horses with mild or moderate cases of PSSM, with proper management, can return to normal performance. Regular exercise without long periods of inactivity is key.
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.
Reviewed in 2023