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PSSM2 New Variant Extension (P8, K1)


→ Exercise intolerance due to muscle wasting, stiffness, and pain.

Important: To order this Panel extension, you must already have tested your horse for the original four EquiSeq variants (P2, P3, P4, and Px). Previously untested horses must order the Six Variant Panel.

Exception – Customers with Icelandic horses may purchase this extension even without previous testing.

Please include your customer number and animal number if possible, to help us quickly locate your sample. You will be contacted if we need you to send a new sample.

Polysaccharide Storage Myopathy (PSSM) is the name of a group of hereditary muscle diseases with similar symptoms. The variants categorized under the umbrella term PSSM2 (P2, P3, P4, P8, Px, K1) lead to exercise intolerance due to muscle wasting, stiffness, and pain.

PSSM2 is not curable and can greatly impact a horse’s quality of life. An accurate diagnosis allows owners to optimize feeding and handling/exercise regimens to support the health of their horses.

Instructions for ordering:

  1. Enter the information of the first horse for which you want to order the test and please include your customer number and animal number if possible.
  2. Add to cart
  3. Go to cart to check out, or
  4. Enter the information for a second horse.
  5. When we receive your order, we will check to see if there is enough of your original sample available to do the test.  If we need more sample we will contact you by email. You do not need to send a new sample unless we contact you.  If you are submitting an Icelandic horse for testing for the first time, please follow the normal instructions for submitting a sample (

In the next days you will receive an email to register through our new Generatio Animal Trust Center (ATC) system.  You will then be able to see your animal’s results as soon as they are available, as long as payment has been received. You can pay through this webshop system today, or through the ATC when you receive your activation email.

No discounts are available on the extension panel.

Enter the name of your animal
Enter your animal's date/year of birth
Enter the breed name
Enter the registration number if known
Enter the microchip number
Is there anything else you would like to tell us about your horse? Reason for testing, symptoms etc.?

General information

  • Two different types of Polysaccharide Storage Myopathy (PSSM) are known so far: Type 1 and Type 2.
  • The diseases have similar symptoms; in the past horses with PSSM1 symptoms that were negative for the PSSM1 GYS1 gene mutation were classified as PSSM2.
  • Research has shown that PSSM2 is not a polysaccharide storage disease, but is actually caused by defects in muscle fibers themselves. The name “PSSM2” is still used because people are familiar with it.
  • “Myofibrillar Myopathy (MFM)” und “Recurrent Exertional Rhabdomyolysis (RER)” are subtypes of PSSM2.
  • Several variants (P2, P3, P4, Px, P8, K1) in the genes  MYOTFLNC, MYOZ3, CACNA2D3, PYROXD1, and COL6A2 have been shown to cause these subtypes.
  • A horse can have more than one P-variant, e.g. n/P3 + n/P4 or n/P8 + n/P3, etc. The combination of different variants leads to more severe symptoms and an earlier age of onset in the affected horse.
  • Research is on-going to identify more genetic variants that may be responsible for other subtypes of PSSM2.

The Equine Myopathy/PSSM2 panel is offered under license from EquiSeq Inc. and is based upon their unpublished research.


  • Changes in temperament/behavior (because of pain)
  • Atactic gait/coordination problems
  • Shifting lameness
  • Muscle wasting (most obvious in hindquarters and shoulder)
  • Local muscle wasting (small divots that could look like kick marks)
  • Frequent tying up/tension
    • Stiff hindquarters
    • Muscle tremors
    • Gait changes (cross firing or disunited canter/bunny hopping/rope walking)

Inheritance and Genotypes

→ PSSM2 is an incomplete autosomal dominant/semidominant hereditary disease. Horses with one or two copies of the variant (n/P or P/P) are affected. Incomplete or semidominant means that animals with one copy (n/P) may show milder symptoms and a later age of onset than animals with two copies (P/P).

Genotype: The horse is: Effects:
n/n normal. The horse does not have any copies of the genetic variant causative for PSSM2 and therefore cannot pass it on to any offspring.
n/P high risk/affected (heterozygous).
The horse has one copy of the P-variant and will pass it on to approximately 50% of its offspring.
P/P high risk / affected (homozygous). The horse has two copies of the P-variant and will pass it on to 100% of its offspring.



  • Although PSSM2 disorders cannot be cured, most affected horses benefit from a diet with high protein content and supplementation of the amino acids lysine, threonine, and methionine.  Fat content should be high and carbohydrate levels should be fairly low. Consult with your veterinarian or equine nutritionist about the appropriate feeding recommendations for your horse.
  • At risk/affected horses (n/P or P/P) should only be bred after careful consideration and with the advise of a genetic expert and veterinarian. Please contact us for consultation and support. No symptomatic animal should be used for breeding due to animal welfare reasons.


The Equine Myopathy PSSM2 panel is based upon unpublished research from EquiSeq.

Further information:
McCue ME et al. (2008). “Glycogen synthase (GYS1) mutation causes a novel skeletal muscle glycogenosis.” Genomics. 91(5):458-66. PMID: 18358695.

McCue ME et al. (2008). “Glycogen synthase 1 (GYS1) mutation in diverse breeds with polysaccharide storage myopathy.” Journal of Veterinary Internal Medicine. 22(0):1228–1233. PMID: 18691366.

McCue ME et al. (2009). “Polysaccharide storage myopathy phenotype in quarter horse-related breeds is modified by the presence of an RYR1 mutation.” Neuromuscular Disorders. 19(0):37–43. PMID: 19056269.

McCue ME et al. (2009). “Comparative skeletal muscle histopathologic and ultrastructural features in two forms of polysaccharide storage myopathy in horses.” Vet Pathol. 46(6):1281-1291. PMID: 19605906.

Maile CA et al. (2017). “A highly prevalent equine glycogen storage disease is explained by constitutive activation of a mutant glycogen synthase.” Biochim Biophys Acta.. 1861(1):3388-3398. PMID: 27592162.

Valberg SJ et al. (2016). “Suspected myofibrillar myopathy in Arabian horses with a history of exertional rhabdomyolysis.” Equine Vet J.. 48(5):548-556. PMID: 26234161.

Lewis SS et al. (2017). “Clinical characteristics and muscle glycogen concentrations in warmblood horses with polysaccharide storage myopathy” Am J Vet Res. 78(11):1305-1312. PMID: 29076373.

Further information is available at EquiSeq .


Here you’ll find the Equine Myopathy/PSSM2 information for downloading and printing:

For German versions:

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