- 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)
- 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) in the genes MYOT, FLNC, MYOZ3 and CACNA2D3 have been shown to cause these subtypes.
- A horse can have more than one P-variant, e.g. P3/n + P4/n or P2/n + P3/n, 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.
Inheritance: incomplete autosomal dominant/semidominant
→ Horses with one or two copies of the variant (P/n or P/P) are affected. Incomplete or semidominant means that animals with one copy (P/n) may show milder symptoms and a later age of onset than animals with two copies (P/P).
Possible genotypes: The following genotypes and effects apply for the variants P2, P3, P4 und Px.
|Genotype:||The horse is:||Effects:|
|n/n||normal.||The horse does not have any P-variants and therefore cannot pass them on to any offspring.|
|P/n||affected (heterozygous).||The horse has one copy of the P-variant and will pass it on to approximately 50% of its offspring. These 50% are at risk of developing PSSM2.|
|P/P||affected (homozygous).||The horse has two copies of the P-variant and will pass it on to 100% of its offspring. All offspring will be at risk of developing PSSM2.
More information about the disease, specific mutations, and inheritance can be found on our
PSSM2 Information Page.
- Althought PSSM2 diseases cannot be cured, most affected horses benefit from a diet with high fat and protein content and/or supplementation of the amino acids lysine, threonine, and methionine. Consult with your veterinarian or equine nutritionist about the appropriate feeding recommendations for your horse.
- Affected horses (P/n 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.
Test information: This Panel test identifies the changes in the MYOT, FLNC, MYOZ3 and CACNA2D3 genes.
The Equine Myopathy/PSSM2 panel is offered under license from EquiSeq Inc.
Unpublished research from EquiSeq.
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 .
Test #: H108
Description: Myopathy Panel (P2, P3, P4, Px)
Samples: Hair, blood
Cost: 238 € (200 € + 19% VAT)
Time: 10 - 14 business days
How to Order
Step 1: Download and fill-in an order form (pdf or fillable Word file).
Other tests may also be ordered on this form.
Step 2: Pull 20-40 hairs from the mane or tail of your horse. Make sure the hairs have roots attached.
Step 3: Tape the hair to a piece of paper with the horse’s name and your name and put into a clean plastic bag or envelope.
Step 4: Decide upon a method of payment: bank transfer or credit card. If choosing bank transfer, payment information will provided later. If choosing credit card, please download and fill out the Credit Card authorization form and include it with your sample.
Step 5: Put the signed form(s) and hair sample into an envelope and send by regular post to:
CAG GmbH – Center for Animal Genetics
Step 6: When the samples are received in the lab, we will send you an email. If you are paying by bank transfer, we will include an invoice with the transfer information.
Step 7: 10-14 business days after the sample arrives in the lab, we will send you your results and receipt by email.