- 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)” and “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. n/P3 + n/P4 or n/P2 + 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.
Inheritance: incomplete autosomal dominant/semidominant
→ 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).
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.|
|n/P||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 (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.
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. and is based upon their unpublished research.
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