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Polysaccharide Storage Myopathy (PSSM)


Polysaccharide Storage Myopathy (PSSM) is a dominant autosomal hereditary condition that can cause a genetic form of tying-up with muscle damage and inability to move. One form of PSSM is in part a result of a single base pair substitution in GYS1 gene, thereby changing the amino acid sequence of the glycogen synthase enzyme. At least 20 breeds have been identified with Type 1 PSSM. The prevalence of the GYS1 mutation in Belgians is as much as 50% of and 8% of the Quarter Horse-related breeds.

Some horses make and store abnormal muscle glycogen and cannot tolerate dietary starches and sugars. Horses with PSSM can be maintained with low-starch and low-sugar rations and precise exercise protocols. In some horses symptoms may begin by 2 to 3 years of age while others can remain subclinical. Clinical signs can include skin twitching, stiffness, firm painful muscles, sweating, weakness, and reluctance to move with light exercise. Occasionally gait abnormalities, mild colic and muscle wasting may also occur. In may cases horse that have tested positive have had no history of 'tying-up' or other symptoms associated with PSSM.


Research conducted at Animal Genetics has identified several additional mutations associated with PSSM1. These DNA mutations form a haplotype that allow us to identify horses with PSSM1. Further ongoing research may provide us with a more comprehensive assay for PSSM and better determine the severity of the disorder in all horses.

An additional genetic mutation in RYR1 gene (MH) influences PSSM and can increase the severity of the symptoms of PSSM in Quarter Horses and related breeds. The combination of PSSM and MH genetic tests are available at Animal Genetics UK.

Not all cases of tying up are caused by the PSSM mutations currently being looked at. A horse that tests N/N for PSSM but exhibits signs of tying-up or muscle pain may be suffering from an another muscle disorder. Please contact us with any examples of this so we can include these animals in further studies. In many cases horses that test positive for these mutations will exhibit only minor problems or may never exhibit any noticeable problems at all. Again, please contact us and let us know so we can add these animals to our current research on PSSM.

Breeds Affected:

Animal Genetics UK offers DNA testing and detection for markers identifying horses that could suffer from PSSM in American Paint horses, Appaloosas, Draft Breeds, Quarter horses (horse with quarter horse blood) and Warmbloods. Animal Genetics is working on identifying the genetic link in other breeds of horses. Please contact us if you have any horses that may be suffering from the disorder.

Additional Tests For Horses:

Malignant Hyperthermia (MH)

Hyperkalemic Periodic Paralysis Disease (HYPP)

Hereditary Equine Regional Dermal Asthenia (HERDA)

Immune Mediated Myositis and MYH1 Myopathy (IMM)

Glycogen Branching Enzyme Deficiency (GBED)

Combination panel (PSSM, MH, HYPP, HERDA,IMM, GBED) 90.00

Test Results:

Animal Genetics UK offers DNA testing for Polysaccharide Storage Myopathy (PSSM). The genetic test verifies the presence of the dominant PSSM mutation and presents results as one of the following:

P1/P1 Affected Positive for the dominant PSSM gene mutation, indicates the animal carries two inherited copies. Homozygous PSSM horses are genetically bound to pass the gene to 100% of their progeny when bred meaning all foals will be have at least one copy of the dominant PSSM gene mutation.
n/P1 Affected Both the normal and PSSM alleles were detected. Horse tested heterozygous for PSSM. The horse is affected with the PSSM genetic disorder and there is a 50% chance that this horse will pass a PSSM allele to its offspring.
n/n Clear Horse tested negative for PSSM and does not carry the PSSM gene mutation. The horse will not pass on the defective gene to its offspring.

    Submit a Sample for Testing:

    To submit a sample for testing please click on ORDER and download a sample submission form. Then follow the sample collection and submission instructions.


    Cost per sample is . Please see our fee schedules below for bulk and combination rates.

    Equine Fee Schedule English Pounds