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Polysaccharide Storage Myopathy Type 1 - PSSM1 - Equigerminal

Polysaccharide Storage Myopathy Type 1 - PSSM1

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DNA test
  • DNA test for the Polysaccharide Storage Myopathy Type 1 (PSSM1). This test verifies the presence of the affected allele at GYS1 locus responsible for PSSM1.


    • 5 mL - blood - K3 EDTA tube. 
    • Collect the blood and send the tube together with the  printable sample submission form.

    Turnaround time

    • Standard processing - Results in 5 working days after sample arrival at the laboratory. Clients organise and support the costs of sending the samples to the laboratory.


    • PREMIUM processing** - Results in 1-2 days after sample arrival. Includes free express delivery** . The laboratory organises Express shipping with pick-up of the package at the client's address and delivery at the laboratory. 


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    For remote/outreach regions EXTRA fees are applied. 


    Why test?

    • This genetic test helps breeders to identify horses that carrying the PSSM allele. Informed choices can be made for breeding selections, and prevent the born of affected foals. Although PSSM cannot be cured since it is a genetic disorder, muscle function can be restored with a change in diet and exercise routine.  Eliminating carbohydrates such as grains and sweet feed from the horse’s diet and adding extra fat seems to alleviate the condition. However, the disorder can lead to death if it’s not identified early enough to make the necessary dietary and exercise changes, so it is vital to identify the presence of PSSM as early as possible.
    • Confirmation of diagnostics: not all cases of tying up are caused by the PSSM mutations currently being looked at. A horse that tests (nn) for PSSM but exhibits signs of tying-up or muscle pain may be suffering from an another muscle disorder.
    • Horses with suspicious symptoms of the disease should also be tested.

    Results description

    The DNA test verifies the presence of the affected allele at GYS1 locus responsible for PSSM1 and the presents results as one of the following:  

    • n/n - Negative for PSSM1. Absence of the affected allele at GYS1 locus responsible for PSSM1.
    • n/P1 - Positive heterozygous for PSSM. Presence of a mutated allele at the GYS1 locus responsible for PSSM1.  The horse is affected with the PSSM1 and can pass the  PSSM1 allele to 50% of their progeny when bred.
    • P1/P1 - Positive homozygous for PSSM1. Presence of two mutated alleles at the GYS1 locus responsible for PSSM1.  The horse is affected with the PSSM1 and will pass the PSSM1 allele to 100% of its offspring.

    Additional information

    Equine 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. This mutation has also been identified in American Paint horses, Appaloosas, Draft Breeds and Warmbloods. 

    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.