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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 (Type1) 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. Several other mutations have also been identified as possibly being associated with some form of PSSM. 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. Symptoms usually begin by 2 to 3 years of age. Some horses are 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. 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. Not all cases of tying up are caused by the PSSM mutation. 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. In many cases horses that test positive for the mutations will exhibit only minor problems or may never exhibit any noticeable problems at all.

PSSM Testing

Animal Genetics 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.



$40.00 US per sample.

Additional Tests For Quarter Horses:
Malignant Hyperthermia (MH)
Hyperkalemic Periodic Paralysis Disease (HYPP)
Hereditary Equine Regional Dermal Asthenia (HERDA)
Polysaccharide Storage Myopathy (PSSM)
Glycogen Branching Enzyme Deficiency (GBED)
Combination panel (PSSM, MH, HYPP, HERDA, GBED) $95.00


Sample Collection

Collect sample by pulling (not cutting) 20-30 mane or tail hairs with roots attached. It is important that you pull the hairs and confirm that the actual root of the hair is being collected. The root contains the genetic material of your horse that is needed for DNA testing. Therefore, cut hairs do not provide an adequate sample of your horse. Place the collected hairs of each horse in a separate zip-lock bag labeling the bags accordingly with the horses name or identification number. Download and complete a submission form for each sample and send along with payment to Animal Genetics for testing.


Results are given using the following symbolic notation:

The horse carries two copies of PSSM1 and is homozygous for PSSM1. The horse is affected with the PSSM genetic disorder and this horse will always pass PSSM1 to its offspring.
Both the normal and PSSM1 alleles were detected. Horse tested heterozygous for PSSM1. The horse is affected with the PSSM genetic disorder and there is a 50% chance this horse will pass a PSSM1 allele to its offspring.
Horse tested negative for PSSM1.




Animal Genetics, Inc.
1336 Timberlane Rd - Tallahassee, FL 32312
Toll Free: 866-922-6436

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