PSSM1 also known as EPSM or EPSSM stands for Polysaccharide Storage Myopathy- type 1; it is a hereditary Glycogen storage condition.

What Is Glycogen?

Glycogen is part of a long chain of Glucose carbohydrate molecules also known as Polysaccharides.

What Is PSSM1?
“Atomic-View-Of-Glycogen-Molecule”

What Does Glycogen Do?

Glycogen serves as a form of energy for animals, fungi and bacteria.

Where Is Glycogen Stored In Horses?

In the muscles and the liver

A muscle biopsy showing normal glycogen storage in the muscle of a horse

How Does Glycogen Work In Normal Horses?

Glycogen is used as a fuel and burned off during exercise. It takes a normal horses 2 to 3 times longer to replenish glycogen in their muscles compared to that of a human.

What Causes PSSM1?

PSSM1 is caused by a mutation in the GYS1 gene. The GYS1 gene is a semi dominant allele that is hereditary a horse only needs one copy of the mutated GYS1 gene to pass it on and be affected.

What Happens When The GYS1 Gene Is Mutated?

The mutation in the GYS1 gene causes these horses to store to much glycogen, meaning their bodies never switch it off so they are constantly producing and storing to much glycogen in their muscles.

What Is PSSM1?
A muscle biopsy from a horse with the GYS1 gene mutation that causes PSSM1, notice how much darker it is, and some areas are completely dark this is because the muscle is storing excess glycogen

What Are The Symptoms Of PSSM1?

Symptoms can occur both under saddle, and not under saddle. A horse may display 1 or more of these symptoms, symptoms include:

Symptoms Of PSSM1
My own horse Tex showing PSSM symptoms of being tucked/sucked up, muscle loss, depression, and standing under himself

What Breeds Are Affected By PSSM1?

ANY BREED can be affected with PSSM1 some breeds it has been detected in so far are:

  • All types of Cob
  • Warmbloods
  • American Quarter horses
  • American Paint horses
  • Heavy horses (Percherons, Comtois etc…)
  • Native ponies (New forest, Dales ponies etc…)

How Can I Test For PSSM1?

  • Hair strand test this is the most simplest and cheapest method, it cost’s less than a routine vaccination!
  • Blood test
  • Muscle biopsy

Where Can I Do A Hair Strand Test In Europe?

Animal Genetics
CAG
Laboklin
Combibreed

Where Can I Do A Hair Strand Test In The USA?

Animal Genetics

Can PSSM1 Be Managed?

Yes these horses need to have a diet of the lowest sugar and starch possible, with high fats and regular exercise PSSM1 horses can lead a long, healthy, and active life. We will be doing a follow up blog on PSSM1 management at a later date.

For more information on PSSM1 check out our YouTube video by clicking this link