Two types of PSSM have been identified: PSSM type 1 (PSSM1) and PSSM type 2 (PSSM2). Recently, genetic research has suggested that there are several variants that fit under the header "PSSM2".


PSSM type 1 was identified in the early 2000's and a genetic test has been on the market since 2008 . It has been found in many breeds, among which Quarter Horses, Paint Horses, (American) Appaloosa's, Morgans, draft horses, Tinkers/Gypsy horses and warmblood breeds which carry bloodlines that have been influenced by cold blooded horses. PSSM type 2 is quite widespread and even though the highest incidence so far has been in the western breeds (Quarters, Paints and Appaloosa's) this is mainly due to where the genetic research was initiated that identified the different genetic variants within the scope of PSSM type 2. Most of these variants can be traced back to thoroughbred lines, which means most warmblood breeds will most likely -and have found to be- affected.


When PSSM is suspected, a good place to start is to do the genetic test for type 1. This inexpensive test is easy to perform by sending in some hair with roots intact to a lab. If this test comes back negative and PSSM is still suspected, the horse owner and/or vet can proceed with a biopsy or a genetic test for the different variants of PSSM type 2. 

PSSM type 1, which is sometimes also referred to as PSSM1, is a sugar storage problem in muscles that is seen in horse breeds such as Quarter Horses, Paint Horses, Haflingers, Tinkers, Morgans, draft horses and other breeds from cold blooded ancestry.

Horses that have PSSM type 1 have a mutation of the GYS1 gene, which is responsible for the storage of the glycogen that is present in skeletal muscles. In horses that have PSSM type 1, the enzyme that regulates this storage is hyperactive, causing abnormal glycogen accumulation in muscles and a disrupted energy release, causing horses to tie up when they exercise. The exact dynamics of this process are still being researched.

PSSM type 2, also referred to as PSSM2 is a genetically determined muscle wasting problem that occurs in in a wide variety of horse breeds.

Horses that have PSSM type 2 do not have the mutation that is specific for PSSM type 1.