Types of PSSM

There are two types of PSSM:  type 1 (the original and actual PSSM) and PSSM type 2, also referred to as PSSM1 and PSSM2. The term “PSSM type 2” was introduced by the team responsible for the discovery of PSSM type 1 when they found that they had horses in their study that did display PSSM symptoms, but did not test positive for the GYS1 gene causing PSSM type 1. Research continued to find the genes responsible for these symptoms. America based company EquiSeq identified 6 variants which they have named P2, P3, P4, Px, P8 and K1. Please see “PSSM type 2 - MIM” for more information. Dr Valberg and her team, who discovered the gene responsible for causing PSSM type 1, went on to identify the MYH1 gene as the cause for Immune Mediated Myositis. Please see “PSSM type 2 – IMM/MYH” for more information.

PSSM type 1 originates from coldbloods and occurs in coldblood infused breeds, such as (but not limited to) Quarter Horses, Paint Horses, American Appaloosa’s, Tinkers, Gypsy Horses, Haflingers, Morgans and the occasional warmblood.

PSSM type 2/MIM can be traced back to the English and Arabian Thoroughbred and as a consequence occurs in many breeds such as (but not exclusively) Quarter Horses, Paint Horses, American Appaloosa’s, Arabians, Haflingers, warmbloods, Tinkers, Gypsy Horses, Thoroughbreds and Iberian Horses (PRE, Lusitano). The symptoms for type 1 and type 2 are comparable but not 100% the same. This is discussed in more detail under “symptoms”. 

IMM/MYH is specific to Quarter Horses and related breeds.

More on PSSM type 1
More on PSSM type 2