WHAT IS PSSM?

 

PSSM is the abbreviation for PolySaccharide Storage Myopathy, a muscle disease (myopathy) involving the abnormal storage of sugars (saccharides) in muscle cells. PSSM is sometimes referred to as EPSM or EPSSM, in which “E” stands for “Equine”. A horse with PSSM has abnormal accumulation of glycogen; the type of sugar stored in muscles. This leads to cramping or weakening of the horse’s muscles, which can lead to a multitude of symptoms, among which stiffness, muscle pain and tying-up (see: symptoms for a more in-depth description).

PSSM type 1 is caused by a genetic mutation of the glycogen synthase 1 (GYS1) gene. Horses that test positive for one or more copies of this gene “have” PSSM type 1/PSSM1.

 

The discovery of the gene causing PSSM1 stems from ongoing research into muscle related disease initiated by the American Quarter Horse Association (AQHA) in 1995. Because of this origin, it has long been thought that PSSM is specific to the Quarter Horse and related breeds. However, further research has shown that the gene that is responsible for PSSM type 1 was  introduced into the AQHA gene pool by crossing coldblooded horses into the breed. Since we are dealing with a very old gene that is very common in coldblooded horses, it should not be a surprise that the disease occurs in other breeds with coldblood influences, such as Haflingers, Tinkers or Irish Cobs, not excluding certain warmblood lines.

 

During research into PSSM type 1 it was found that a number of horses that showed PSSM symptoms tested negative for the gene responsible for PSSM type 1. These horses were labelled as having PSSM type 2, or PSSM2, but the cause for the disease was not known. Much research has been done into the discovery of the genes responsible for PSSM type 2 since and it turns out there are several different mechanisms that can cause similar muscle related symptoms. EquiSeq, a company based in the USA, has identified 6 genes that according to their research cause muscle integrity issues. Originally, these were categorised as PSSM type 2, but they have since been relabelled Muscle Integrity Myopathy (MIM), which more accurately describes the underlying cause. Professor Valberg, who, together with her team, was responsible for discovering the gene responsible for PSSM type 1, has since found a gene that is responsible for yet another muscle disease that has been labelled Immune Mediated Myositis or IMM. See “PSSM types” for more information.

 

PSSM2/MIM as defined by EquiSeq is mostly seen in Thoroughbreds, Arabians and breeds that are derived from those breeds, including most warmbloods and the Iberian breeds (PRE, Lusitano).

IMM is a muscle disease specific to Quarter Horses and related breeds. Especially cutting and cowhorse bloodlines seem to be affected, which also affects reining bloodlines.