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

Until recently, PSSM type 2 could only be diagnosed by muscle biopsy taken from skeletal muscle in the hamstring (semimembranosis) and this is still the official diagnostic method. However, over the past few years, research has revealed a number of genes that are at least in part responsible for symptoms associated with PSSM type 2. 

USA based company EquiSeq has  identified a number of genes that are considered to be responsible for PSSM type 2, which introduces the possibility of genetic testing. EquiSeq's findings have not been published for peer review yet, which means that their tests are not considered academically validated. It does, however, offer the horse owner the possibility to test their equines on the presence of these genes without having to perform a biopsy. Experiments in the management of horses that  carry one or more of the genes that were discovered by EquiSeq have brought on a considerable improvement in the horses' quality of life, which is why we have decided to include this option on this website. 

Dr Valberg, who also discovered the gene responsible for PSSM type 1, has published her findings on the discovery of MYH1, a gene responsible for a rapid onset of muscle wastage along the topline in Quarter Horses and related breeds that has been termed Immune Mediated Myositis (IMM). The test for this gene is available through UC Davis in the USA and via Laboklin and Center for Animal Genetics in Germany.

Events that cause a negative nitrogen balance, such as a viral infection or an injury that required stitching, seem to trigger symptoms of PSSM type 2 or make them worse.