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Morgan standing in a foggy field

AMHA to Sponsor PSSM1 Study

May 16, 2022


The Registry Committee and the AMHA Board of Directors, during their recent meetings, approved sponsoring a study to determine the prevalence of Type 1 Polysaccharide Storage Myopathy (PSSM1) in Morgan horses. The Veterinary Genetics Laboratory (VGL ) at U. C. Davis will screen a random set of 300 samples from within the last 10 years. The identity of the horses will be anonymous and the lab will not provide individual genotypes.

PSSM1 is a muscle disease characterized by accumulation of abnormal complex sugars (glycogen) in skeletal muscles. The accumulation of abnormal sugars can cause breakdown of muscle fibers (rhabdomyolysis) which leads to muscle pain, weakness, skin twitching, sweating, and reluctance to move.

The last study of the prevalence of PSSM1 in Morgan horses was done more than 10 years ago. That study indicated that the prevalence of PSSM1 in Morgans was two out of 214 samples or 0.9 percent.

The VGL informed AMHA that they have recently screened 27 Morgan horses for PSSM1 and all were negative (N/N). This was not enough for the VGL or the Registry Committee to make an informed recommendation regarding breeders’ concerns of PSSM1 in Morgan horses, and the Director of VGL proposed that AMHA sponsor a larger study in order to establish more comprehensive prevalence data. 

This study is expected to take a month. VGL does not offer PSSM2 testing as it has not been scientifically validated yet.