Study on the Prevalence in PSSM1 in Morgan Horses Completed
This summer, UC Davis Veterinary Genetics Laboratory (VGL), at the request of the AMHA Registry, completed a study on PSSM1 in Morgan horses. This study was graciously sponsored by a grant from the USA Equestrian Trust. Based on VGL records, prior to this study only 27 Morgan horses were tested for this variant, and all were negative (R.R. Bellone, personal communication). Previously published research, conducted by others in 2010, suggested that the allele frequency was low, with 2 out of 214 horses having the PSSM1 variant (McCue et al, 2010). To estimate the current frequency, VGL randomly selected 300 individuals and tested them for the PSSM1 variant. This was an anonymized study, thus the Registry does not know which horses were tested or the specific results for each horse.
The summary of the VGL report included the testing of samples from 15 male and 15 female Morgans with foaling years between 2012-2021, which were genotyped for PSSM1. Nine samples failed to produce high quality results. In the 291 remaining samples, no homozygotes for PSSM1 were detected. 2 out of 291 samples had one copy of the PSSM1 variant. Based on these data the allele frequency was estimated to be 0.35% in the population (95% confidence interval is 0.01 to 1.33%, similar to a study done in 2010). Using the estimate from this study, if 1750 horses are registered annually, 12 are expected to be heterozygous of PSSM1. Only 1 in 100,000 horses in the Morgan population would be expected to be homozygous. Homozygous horses are thought to be more severely affected as compared to those with one copy of PSSM1.
The VGL did recommend genetic testing but given the low frequency suggested that mandatory testing was not necessary. The VGL also recommended that as a breed we continue to monitor the frequency of the mutation and adjust recommendations as necessary. Genetic testing is recommended for horses who have symptoms of disease or if there is a known affected individual in the line. Genetic testing can help to confirm clinical diagnosis and results should be shared with clinicians to develop a diet and exercise regimen to best manage symptoms.
After discussion during the September Board of Directors and Registry Committee meetings, and under advice from the Registry’s legal counsel, the Committee moved to not offer PSSM1 testing through AMHA nor allow the use of DNA on file with AMHA for individual requests made privately through VGL.
What is PSSM1?
Polysaccharide storage myopathy (PSSM1) is characterized by the abnormal accumulation of the normal form of sugar stored in muscle (glycogen) as well as an abnormal form of sugar (amylase-resistant polysaccharide) in muscle tissue. By definition horses with PSSM1 have a distinctive genetic mutation in the gene.
Horses with PSSM1 can have signs typically associated with tying-up. These signs are most commonly muscle stiffness, sweating, and reluctance to move in conjunction with increased serum creatine kinase (CK) activity. The signs are most often seen in horses when they are put into initial training or after a lay-up period when they receive little active turn-out. Episodes usually begin after very light exercise such as 10-20 minutes of walking and trotting. Horses with PSSM1 can exhibit symptoms without exercise.
