Geneticist Dr. Molly McCue at the University of Minnesota, in conjunction with Dr. James Mickelson and Dr. Stephanie Valberg of the Comparative Neuromuscular Diagnostic Laboratory, have announced the availability of a genetic test for equine polysaccharide storage myopathy (PSSM/EPSM).
They have uncovered a genetic mutation that accounts for over 90% of cases of PPSM/EPSM across all breeds. The mutation involves an enzyme involved in the manufacturing of glycogen, the storage form of glucose in muscle cells. A second mutation that they are calling the modifying gene has been identified in Quarter Horses and Quarter-Horse-related breeds. A Quarter Horse positive for both mutations will have more severe signs of the disease.
The PSSM/EPSM mutation is believed to be a conserved ancient mutation that has its origins prior to the evolution of the modern horse. It’s a dominant gene, which means that if either the mare or the sire carry at least one copy of the gene, there is a 50% chance of getting an affected foal. Horses carrying two copies of the gene will have more severe signs and will pass the gene on to all of their offspring.
Testing can be done on either blood or hair. Cost of testing is $65. For further details, call the Equine Center at 612-625-6700.
Equine Polysaccharide Storage Myopathy is a muscular condition first diagnosed in draft breeds but has since been found in many others and is particularly common in the Quarter Horses where selective breeding for heavy muscling may have helped concentrate the gene. Symptoms vary widely, including muscle fasiculations (twitching), stiff or hard muscles, muscle pain (may be localized, e.g. back), to tying up of various degrees or progressive muscle weakness (usually drafts).
Bottom Line: Until now, biopsy was the only way to accurately diagnose the disease and many horses were assumed to have PSSM/EPSM without proper diagnostics because of owner reluctance to do muscle biopsy. The availability of this test should greatly reduce misdiagnosis of this condition in all horses.