Skeletal muscle disease in Quarter Horses takes many forms, including both type 1 (PSSM1) and type 2 (PSSM2) polysaccharide storage myopathy. These conditions lead to accumulation of abnormal polysaccharide in the muscle. PSSM1 has a genetic component (GYS1). However, PSSM2 in Quarter Horses has been defined as a histopathological description rather than as an etiology.
A retrospective study characterized the histopathological and biochemical features of PSSM2 associated with exertional rhabdomyolysis in Quarter Horses. Additional evaluation looked at responses of PSSM2-QH to diet and exercise. [Valberg SJ, Williams ZJ, Finno CJ, et al. Type 2 polysaccharide storage myopathy in Quarter Horses is a novel glycogen storage disease causing exertional rhabdomyolysis. Equine Veterinary Journal Aug 2022; doi.org/10.1111/evj.13876.]
The study pulled cases of Quarter Horse PSSM2 from records at Michigan State University from December 1993 and November 2021. Horses were Quarter Horse, Paint or Appaloosa at least two years old. The horses had a history of rhabdomyolysis, were negative for GYS1 mutation (that is associated with PSSM1) and were diagnosed with moderate polysaccharide aggregations in thigh or gluteal muscle biopsy samples. Control horses for the study were at least two years old, had no history of myopathy, were negative for GYS1 mutation, and had no abnormal histopathology in gluteal muscle biopsies.
PSSM2 Study Participants
The study included 64 PSSM2, 30 PSSM1, and 185 controls. All were Quarter Horse mean age ranged between 8-9 years old. Of the 64 PSSM2 horses, 55 had experienced exertional rhabdomyolysis. Two had tied up in pasture, and seven had both exertional and non-exertional rhabdomyolysis. The horses demonstrated such clinical signs as low-grade lameness, stiffness, fasciculations, and/or muscle atrophy.
The study initially contacted 31 owners, but only 19 responded to follow-up. Of these, 15 horse owners (79%) complied with the dietary recommendations. Fourteen exercised their horse daily or turned out if no exercise. When owners followed both recommendations, 13 horses had no additional episodes of rhabdomyolysis, and 12 of the 13 returned to previous performance. One horse switched from barrel racing to pleasure riding to prevent rhabdomyolysis. The owners felt that exercise was very instrumental in managing PSSM2-QH. The optimal diet that helped with management relied on low non-structural carbohydrates with fat supplementation.
Six horses did not receive the diet and exercise regimes. Of these, two were euthanized due to poor performance or uncontrollable behavior when ridden. One had navicular issues, that precluded exercise. One needed dantrolene prior to exercise and was exercised regularly. Another one was donated to a rescue, and one hadn’t returned to work following an injury.
The study evaluated pedigrees in 30 PSSM2 Quarter Horses. Three 3-6 generation families descended from three sires deemed desirable in barrel racing and working cow horse/roping. PSSM2-QH is prevalent in 45% of barrel racing horses and 21% of working cow horses. This suggests (but does not definitively identify) that there is a distinct familial basis for PSSM2-QH. This is especially relevant considering that origins of the Quarter Horse breed in 1940 stemmed from a small number of stallions. The study did not identify an X-linked inheritance pattern since distribution between male and female horses was equivocal.
The authors conclude, “PSSM2-QH appears to be a novel glycogenosis potentially caused by variants in genes not currently associated with skeletal muscle glycogen storage disease or in genes that either alter the regulation of glycogen-related genes or cause post-translational modification of their product. Alternatively, there could be multiple genes that together contribute to PSSM2-QH.” The histopathology of PSSM2-QH is less severe than PSSM1-QH and the polysaccharide is more sensitive to amylase digestion and has less filamentous material than that of PSSM1-QH.
Conclusions about PSSM2 in Quarter Horses
In conclusion, there is as yet no genetic test for diagnosing PSSM2-QH, but rather, diagnosis relies on histopathological characteristics of muscle biopsies. That said, the specific appearance of polysaccharide in biopsy samples may degrade during shipping and may not be present in all PSSM2-QH cases. It is best to ship samples well-chilled to the lab and/or fix them in formalin to preserve glycogen during shipping in order to achieve the best opportunity for a definitive diagnosis.