Familial basis of exertional rhabdomyolysis in Quarter Horse-related breeds

Objectives - To trace pedigrees from affected horses, identify likely contributing founder horses, and determine the conditional probability of founder genotypes. Design - Muscle biopsy records from the Neuromuscular Disease Laboratory at the University of California-Davis and the University of Minnesota were searched to identify horses with a polysaccharide storage myopathy and exercise intolerance/rhabdomyolysis. Pedigrees containing 5 to 6 generations were obtained where possible. Animals - 13 Quarter Horses, 4 American Paint Horses, 3 Appaloosas, and 3 Quarter Horse crossbreds (16 mares, 4 geldings, and 3 stallions) were identified with polysaccharide storage myopathy. Pedigrees were available for 18 horses. Procedure - Inbreeding coefficients, founder contributions, and conditional probability of founder genotypes were calculated. Results - Three stallions (A, B, and C) were featured prominently in the pedigrees Stallions A and B descended from a common sire. On average, A contributed 8 8% (range, 0 to 23%) of the genes in affected horses, B contributed 4.2% (range, 0 to 14%), and C contributed 3.0% (range, 0 to 14%). The sire and dam of 4 horses were descendants of stallion A, the sire and dam of 1 horse were descendants of stallion B. and the sire and dam of 11 horses were descendants of a combination of stallions A and B. The pattern of inheritance resembled an autosomal recessive disorder. Assuming this pattern of inheritance, the conditional probability that these founders were carriers or recessive for the trait was > 99.29% for stallions A and B and 92% for stallion C. Conclusions - Results support a familial basis for polysaccharide storage myopathy and associated exertional rhabdomyolysis in Quarter Horse-related breeds. The strong contribution of particular founder stallions to the gene pool in some lines of Quarter Horses may explain the high incidence of exertional rhabdomyolysis in these horses.