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.