Familial basis of exertional rhabdomyolysis in quarter horse-related breeds.

This research investigated the genetic foundation of exertional rhabdomyolysis, a muscle disease, in horses related to the Quarter Horse breed. It found a strong possibility that particular founding stallions in Quarter Horse pedigrees significantly contribute to the high incidence of this disease, supporting the idea that the condition has a familial or genetic basis.

Research Methodology

• The researchers started by determining horses affected with a condition known as polysaccharide storage myopathy (PSSM), characterized by muscle weakness and exercise intolerance, which is often linked to exertional rhabdomyolysis.
• The team analyzed muscle biopsy records from the Neuromuscular Disease Laboratory at the University of California-Davis and the University of Minnesota, identifying 23 horses related to the Quarter Horse breed, who suffered from PSSM.
• Where possible, the researchers obtained pedigrees of these horses, reaching back to five or six generations.
• The team then calculated inbreeding coefficients, founder contributions, and conditional probability of founder genotypes to identify patterns and establish a potential familial link.

Research Findings

• The scientists identified three stallions (referred to as A, B, and C) that were commonly present in the pedigrees of the examined horses. Stallions A and B shared a common sire (or father).
• On average, Stallion A contributed to 8.8% of the genes in affected horses, B contributed 4.2%, and C contributed 3.0%.
• The researchers also discovered that several of the horses had parents who were descendants of these stallions. Specifically, the sire and dam of four horses were descendants of stallion A, one horse had parents descending from stallion B, while in 11 horses, the parents were descendants of a combination of Stallions A and B.
• The researchers noted that the observed pattern of inheritance resembled that of an autosomal recessive disorder, assuming which, the likelihood that these founder stallions were carriers or recessive for the trait was estimated at >99.29% for Stallions A and B and 92% for Stallion C.

Conclusions

• The study’s findings support the idea that PSSM and associated exertional rhabdomyolysis in Quarter Horse-related breeds have a familial or genetic basis.
• The significant genetic contribution of certain founding stallions to some lines of Quarter Horses, as the researchers discovered, could explain the high incidence of exertional rhabdomyolysis in these horses.