Heritability of recurrent exertional rhabdomyolysis in Thoroughbred racehorses

Objective-To determine the likely mode of inheritance and identify probable foundation horses for recurrent exertional rhabdomyolysis (RER) in Thoroug hbred (TB) racehorses. Animals-4 families of TB racehorses with a high prevalence of RER, consisti ng of 3 to 53 horses/family, were used to determine mode of inheritance. Si xty-two TB horses with RER and 34 control TB racehorses without RER were us ed to identify probable foundation horses for the RER trait. Procedure-RER was diagnosed by a veterinarian and verified by detecting hig h serum creatine kinase activity. Pedigrees dating from 1930 for all horses were entered into a database. Pedigrees of horses in 4 families were visua lly inspected for a pattern of inheritance and used for calculation of foun dation horse contributions and inbreeding coefficients. The Markov chain Mo nte Carte technique was used to analyze pedigrees of the 62 affected and 34 control horses for the conditional probability of foundation genotypes. A dominant mode of inheritance with variable expression model was used. Results-Pedigree analysis supported an autosomal dominant mode of inheritan ce with variable expression. All affected horses from the 4 families shared a common ancestor. This ancestor and 5 other stallions had a conditional p robability of 1.00 for being affected. All 6 stallions shared a common male ancestor within 3 to 5 generations. Conclusions and Clinical Relevance-On the basis of this study, the RER trai t has been in TB racehorses for more than 70 years and may be inherited as an autosomal dominant trait with variable expression.