Breed predisposition and heritability of atrial fibrillation in the Standardbred horse: a retrospective case-control study.

OBJECTIVES: To assess evidence for genetic contributions to atrial fibrillation (AF) in the Standardbred horse. ANIMALS: Equine referrals to the Ontario Veterinary College Health Sciences Centre (OVCHSC) for 1985-2009, and age and gait matched breed registry controls. METHODS: Breeds presenting ≥ 5 times annually were tabulated (admission year and diagnosis; total 40,039; AF 396; no AF 39,643), and breed and year effects examined. Heritability and inbreeding coefficients were determined for Standardbred AF cases and racing contemporaries, and odds ratios for AF were calculated for frequently occurring sires. RESULTS: Year and breed effects on diagnosis were highly significant (Chi-Square 212.85, p < 0.0001, and 304.25, p < 0.0001, respectively). Year effect on diagnosis by breed was significant from 1997, and due to Standardbred admissions each year. Quarterhorses were significantly less likely to present with AF (OR 0.0578-0.6048), Standardbreds were more likely (OR 4.3874-10.9006). Heritability of AF on the underlying scale (h²(u)) was estimated at 29.6 ± 3.9% and on the observed binomial scale (h²(o)), at 9.6%. For horses born in 1994 or later, h²(u) was 31.1 ± 4.3% and h²(o), 10.1%. Of 22 first generation sires appearing ≥ 10 times in the case/control file, seven pacing and one trotting sire produced affected horses more frequently than expected (OR 2.66-66.32). Inbreeding was not a factor. CONCLUSIONS: There is genetic liability to AF in Standardbred horses, likely due to more than single genes with simple Mendelian inheritance. Genomic studies are required.