Estimates of heritability of atrial fibrillation in the Standardbred racehorse

Summary Background The number of Standardbred racehorses admitted to the Ontario Veterinary College Teaching Hospital (Guelph, Canada) for treatment of atrial fibrillation (AF) has been on the rise since the early 1990s. A small number of sires have been contributing to a large proportion of cases, indicating there may be a genetic predisposition to the arrhythmia in this breed. Objectives The objectives of this study were to determine the heritability of AF in Standardbred horses and whether heritability of the arrhythmia differs across gaits and/or sexes. Study design Heritability study based on retrospective review of clinical records and publicly available pedigree and racing records. Methods Standardbred horses admitted to hospital for treatment of AF that were born between 1978 and 2007 comprised the affected case population (n = 204). Five randomly selected racing contemporaries for each case, assumed to not suffer from the arrhythmia, comprised the control population (n = 1017). Racing contemporaries were identified by examining the race records of affected horses within the 6 months prior to their admission, and randomly selecting sex‐ and gait‐matched horses from these races. Heritability was estimated from the sampled horses as a whole (n = 1221), as well as for both sexes and gaits, using a generalised linear mixed model. Results Heritability of AF on the underlying liability scale was estimated to be (±s.e.) 0.30±0.04 in the entire data set; 0.30±0.06 in males; 0.24±0.08 in females; and 0.32±0.05 in pacers. After conversion to the observed scale, heritability estimates were 0.14, 0.15, 0.09 and 0.15, respectively. Main limitations There were insufficient data to estimate heritability of AF for trotters. Conclusions Modest heritability estimates were found for AF in the Standardbred horse, particularly in males and pacers, which support the hypothesis that there is a genetic contribution to the arrhythmia in this breed. The Summary is available in Chinese – See Supporting Information.