Objective: To determine prevalence of atrial fibrillation (AF) immediately after racing among racehorses that finished well behind the winners and examine potential risk factors for AF in these horses.
Design: Case-control study.
Animals: 39,302 racehorses representing 404,090 race starts in races sanctioned by the Japan Racing Association between 1988 and 1997.
Procedure: Horses that finished > or = 4 (turf races) or 5 (dirt races) seconds behind the winner or that did not complete the race were examined for AF within 5 minutes after the race. Logistic regression and chi2 analyses were used to determine whether sex, age, race distance, race surface, year, or development of epistaxis was associated with development of AF.
Results: Estimated minimum frequency of AF was 0.03% (123 instances of AF following 404,090 race starts), and estimated minimum prevalence of AF among racehorses was 0.29% (115 horses with AF among 39,302 racehorses). Estimated frequency of AF among horses that finished slowly or did not finish was 1.39% (120 instances of AF among 8,639 examinations), and estimated prevalence of AF in horses that finished slowly was 1.23% (92 instances of AF among 7,500 horses) or 1.01% when only the first time a horse finished slowly was considered (76 instances of AF among 7,500 horses). Atrial fibrillation was paroxysmal in most horses. Among horses that finished slowly, 4-year-old and older horses and horses that raced on turf were more likely to develop AF.
Conclusions and clinical relevance: Results suggest that the likelihood of AF among racehorses that finish slowly is related to age and racing surface.