An accessory bypass tract masked by the presence of atrial fibrillation in a horse.

Accessory bypass tracts are rarely documented in horses. Here, we present a case of an accessory bypass tract which was initially masked by the presence of atrial fibrillation. Evidence of ventricular pre-excitation was recognized after cardioversion to normal sinus rhythm and the horse was diagnosed with Wolff-Parkinson-White Syndrome. In people, atrial fibrillation in the presence of an accessory bypass tract is considered dangerous due to the risk of sudden cardiac death. Although we did not consider this horse safe to ride, he continues to compete successfully and has not had recurrence of clinically significant tachyarrhythmias.