Quadricuspid aortic valve in a 16-year-old Quarter horse

•Quadricuspid aortic valve is a rare congenital heart defect.•Echocardiography is a useful tool for quadricuspid aortic valve diagnosis.•Prevalence of this congenital heart defect remains unknown in the horse. A 16-year-old horse, 400 kg, male, Quarter horse gelding was examined for a cardiac murmur that had not been previously heard. Physical examination revealed a regularly irregular pulse and a grade III/VI, decrescendo, diastolic murmur with a point of maximum intensity over the left heart base. Base-apex standard electrocardiographic examination at rest showed sinus rhythm with second-degree atrio-ventricular blocks. Echocardiography identified the presence of four aortic valve cusps, two equal larger and two unequal smaller cusps. Color flow Doppler examination showed a diastolic regurgitant jet emerging from the central region of closed aortic valve. Based on clinical and echocardiographic findings a diagnosis of mild to moderate aortic valve insufficiency secondary to quadricuspid aortic valve was made. This report describes a rare congenital heart defect that can be detected by transthoracic echocardiography in the horse.