Complete atrioventricular septal defect in Thoroughbred foals: Echocardiographic and postmortem findings.

Complete atrioventricular septal defect (AVSD) is rarely reported in horses. We retrospectively reviewed three Thoroughbred foals (2017-2023) with necropsy-confirmed complete AVSD and summarized their clinical presentation, transthoracic echocardiographic findings, and postmortem anatomy. All foals showed early signs of congenital cardiac disease including weakness, failure to thrive and cardiac murmurs. Echocardiography demonstrated a common atrioventricular junction with a single atrioventricular valve and combined atrial and ventricular septal defects; all were classified as Rastelli type C. Postmortem examination confirmed a common atrioventricular valve with atrial and ventricular septal components in each foal. Despite variable age at presentation, all foals had marked cardiovascular compromise and prognosis was poor. Complete AVSD should be considered in foals with early-onset debilitation or murmurs, and transthoracic echocardiography provides a reliable antemortem diagnosis.