Sudden death in a Thoroughbred stallion: cardiac tamponade due to transverse aortic rupture with bone metaplasia and calcification.

A 17-year-old Thoroughbred stallion died suddenly after grazing. Necropsy revealed massive hemorrhage in the pericardial sac, suggesting cardiac tamponade. Aortic perforation was observed at the aortic origin, and hemorrhage was observed in and around the epicardium. Superficial observation of the aortic lumen revealed a transverse tear of the aortic wall at the bases of the right semilunar valve and septal semilunar valve. Fibro-osseous changes with calcification were histopathologically observed at the site of the rupture. A nodular goiter was observed in the left thyroid gland. The horse did not engage in strenuous exercise or activity likely to significantly elevate blood pressure during grazing. These results suggest that sudden death was caused by cardiac tamponade due to bone metaplasia and transverse aortic rupture with calcification, potentially triggered by a mild increase in blood pressure associated with normal activity during grazing.