You’ve probably heard horror stories like these: A breeding stallion completes servicing a mare, slides off, goes down to the ground, gasps a few times, and dies. A 20-year-old pony being ridden by his young owner staggers sideways, falls to his knees, and dies moments later. The trail horse starts lowering his head, stumbles a bit, and drops dead. The Belgian draft horse is found dead in the pasture. The racehorse fatally collapses during a gallop two days after a race.

These tales aren’t hypothetical, but the real thing. They are a few of the case histories seen by Bill Johnson, DVM, Diplomate ACVP (American College of Veterinary Pathologists), associate professor, clinical diagnostic pathology, University of California, Davis. Necropsy proved that all of these horses were killed by the same cause: an aortic rupture