Q.I read about horses that have died of apparent “heart attacks” during competitions, breeding, and even out in the field. But I have had veterinarians tell me there is no such thing as a horse having a heart attack. What are people talking about when they report a horse has died of a heart attack?

—Molly, via e-mail

A.In general, most horses that collapse and die suddenly are the victims of an aortic aneurysm, which is a weakening in the wall of the aorta—the major vessel leading from the heart. This area of weakened wall can rupture and the horse then bleeds out internally. In other words, there are no external signs of bleeding, except that the mucous membranes of the mouth, nose, eyes, rectum, and genitalia become very pale.

People can develop a similar weakening of the wall of major vessels, and surgical intervention to strengthen the area is usually indicated if the aneurysm is diagnosed prior to its rupture. So far, we have been unsuccessful in developing successful ways to either diagnose or treat these aneurysms in the horse