http://cal.vet.upenn.edu/projects/eqairway/index.htm

In several studies of poor performance in horses, the four most common systems implicated included the Upper Respiratory Tract (URT), along with the musculoskeletal, cardiac, and Lower Respiratory Tract (LRT).The URT system has been implicated in 47-49% of cases affecting poor performance, followed by the cardiac system in 22.3% of cases; and the musculoskeletal system in 15-20% of cases.