Aorto-iliac thrombosis (AIT) is a progressive vascular disease which causes blockage of the blood vessels to the hindlimbs of the horse. The disease is characterized by hindlimb lameness following exercise that may also remain cool when the horse is sweating.

Horses may demonstrate severe discomfort during or after exercise by pawing and showing colic-like symptoms. These symptoms tend to pass within an hour. Horses that are forced to work through the pain develop a more severe lameness and may require significantly more time for the clinical signs to resolve.

Early treatment is essential. As the thrombus increases in size, reduced performance occurs progressively with less strenuous activity.