A thrombus is a clot in the cardiovascular systems formed from the constituents of blood. It may occlude the vessel or attach to the vessel without obstructing the lumen. In horses a saddle thrombus refers to a clot that completely or partially occludes the blood vessels that feed the rear legs resulting in pain, lameness, and weakness that may lead to complete paralysis. Reports of the prevalence of this condition are conflicting. One early report states that aortic-iliac thrombosis occurs in 1–2% of racing Thoroughbreds in South Africa. In a recent survey of 28 experienced equine veterinarians from all over the world only 44 cases had been recognized and subsequently confirmed ultrasound (1997). This article is about the clinical findings, diagnosis, treatment and prognosis of this disease.

The exact aetiology of aortic-iliac thrombosis is unknown and there are probably several causes:

• Mechanical damage to the termail aorta and iliacs arteries during locomotion.
• Turbulent blood flow at the aortic bifurcation may also predispose to intimal damage.
• Larval migration has not been shown to be a direct cause as the predilection site for S. vulgaris larval migration is the cranial mesenteric artery and larvae rarely migrate in the aortic intima as far caudally as the aortic quadrification or the iliac arteries. However, larval migration could indirectly cause aortic-iliac thrombosis by giving rise to thromboemboli, which subsequently organize and lead to arterial occlusion and further thrombosis.
• Vasculitis elsewhere predisposes to clotting abnormalities throughout the vasculature.