A thrombus is a blood clot that may form when the blood flow of an artery or vein is restricted. It frequently causes obstruction to blood vessels at its site of origin. The clot can be classified based on its location and the syndrome it produces. Examples of this condition in horses include venous thrombosis (a blood clot in a vein often associated with having a catheter inserted into it for a long period of time) and pulmonary arterial thrombosis (a blood clot in the pulmonary artery). All or part of a clot may break off and be carried through the bloodstream as an embolus that lodges someplace else at a point of narrowing. Blockage of a blood vessel can also occur when foreign material (for example, bacteria, air, or fat) is carried into the bloodstream. Some clots are infected. Blood clots generally result in an inadequate supply of blood reaching nearby tissues.

A clot in the jugular vein usually resolves with appropriate therapy.

An aneurysm is an enlargement of a blood vessel caused by weakening of the middle layer of the blood vessel. Disruption of the tissue layer lining the inside of the blood vessels associated with an aneurysm can cause formation of a blood clot with subsequent blockage of a blood vessel by the clot.

Signs and Diagnosis

A sudden onset of difficulty breathing is often associated with a clot in the lungs. Infective clots in the heart are associated with endocarditis (see Heart and Blood Vessel Disorders of Horses: Infective Endocarditis). Clots in the heart that are not infective are associated with myocardial (heart muscle) disease. Tissue death due to loss of blood supply in the kidneys or reproductive system can produce blood in the urine or abdominal pain. Blockage of blood vessels to other organs of the abdomen may cause similar signs.

In horses, cranial vena cava thrombosis may result from blockage of blood vessels due to a clot in a jugular vein or extension of an inflamed lining of the right atrium. Jugular vein thrombosis in horses often follows catheterization or an injection and will cause swelling, heat, and pain of the affected area with thickening of the jugular vein. Blockage of both jugular veins by clots can cause fluid accumulation and swelling of the head and neck due to the difficulty of blood returning to the heart. Ultrasonographic examination of the affected vein can determine the extent of the clot and degree of obstruction. Horses with colitis (inflammation of the colon) and other gastrointestinal disorders are at increased risk for developing jugular thrombosis.

The larvae of the worm Strongylus vulgaris migrate through the horse's arteries and can cause inflammation of the walls of an artery. This can develop into blood clots and aneurysms in horses infested by these parasites. Blockage or death of intestinal tissue can occur. Signs are those of colic, constipation, or diarrhea. The colic usually keeps returning, and attacks may be severe and prolonged. With the recent introduction of newer drugs that destroy or flush out parasitic worms and improved treatment plans, this is becoming an uncommon disorder.

Blood clots of the aorta and the iliac arteries (with or without aneurysm) produce a characteristic syndrome in horses. Affected horses appear normal at rest; however, exercise results in weakness of the hind limbs with lameness on one or both sides, muscle tremors, and sweating. Severely affected horses may show signs of exercise intolerance, weakness, and an unusual lameness that resolves after a short rest. In severe cases, the hindquarter muscles weaken and waste away, and lameness may occur after only mild exercise. Severe paralysis of both hind limbs and an inability to rise may also occur. Affected horses are anxious, appear painful, and rapidly go into shock. The hind limbs are cold, and no pulse is felt in either iliac artery. Ultrasonographic examinations can be helpful in evaluating blood flow in the aorta and iliac arteries.

Treatment

Embolic pneumonia caused by endocarditis is treated with longterm antibiotics (given for several weeks) and, in some cases, anti-inflammatory drugs and drugs to reduce fever. The outlook for recovery is guarded at best, and the performance of recovered horses is often decreased.

Treatment of blood clots in veins is usually limited to supportive care, including hydrotherapy of accessible veins, anti-inflammatory drugs, and injectable antibiotics to control infection. Surgical removal of jugular veins that have been blocked by blood clots has been performed successfully in horses, but unless both veins are severely affected, the inflammation will resolve with appropriate medication. Thrombosis of the cranial or caudal vena cava generally does not respond to treatment and the outlook is poor.

In horses, aneurysms due to Strongylus vulgaris rarely rupture; the chief concern is blood clots within the intestines that can cause colic. Generally, removal of the clot is impractical, as another would likely form. Antibiotics and drugs used to kill the migrating larvae are of considerable value. The best approach to cranial mesenteric and aorticiliac thrombosis in horses is prevention and control of strongyles (see Digestive Disorders of Horses: Large Strongyles).

Last full review/revision July 2011 by Davin Borde, DVM, DACVIM; Benjamin J. Darien, DVM, MS, DACVIM; Ase Risberg, VMD