A thrombus is a blood clot that may form when the blood flow of an artery or vein is restricted, when the lining of a blood vessel is damaged, or when another condition causes a horse to produce clots excessively. They can form in the heart or within blood vessels and may further obstruct blood flow. A 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). In horses, blood clots are most likely to form within veins. Serious blood clots are most commonly encountered in animals with underlying diseases that affect the blood’s ability to clot. 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 and can seed infection to other parts of the body. Blood clots generally result in an inadequate supply of blood reaching nearby tissues. Blood clots within the blood vessels of a limb can cause lameness and gangrene in horses. This has been seen because of conditions that cause excessive clotting in adults and bacterial blood infections (septicemia) in foals.
An aneurysm is an enlargement of a blood vessel caused by weakening of the middle layer of the blood vessel. They are rare in horses. 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. Aneurysms cause no signs unless they rupture and bleed or they develop a thrombus. A rupture of an aneurysm in the aorta at a place called the sinus of Valsalva usually leads to sudden death and is most commonly seen in stallions during breeding. A rupture can also occur in horses with fungal infections in the guttural pouch.
A sudden onset of difficulty breathing is often associated with a clot in the lungs. Infective clots in the heart are associated with 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 (such as the spleen) may also cause abdominal pain.
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. A blood clot within the cranial vena cava can cause similar signs. 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 with a complete blockage. 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.
The treatment for thrombosis depends on the cause. Endocarditis is treated with longterm antibiotics (given for several weeks to months) and, in some cases, anti-inflammatory drugs and drugs to reduce fever. The outlook for recovery is guarded at best, and the performance of horses that recover 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 secondary infections. 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 requires more prompt and thorough treatment, which could include drugs to break up clots as well as surgery followed by drugs to reduce clotting.
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 aortic-iliac thrombosis in horses is prevention and control of strongyles.
Also see professional content regarding blood clots and aneurysms in horses.