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Blood Clots and Aneurysms in Cats

By Davin Borde, DVM, DACVIM, Staff Cardiologist, Veterinary Heart Institute
Jorge Guerrero, DVM, PhD, DEVPC (Ret), Adjunct Professor of Parasitology, Department of Pathobiology, School of Veterinary Medicine, University of Pennsylvania
Michelle Wall, DVM, DACVIM,
Clay A. Calvert, DVM, DACVIM, Professor, Department of Small Animal Medicine and Surgery, College of Veterinary Medicine, University of Georgia
Benjamin J. Darien, DVM, MS, DACVIM, Associate Professor of Internal Medicine, School of Veterinary Medicine, University of Wisconsin

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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. 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. Blood clots generally result in not enough blood reaching tissues supplied by the blocked blood vessel. Some clots are infected and can spread bacteria and cause localized infection.

A thrombus (blood clot) may form when the blood flow of an artery or vein is restricted.

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

Signs and Diagnosis

A sudden onset of breathing difficulty may be a sign of a blood clot in the lungs, and some cats may cough up blood or bloody mucus. Infective clots in the heart are associated with endocarditis, an inflammation of the membranes lining the heart cavity (see Acquired Heart and Blood Vessel Disorders in Cats : Infective Endocarditis). Clots in the heart that are not infective are associated with heart muscle disease. Blood in the urine or abdominal pain can indicate blockage of certain blood vessels.

Heartworm disease may lead to blood clots in arteries of the lungs. Blood clots in the pulmonary artery most commonly produce difficulty breathing and an increased breathing rate. Affected cats often seem normal until they have a sudden onset of respiratory distress. Chest x-rays may show changes such as an enlarged main pulmonary artery and right heart, not enough blood getting to the affected region, an accumulation of fluid in the chest cavity, or bleeding or tissue death within the lungs. Additional tests are essential for the diagnosis of underlying diseases.

Blood clots associated with the aorta are a frequent complication of cardiomyopathy (see Acquired Heart and Blood Vessel Disorders in Cats : Disorders of the Heart Muscle (Cardiomyopathy)). Clots may be located in the left atrium, ventricle, or both. Clots that dislodge form emboli, which may obstruct the aorta where it branches. This happens most commonly where the aorta splits into 3 branches near the hips. Signs include paralysis and pain of the back legs, cold limbs, and signs related to congestive heart failure. Incomplete blockage of the aortic branches may cause mild neurologic signs in both hind limbs or muscle weakness in only one.

Aneurysms cause no signs unless bleeding occurs or an associated clot develops. Spontaneous bleeding from aneurysms is rare, and signs usually relate to blood clots. Ultrasonography and angiography may be helpful in confirming a diagnosis.


Treatment of pneumonia caused by a blood clot due to endocarditis includes longterm antibiotics, a treatment program lasting several weeks. Some cases require anti-inflammatory drugs to reduce fever. The outlook for recovery is guarded at best.

Treatment of blood clots in veins is usually limited to supportive care, including hydrotherapy of accessible veins, anti-inflammatory drugs, and antibiotics to control secondary infection. Thrombosis of the large veins that empty into the right atrium generally does not respond to treatment and the outlook is poor.

Surgical removal of clots in the aorta may be attempted, but most authorities recommend medical treatment only, including analgesics, anticoagulants, careful use of intravenous fluids (to maintain hydration and blood pressure but not worsen congestive heart failure), and specific treatment for underlying heart disease.

Many cats with aortic thromboembolism die despite treatment or fail to regain hind limb function. Some cats that survive the initial cardiovascular crisis recover the ability to walk after several weeks, but may have some permanent damage (such as abnormal tightening or shortening of muscle and a degenerative disorder affecting the nervous system). The longterm outlook often depends on the severity of underlying heart disease.

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