Merck Manual

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Pet Owner Version

Blood Clots and Aneurysms in Cats


Suzanne M. Cunningham

, DVM, DACVIM-Cardiology, Department of Clinical Sciences, Cummings School of Veterinary Medicine, Tufts University;

Kursten V. Roderick

, DVM, Tufts University

Reviewed/Revised Jul 2018 | Modified Oct 2022
Topic Resources

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 cat to produce clots excessively. It can cause a partial or full obstruction to blood vessels at its site of origin. Blood clots can also form in the heart, especially in cats with enlargement of the left atrium. 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.



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. Aneurysms are rare in cats.

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, have trouble breathing, or die suddenly. Heartworm disease, protein-losing kidney or intestinal diseases, cancer, and immune-mediated hemolytic anemia (a disease in which red blood cells are destroyed by the immune system) are associated with blood clots in the lungs.

Blood clots that form in the heart (called cardiogenic embolism or arterial thromboembolism) are a frequent complication of cardiomyopathy Disorders of the Heart Muscle (Cardiomyopathy) Cats can develop many different cardiovascular diseases. The ones discussed below are the most common. This acquired disease is characterized by a thickening of the heart valves. Degenerative... read more , hyperthyroidism Hyperthyroidism The thyroid gland is a 2-lobed gland in the neck. It produces 2 iodine-containing hormones, T3 and T4, which affect many processes in the body. In general, the thyroid hormones regulate metabolic... read more , or other heart diseases in cats. Clots may be located in the left atrium, ventricle, or both. Clots that dislodge form emboli, which may obstruct the aorta where it branches (called a "saddle thrombus"). 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. Emboli from the heart can also block other important arteries in the body, including those that supply the kidneys, the right front limb, brain, heart, and other internal organs.

Infective clots in the heart are associated with endocarditis Infective Endocarditis Cats can develop many different cardiovascular diseases. The ones discussed below are the most common. This acquired disease is characterized by a thickening of the heart valves. Degenerative... read more , an inflammation of the membranes lining the heart cavity. 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 Heartworm Disease in Cats Heartworm disease is a potentially fatal, but preventable, infection caused by a worm parasite, Dirofilaria immitis. The organism is transmitted by mosquitoes, which carry the heartworm... read more 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.

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 for blood clots in the aorta usually involves pain medication and medications to reduce clotting. Although medications are available to break down clots, similar results can be seen by allowing the cat's body to break them down on its own. Surgical removal of clots in the aorta may be attempted.

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). Cats with only one limb affected do better than those with signs in both limbs. The longterm outlook often depends on the severity of underlying heart disease. Veterinarians may prescribe a medication that prevents the formation of blood clots in cats with heart disease.

Treatment of infections 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.

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