Merck Manual

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Pet Owner Version

Blood Clots and Aneurysms in Dogs


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 Jun 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 dog to produce clots excessively. They can form in the heart or within blood vessels. 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. Some clots are infected. Life-threatening blood clots are most commonly encountered in animals with underlying diseases that affect the blood’s ability to clot. If left untreated or uncontrolled, these conditions can result in a tendency to bleed and/or a life-threatening condition in which small blood clots develop throughout the bloodstream, blocking small blood vessels and depleting the platelets and clotting factors needed to control bleeding.

Blood clot

Blood clot

Thrombus formation can occur in both large and small arteries and veins. Blood clots generally result in an inadequate supply of blood reaching nearby tissues. In addition, pus-filled clots can spread of bacteria and localized infection. Blood clots can affect the heart or the central nervous system.

An aneurysm is an enlargement of a blood vessel caused by weakening of the middle layer of the blood vessel. Disruption of the inner layer 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 dogs and cause no signs unless abnormal bleeding or clotting occurs.

Signs and Diagnosis

A sudden onset of breathing difficulty may be a sign of a blood clot in the lungs, and some dogs may cough up blood or bloody mucus. Infective clots in the heart are associated with endocarditis Infective Endocarditis Dogs can have a variety of cardiovascular diseases. The most common ones are discussed below. Degenerative valve disease is the most common heart disease in dogs and accounts for about 75% of... 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 (within the genital or urinary systems, for example) or the loss of blood supply caused by clots. Blockage of blood vessels to abdominal organs may cause similar signs, although dogs may vomit or be unable to control their bladder and bowels.

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 dogs often seem normal until they have a sudden onset of respiratory distress. The changes may be seen on x‑rays of the chest. Additional tests are essential for the diagnosis of underlying diseases.

Bacterial infection of the lining of the heart cavity can lead to blood clots in the lungs and pneumonia. Other diseases associated with blood clots in the lungs include diabetes mellitus, kidney disease, increased adrenal gland hormones (hyperadrenocorticism), immune-mediated hemolytic anemia (a disorder in which the dog’s immune system destroys its own red blood cells), protein-losing diseases of the intestines or kidneys, and cancer. Medical devices placed within blood vessels, such as pacemakers or catheters, can also lead to the formation of blood clots.


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

Your veterinarian will use medications to treat blood clots in the arteries of the body or embolisms in the vessels of the lungs. In addition, dogs with immune-mediated hemolytic anemia are often prescribed medication to prevent the formation of clots.

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