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. 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.
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.
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, an inflammation of the membranes lining the heart cavity (see Heart and Blood Vessel Disorders of Dogs: 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 (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. 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.
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), and cancer.
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 given by mouth or injection 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.
Last full review/revision July 2011 by Davin Borde, DVM, DACVIM; Clay A. Calvert, DVM, DACVIM; Benjamin J. Darien, DVM, MS, DACVIM; Jorge Guerrero, DVM, PhD, DEVPC (Ret); Michelle Wall, DVM, DACVIM