When bleeding occurs in an organ or body part, a process is set in motion to stop the bleeding. This is called hemostasis. In order to work, hemostasis requires an adequate number of platelets, the right amount of blood clotting proteins (often referred to as factors), and blood vessels that constrict properly. When an injury occurs, the wall of the blood vessel breaks. A normally responsive blood vessel will narrow so that blood flows more slowly, allowing the clotting process to begin. Platelets also rush to the broken wall where certain proteins change the platelets' shape from round to spiny so that they can stick to blood cells, the broken vessel wall, and to each other. Other proteins form long strands called fibrin. These fibrin strands form a net that traps and helps hold together the platelets and blood cells, creating a clot that plugs the break in the vessel wall. After the clot has formed and stabilized, other proteins stop the clotting process and eventually dissolve the clot.
Bleeding disorders may be present at birth (congenital) or occur later. Defects in blood clotting proteins usually show up as delayed bleeding and bruising deep in tissues, while platelet defects usually show up as superficial small bruises, nosebleeds, black stools caused by bleeding into the bowels, or prolonged bleeding at injection and surgery sites.
Bleeding disorders include coagulation protein disorders, platelet disorders, and vascular disorders. The ones that are most common in cats are described below (see Blood Disorders of Dogs: Bleeding Disorders of Dogs).
Congenital Clotting Protein Disorders
If clotting protein levels are too low, or if they do not work properly, signs appear at an early age. Severe defects are usually deadly. Animals may be stillborn or die shortly after birth. Insufficient production of clotting proteins may worsen a clotting defect. If activity of any particular clotting protein is 5 to 10% of normal, the newborn may survive, but signs usually appear before 6 months of age. It is during this time, when numerous routine procedures (for example, vaccination, castration, or spaying) are usually done, that a bleeding tendency may be noticed by your veterinarian.
Hemophilia A (Factor VIII deficiency) is the most common inherited bleeding disorder in cats. Affected cats may show prolonged bleeding after surgery or injury but rarely bleed spontaneously, probably because of their agility and light weight. The diagnosis is harder to confirm in animals less than 6 months old because their livers may not yet have produced enough of the clotting proteins. Treatment requires repeated transfusions of whole blood or plasma until bleeding has been controlled.
Hemophilia B (Factor IX deficiency) is diagnosed less often in cats than hemophilia A. It has been reported in Himalayan cats, a family of Siamese-cross cats, and a family of British Shorthaired cats. Signs are similar to those of animals with Factor VIII deficiency. Animals with extremely low Factor IX activity (less than 1%) usually die at birth or shortly thereafter. Animals with 5 to 10% of normal Factor IX activity may suddenly develop blood clots, bleeding in the joints, oozing of blood into the body cavity, or organ bleeding. Gum bleeding during teething or prolonged bleeding after surgery can occur. Some animals have no symptoms until injury or surgery. Treatment requires transfusion with fresh or fresh-frozen plasma. Often, internal bleeding into the abdomen, chest, central nervous system, or muscles occurs, and may not be noticed until a crisis happens.
Acquired Clotting Protein Disorders
Most clotting proteins are produced in the liver. Therefore, liver disease can lead to decreased production of clotting proteins, particularly Factors VII, IX, X, and XI. Small to large decreases in clotting proteins can result after severe liver disease.
Cats that eat rat poison may have blood clotting problems because the poison reduces the liver's production of clotting proteins (see Poisoning: Rodenticide Poisoning). Affected animals may have blood clots and bruises in multiple tissues. If you suspect your cat has eaten any type of rat or mouse poison, this is an emergency and an immediate trip to your veterinarian is recommended.
Disseminated intravascular coagulation (DIC) is a condition in which small blood clots develop throughout the bloodstream, blocking small blood vessels and destroying the platelets and clotting factors needed to control bleeding. It usually develops after numerous triggering events such as severe infections, heat stroke, burns, tumors, or severe injury. In many cases, the signs are uncontrolled bleeding and the inability to form a normal clot. Death is caused by extensive blood clots or collapse of circulation, leading to the failure of one or several organs. If the animal survives this crisis, a chronic form of DIC can occur. Your veterinarian will determine and attempt to correct the underlying problem. Intravenous fluids are extremely important for maintaining normal circulation. DIC is a very serious disorder and is often fatal.
Disorders of platelets include having too few platelets (thrombocytopenia) or having platelets that do not work properly. Each type of disorder can be either congenital (present at birth) or acquired later in life. Thrombocytosis is having too many platelets in response to a physiologic or disease process or, rarely, as a component of cancer.
Acquired thrombocytopenias are reported frequently in cats and dogs and rarely in other species. Numerous causes have been identified; in most cases the immune system destroys platelets.
Thrombocytopenia caused by drugs occurs in cats. Some drugs and classes of drugs (including estrogen and some antibiotics) suppress platelet production in the bone marrow. Other drugs (including aspirin, acetaminophen, penicillin, and others) destroy platelets circulating in the bloodstream. Drug reactions are rare and unpredictable. Platelets usually return to normal shortly after the drug is discontinued. Drug-induced bone marrow suppression may last longer, however. If your cat is taking one of these drugs, your veterinarian will likely monitor the blood count to check for any serious reductions in the number of platelets.
Cats are inquisitive and investigate the world with their tongues. They may lick or eat birth control pills, aspirin, pain pills, or other medications. This is another example of why medications should be carefully stored. This simple precaution can save your pet's life.
Congenital Platelet Function Disorders
Cats with Chédiak-Higashi syndrome (see Blood Disorders of Cats: Chédiak-Higashi Syndrome) have abnormal white blood cells, melanocytes (pigment-producing skin cells), and platelets. The defect in melanocytes causes a pale coat color. White blood cells do not work properly, and platelets do not clump together or separate as they normally should when blood clots are formed or broken down. Prolonged bleeding in blue smoke Persian cats occurs after surgery or when a blood sample is taken.
Von Willebrand's disease is caused by a defective or deficient von Willebrand's factor (also called Factor VIII-related antigen). It is more common in dogs; however, it has also been reported in cats.
Acquired Platelet Function Disorders
Several diseases have been associated with acquired platelet function disorders. In 2 studies of cats with lowered numbers of platelets circulating in their blood, up to half had infectious diseases, including feline leukemia, feline infectious peritonitis, feline distemper (panleukopenia), or toxoplasmosis (a disease caused by the protozoan parasite Toxoplasma gondii). Acquired defects of the immune system, longterm kidney disease, liver disease, and certain drugs can also cause disorders in platelet function. Your veterinarian can diagnose these disorders with blood tests and other tools.
Blood Vessel Disorders
Ehlers-Danlos syndrome, also known as cutaneous asthenia, is caused by a defect (present at birth) in the development of connective tissue proteins found in skin. This causes weak structural support of blood vessels and can result in blood clot formation and easy bruising. The disorder has been reported in cats but is rare. The most striking sign is loose skin that stretches to a greater than normal degree and tears easily. There is no treatment.
Blood Clotting Disorders
Certain diseases in animals have been associated with increased risk of blood clots. Cats with cardiomyopathy (a disease of the heart muscle) can form blood clots in large arteries. Injury to the wall of the heart and turbulent blood flow through the heart chambers and valves caused by poor functioning of the heart are thought to start blood clot formation. Cats with heart disease caused by high levels of thyroid hormone are often given drugs that lessen the signs of heart problems. These drugs appear to protect against increased risk of blood clots by changing how likely the platelets are to form a clot. Your veterinarian can prescribe the most appropriate medication for your cat.
Last full review/revision July 2011 by Peter H. Holmes, BVMS, PhD, Dr HC, FRCVS, FRSE, OBE; David J. Waltisbuhl, BASc, MSc; Michael Bernstein, DVM, DACVIM; Karen L. Campbell, MS, DVM, DACVIM, DACVD; Nemi C. Jain, MVSc, PhD; Wayne K. Jorgensen, BSc, PhD; Sarah E. Payne, DVM, DACVIM