Polycythemia Vera

By Tammy Hunter, DVM; Ryan Llera, BSc, DVM; Robin Downing, DVM, CVPP, DAAPM; David Kerr, DVM

What is polycythemia vera, and what are the symptoms?

Polycythemia vera, or true polycythemia, is a rare disease of dogs and cats in which the bone marrow produces too many red blood cells (RBCs). This is the opposite of anemia, in which there are too few red blood cells.

Blood is composed of cells (RBCs, white blood cells, and platelets) and fluid (plasma). Normally, the RBCs account for 35% to 55% of the blood volume (except for greyhounds, whose RBCs account for 50% to 65% of blood volume). White blood cells (WBCs) account for 45% and plasma accounts for 65%. In these proportions, blood flows easily through arteries, veins, and capillaries to all parts of the body.

In dogs and cats with polycythemia vera, red blood cells may comprise 65% to 75% of the total blood volume. When this happens, blood becomes very thick and has difficulty moving through the small blood vessels in the body. Slower blood flow means fewer nutrients and less oxygen delivered to the tissues. The muscles and brain require the most nutrients and oxygen, so animals with thicker blood from polycythemia feel tired, sluggish, and weak, and may even have seizures.

If left untreated, polycythemia vera can affect the heart. This disease can develop slowly over many months if it is a primary disease, so the symptoms (clinical signs) appear gradually and may be easy to overlook in the early stages. Some cases of polycythemia are secondary, or related to another disease.

What causes polycythemia?

Polycythemia can commonly be seen with conditions such as severe dehydration, vomiting, diarrhea, and hyperventilation. There are several secondary causes of polycythemia, including congenital heart disease, tumors in the kidney, hydronephrosis (fluid swelling of the kidney), hyperadrenocorticism (Cushing's disease), various types of organ tumors, and certain types of bone marrow cancer. Following a diagnosis of polycythemia, these secondary causes are explored and treated.

If secondary causes of polycythemia are absent, then the diagnosis is polycythemia vera (“true” or primary polycythemia). The cause of this primary disease remains a mystery.

Is there a treatment for polycythemia vera?

Treatment is aimed at reducing red blood cell numbers. This thins the blood, making it easier for nutrients and oxygen to be transported throughout the body. Better oxygenation and tissue nutrition help reduce the tiredness and weakness often associated with the disease. Two treatment techniques are used, generally in conjunction with one another, to reduce the number of circulating RBCs in an animal with polycythemia: (1) removing some of the blood and (2) administering medication to slow down production of RBCs in the bone marrow.

Because the medication used to reduce red blood cell production in the bone marrow takes time to produce an effect, the fastest way to reduce the number of circulating RBCs is to physically remove them through a procedure called phlebotomy

Phlebotomy involves placement of an intravenous (IV) catheter, through which a calculated volume of blood is removed. This procedure is similar to that used when people donate blood. The body quickly replaces the fluid removed with the red blood cells, usually within six hours. Occasionally, the procedure must be repeated until the desired level of RBCs is reached.

Phlebotomy requires the pet to be admitted to the hospital for a number of hours. As the blood is removed, intravenous fluids are replaced with a balanced solution to help dilute the blood cell concentration.

What is the medication used to treat polycythemia vera?

Hydroxyurea is the medication used in conjunction with phlebotomy to treat polycythemia vera. It works by slowing the bone marrow's production of red blood cells. Because RBCs live an average of 120 days, it takes time to see the effects of decreased production, which is why phlebotomy is also part of the treatment. Once phlebotomy has been performed and the pet is feeling better, hydroxyurea is started. The medication is initially given at a fairly high dose.

A complete blood count (CBC) is evaluated weekly for the first month, then monthly for three months, then every three months to check the bone marrow's response to therapy. Over time, as the red blood cell numbers decrease, the amount of hydroxyurea and the frequency of administration are reduced. Some pets can be weaned off the medication after one to two years, although other pets need to stay on the medication for life.

Are there side effects associated with hydroxyurea?

Hydroxyurea can cause vomiting, loss of appetite, and diarrhea. Rarely, it can cause hair loss, sores in the mouth, brittle toenails, and a predisposition to urinary tract infection. In addition to suppressing red blood cell production, hydroxyurea can occasionally suppress white blood cell production, so medication's effect on the body must be closely monitored.

Hydroxyurea should be handled with care. To avoid contact with your skin, consider wearing disposable gloves when administering the medication, and always thoroughly wash your hands afterward.

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