You may hear your veterinarian refer to the overproduction of red blood cells in cats as "polycythemia." In polycythemia, the blood becomes thick and viscous due to the saturation of red blood cells in the plasma. This thickened blood does not flow easily through the smaller veins and capillaries in the body, resulting in poor oxygen delivery to tissues. Left undiagnosed and untreated, polycythemia can lead to severe coronary symptoms, even heart failure.

The function of red blood cells is to deliver oxygen to the body's tissues. Red blood cells are produced in your cat's bone marrow when a hormone, erythropoietin, signals there is too little oxygen in the blood. It is very common for cats to have too few red blood cells, resulting in anemia. What is less common the opposite condition, the overproduction of red blood cells in cats.

Since red blood cells impact all systems of a cat's body, the symptoms of polycythemia can manifest seemingly random and unrelated symptoms. The symptoms of polycythemia in cats include:

The symptoms of polycythemia progress gradually and may be difficult to notice for months. It is important you consult your veterinarian any time your cat is exhibiting unexplained physical or behavioral changes. 

Polycythemia vera, or "true" polycythemia, arises spontaneously in cats. The cause of this primary form of overproduction of red blood cells in cats is not well understood at this time. 

Polycythemia can also arise secondary to a preexisting condition, such as:

If none of these secondary conditions is present, it is likely the cause of overproduction of red blood cells is polycythemia vera. 

Your veterinarian will likely begin the diagnostic process with a complete physical exam and collection of a comprehensive history to better understand the cat’s symptoms. Your veterinarian's examination will include mucous membranes, which may appear red. He may also palpate your cat's torso to estimate capillary refill time, or the time it takes blood to fill the capillaries. 

Your veterinarian will also collect urine and blood samples from your cat. Examination of the blood sample will show elevated red blood cells, packed cell volume (PCV) and hemoglobin levels in cats with overproduction of red blood cells. The urine sample is typically used to ascertain whether polycythemia has begun to affect other organ systems or to rule out other diagnoses. 

Since the overproduction of red blood cells in cats often impacts the heart and circulatory system, you can expect your veterinarian to thoroughly examine your cat's heart. This may include auscultation of the heart and thoracic cavity, x-rays and other imaging techniques or further blood tests. 

Treatment of polycythemia progresses in two stages. First, red blood cells will be removed directly from the cat's bloodstream treat the acute overproduction of red blood cells. Second, the cause of polycythemia discovered by your doctor will be addressed, or, in the case of polycythemia vera, preventative treatment may be prescribed. 

Phlebotomy is the removal of blood from the body. Veterinarians perform phlebotomy, replacing any fluid lost to prevent dehydration or hypotension, on cats with polycythemia in an effort to thin the blood and improve circulation. Depending on the viscosity of the blood, 5 to 20 ml of blood will be removed for each kilogram of the cat's weight. The cat's PCV (packed cell volume) level is monitored throughout the procedure to determine when the goal red blood cell density has been reached, typically below 50% of whole blood. Iron supplements may be administered at the time of phlebotomy or prescribed for home use to prevent anemia.

Hydroxyurea, a drug that suppresses the production of red blood cells, will also be prescribed for cats suffering from polycythemia. A high dose is prescribed initially, tapering off until remission. 

If polycythemia is accompanied by another diagnosis, such as cancer, the precipitating condition will also be addressed. In the case of a tumor, for example, resection may be appropriate.

Recovery and management for cats with polycythemia may include additional phlebotomy, iron, and fluid replacement and hydroxyurea administration. Unfortunately, hydroxyurea has serious side effects, such as vomiting, poor appetite, diarrhea, mouth sores, and urinary tract infections. While some cats may only need hydroxyurea for months or years, others may require the medication for life. 

These cats will also require regular follow-up appointments with a veterinarian. Initially, follow-up appointments may be frequent until the condition is appropriately managed. A complete blood count will be required for all cats undergoing phlebotomy or hydroxyurea treatment every three months to prevent anemia and calibrate medications dosages.