Blood thickening typically occurs in older cats and can give vague symptoms that are often seen in other conditions. Because of this, it's important for cat owners to be aware of their cat's normal habits and seek help when abnormal symptoms or habits occur.

Polycythemia vera, or erythrocytosis, is a condition that causes the blood to thicken due to an abnormal increase in the concentration or number of red blood cells in the cat's body. This increase normally occurs in the cat's bone marrow where the blood cells are produced. There are several different types of the condition. Correctly identifying the specific type of polycythemia vera in the cat is essential in proper treatment.

Symptoms typically develop slowly over time and become chronic.

There are three main types of polycythemia vera, which can be broken down further into subtypes.

Though it's not always possible to identify the cause of polycythemia vera in cats, accurately describing the cat's symptoms can help the veterinarian determine the cause. This is because each type of polycythemia displays slightly different symptoms and each is attributed to a different cause.

The veterinarian will ask the cat's health history. This will include the cat's specific symptoms, when the symptoms first began, and if symptoms approve upon drinking water. The veterinarian will run several labs, which include a urinalysis, complete blood count and a biochemistry profile. The complete blood count will show an increase in red blood cells and may show an increase of white blood cells.

An echocardiogram is an ultrasound of the heart and will be done in order to look for any abnormalities in the heart muscle. Other ultrasounds and x-rays, such as of the kidneys and lungs, may also be performed to rule out other causes and determine the type of polycythemia. The veterinarian may also take a small sample of the blood marrow, or a bone marrow biopsy, and send it off to an outside lab for further evaluation.

Treatment of polycythemia vera depends on the type that the cat has been diagnosed with having.

Intravenous fluids will be given to cats who are experiencing blood thickening due to dehydration or blood loss. The cat's organs, such as the kidneys and heart, will be monitored during fluid therapy in order to ensure they are responding well to the increase in fluids.

In this procedure, the veterinarian will remove some of the blood from the cat's body through one of the central veins. To prevent a severe drop in blood pressure while the blood is removed, saline will simultaneously be administered to the cat. The procedure may need to be done more than one time. In some cases, the application of leeches has been used in order to remove excess blood from the cat's body, allowing it to thin slowly and preventing a sudden change in blood pressure.

Cats with primary absolute polycythemia will be prescribed hydroxyurea, an antineoplastic medication which prevents the blood marrow from forming excess red blood cells. 

In secondary absolute polycythemia, the primary condition must be treated in order to allow the blood to thin to its proper concentration. Therapeutic phlebotomy may be performed until the primary condition is under control.

With proper treatment and care, most cats live with polycythemia vera for many years. It's important to follow-up with the veterinarian for continued treatment and evaluation of the cat's condition. Because hydroxyurea is a highly toxic medication, continued labs will need to be performed to evaluate its effect on the cat's body. Make sure that the cat stays properly hydrated and doesn't exercise to the point of exhaustion. Any change in symptoms should be reported promptly to the veterinarian to prevent the disorder from becoming fatal.