Pet Factsheets


Polycythaemia (also called erythrocytosis) is a rare condition in cats where there are too many red blood cells in the body. The signs of disease are often vague so you might not recognise that there is a problem with your pet for some time. The problem might be diagnosed on a single blood sample but confirming the diagnosis can sometimes require repeated evaluation, and determining the underlying cause can be very challenging.

What is polycythaemia?

Polycythaemia means an increase in the number of red blood cells in the body. The increased number of cells makes the blood thicker and more likely to clot which can cause a variety of problems. Polycythaemia can be seen in animals which are severely dehydrated, but in this case there are not too many red blood cells but too little fluid in the blood. It occurs naturally in animals living at high altitude where there is less oxygen in the air so more red blood cells are needed to supply oxygen to the tissues. Other cases are caused by a disease of the bone marrow or elsewhere.

How would I know if my cat has polycythaemia?

The signs of polycythaemia can be very vague. Signs can be seen in cats of a variety of ages between 2 to 8 years old. Often the first sign observed by owners is increased drinking or urination. Your pet could also be more lethargic than usual, have episodes of weakness, or collapse unexpectedly. You may notice that the mucous membranes in your pet’s mouth or eyes are ‘brick red’ in colour instead of pink. As the disease progresses more serious signs such as seizures or blindness may develop, or you may notice spontaneous nose bleeds or blood in your pet’s urine or vomit. The blood in cats with polycythaemia is thicker than normal and this can cause clots to form which can cause signs of stroke or difficulty breathing.

What causes polycythaemia?

There are 2 types of polycythaemia. In primary polycythaemia there is a problem in the cells produced in the bone marrow that go on to become red cells. This is a form of leukaemia. In secondary polycythaemia there is a problem elsewhere in the body that results in excess red cell production.  

Red blood cells have a vital role in transporting oxygen from the lungs to all the tissues in the body. The production of red cells in the body is controlled by a hormone called erythropoietin (also known as EPO) which is released from the kidneys. In a normal animal when red blood cells are lost from the circulation EPO is released to stimulate the bone marrow to make more red blood cells. Once red blood cell levels have recovered EPO levels drop.  

In animals with chronic lung disease, or certain heart conditions, there may be insufficient oxygen getting to the tissues and the body interprets this as a lack of red blood cells and more EPO is released. This is an example of an appropriate response to the lack of oxygen, and a small increase in red cells may be helpful. However, although more red cells are produced this doesn’t help get more oxygen to the tissue as the underlying defect is not corrected, and so EPO levels remain high and more and more red cells are made.  

In some animals with kidney disease too much EPO is produced and some tumours may produce substances which act like EPO. In this case there is an inappropriate production of excess red cells when there is no need for them at all.

How would my vet know if my cat had polycythaemia?

Your vet is unlikely to suspect a diagnosis of polycythaemia from the history of your cat’s illness because it is an uncommon disease and the same signs can be caused by many other diseases. They will need to take blood samples which will show high levels of red blood cells. The hormone EPO can also be measured (although this test is not on routine blood screens). If EPO levels are low your cat is more likely to have a bone marrow problem and if they are high your vet will want to look for disease elsewhere. 

Your vet will probably want to do some other tests to see whether there is an underlying cause for the polycythaemia. These may include X-rays and an ultrasound of your cat’s kidneys and heart to check for disease. If any masses are identified or other abnormalities detected further, more specific, tests will be needed. 

If there are any other changes in the blood picture or abnormal cells are found your vet may need to take a sample of the bone marrow to see if there are any signs of leukaemia or other problems with cell development. 

Can polycythaemia be treated?

The treatment for polycythaemia depends on the underlying cause. If the underlying cause can be treated, eg a tumour is identified and can be removed, or a congenital heart defect is corrected, then the red cell numbers will reduce naturally. Animals with polycythaemia are at higher risk than healthy cats during surgery because of the risk of blood clots and poor blood supply to the tissues. For this reason the condition should be stabilised before surgery, usually by removal of some blood, a process known as phlebotomy. In phlebotomy a cannula is placed in a vein and an appropriate volume of blood is removed. The volume of blood removed is then replaced with intravenous fluids to dilute the remaining blood. The process will need to be carried out in the veterinary surgery but should only take a couple of hours. 

In other conditions, where the cause cannot be treated, eg some congenital cardiac diseases, the high levels of red cells can be reduced by regular phlebotomy to remove excess red blood cells. Initially this may have to be done every few weeks to get the disease under control but then it may be possible to keep the disease under control with phlebotomy only every 3 months. In some cases removal of blood can also be achieved by use of medical-grade leeches. 

There are some drugs that can be given as daily tablets to reduce the number of cells in the bone marrow that can produce red blood cells. The most commonly used of these is called hydroxyurea. However, these drugs must be given with care and your cat will need to be monitored while they are on them, because they can sometimes affect the production of other blood cells. Red blood cells can live for 4 months in the circulation so even when you start treatment to stop production of new red cells you have to wait a long time to see the effect of this. This is why phlebotomy is an important early treatment. 

If you are worried about any aspect of your cat’s health you should always visit your veterinary surgeon for advice. If polycythaemia is diagnosed and treated early many cats can live normal lives for many years with simple treatment.