Also known as erythrocythemia, polycythemia (PV) is a rare blood disorder in which there is an abnormally high number of red blood cells in the blood.
The blood is composed of plasma (the clear fluid component of blood which contains antibodies, electrolytes and other proteins), red blood cells, white blood cells and platelets. Red blood cells are the most common type, amounting to 35-45% of whole blood, polycythemia is defined as a red blood cell count above 55%. Brachycephalic breeds such as Persians and Exotics, as well as cats who live at higher altitudes, may have a higher packed cell volume than other cats.
There are three classifications of polycythemia in cats:
The most common cause of polycythemia is most commonly due to dehydration. Red blood cell counts are normal, however, plasma levels are decreased resulting in an elevated packed cell volume (PCV). This test measures the volume of red blood cells compared to the fluid portion of the blood.
The spleen is an organ which in part is responsible for storing red blood cells, if the spleen contracts as a result of epinephrine secretion, a large number of red blood cells can be released into the blood stream. This usually occurs in dogs.
Also known as true polycythemia, absolute polycythemia results in an over production of red blood cells. It may be primary (polycythemia rubra vera) or secondary.
Primary polycythemia is due to overproduction of red blood cells in the bone marrow (known as myeloproliferative neoplasms).
Secondary may be due to an erythropoietin secreting tumours or inappropriate administration of erythropoietin (when treating anemia caused by kidney disease). Erythropoietin (EPO) is a hormone produced by the kidney which promotes the formation of red blood cells). Other causes of secondary polycythemia may include heart disease and lung disease which cause hypoxia (low levels of oxygen in the blood), which triggers an increase in EPO production.
The average age in cats with polycythemia is 8-9 years old, with male cats being commonly affected. Short-nosed breeds will often have a higher packed cell volume than other cats.
What are the symptoms of polycythemia in cats?
When all blood components are at normal levels, blood is able to flow freely blood vessels throughout the body. However, if red blood cell levels are higher than normal, the blood becomes thicker (known as hyperviscosity), making it more difficult to travel through the smaller blood vessels. This results in reduced oxygen levels reaching the tissues. Symptoms can vary depending on the underlying cause and the type of polycythemia.
Cats with polycythemia vera may not display symptoms until a high degree of hyperviscosity has been reached.
One of the most common symptoms of polycythemia is the development of neurological disorders such as seizures, ataxia (unsteady gait), behavioural changes and blindness
Other symptoms may include:
Hyperemic mucus membranes (dark/red brick colouration to the gums)
The spleen is an organ responsible for removing damaged or abnormal blood cells from the circulating blood. In cats with polycythemia, the spleen has to work harder, and over time can become enlarged (splenomegaly) in cats with primary polycythemia.
Relative polycythemia is most often due to dehydration which may be associated with prolonged vomiting, diarrhea, increased urination, sickness in which may have resulted in your cat taking in fewer fluids or lack of available drinking water.
Cats with secondary polycythemia due to kidney tumours, heart or lung disease may display additional symptoms related to these diseases.
How is polycythemia diagnosed?
You and your veterinarian may not be aware your cat has polycythemia until it is found during a routine complete count where the packed cell volume is greater than 55%.
Your veterinarian will perform a complete physical examination of your cat and obtain a medical history from you including medical conditions your cat has as well as any medications he is taking. Routine tests in addition to a complete blood count will include biochemical profile and urinalysis.
It will be necessary to determine what type of polycythemia your cat has, relative, transient or absolute.
Relative polycythemia will be picked up during routine urinalysis.
Absolute polycythemia will require additional testing which may include:
Serum EPO level
Arterial blood gas levels
Abdominal ultrasound, x-ray or contrast radiography to evaluate the kidneys for signs of kidney disease or tumours as well as evaluate the spleen for signs of enlargement
Thoracic x-rays or ultrasound to look for causes of hypoxic disease such as heart or lung disease
How is polycythemia treated?
Treatment depends on the type of polycythemia your cat has.
Relative polycythemia is treated with fluid therapy to correct dehydration
Addressing the underlying cause which may include:
Surgical removal of tumours.
Managing heart disease or lung disease.
Therapeutic phlebotomy (fle-bot-omy) involves the removal of between 10-20 mL per kg of your cat’s blood, this helps to decrease blood viscosity by removing excess red blood cells.
One cat was treated with leech therapy as hyperviscosity made it impossible to perform a phlebotomy, although it would appear this is an isolated case.
Cats with polycythemia are at increased risk of developing blood clots and in some cases a low dose of aspirin may be prescribed. As cats are not able to metabolise aspirin very well, close monitoring is necessary.
Iron supplements may be necessary for some cats who may develop low iron levels due to phlebotomy.
Where an underlying cause can not be treated and phlebotomy is not enough to control the red blood cell counts, hydroxyurea (Hydrea) may be prescribed. This medication slows down the production of red blood cells in the bone marrow. Side effects may include thrombocytopenia (decreased platelet count), leukopenia (decreased white blood cell count) due myelosuppression, anemia, vomiting, diarrhea and loss of appetite.
Many cats will require lifelong monitoring of PCV and phlebotomies every 6-8 weeks. Some cats may be able to be weaned off hydroxyurea but others may need to stay on the drug for life, however, the dose will be tapered to the lowest possible maintenance level.
Only administer medications prescribed by your veterinarian.
The prognosis for cats with absolute polycythemia is guarded, although some cats have lived for several years after diagnosis.
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