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Erythrocytosis and Polycythemia in Dogs


John F. Randolph

, DVM, DACVIM-SAIM, Department of Clinical Sciences, College of Veterinary Medicine, Cornell University

Reviewed/Revised Dec 2017 | Modified Oct 2022

Erythrocytosis is an increase in the amount of red blood cells in the bloodstream. The condition is also referred to as polycythemia, but polycythemia can also imply an increase in all types of blood cells (including white blood cells and platelets). Erythrocytosis can be relative, transient, or absolute.

Relative erythrocytosis occurs when a decrease in the volume of plasma (the liquid part of blood) results in an apparent increase in red blood cell numbers. Relative polycythemia can be caused by anything that causes fluid loss from the blood, such as dehydration from vomiting or diarrhea.

Transient erythrocytosis is a type of relative polycythemia that occurs when excitement or fear causes the spleen to contract, resulting in the release of large numbers of red blood cells into the circulation. Treatment consists of giving fluids to the animal and addressing the underlying cause.

Absolute erythrocytosis is a real increase in red blood cell numbers resulting from increased production. This can be due to a bone marrow disorder in the case of primary erythrocytosis (polycythemia vera) or caused by excessive release of hormones that stimulate red blood cell production (secondary erythrocytosis). Signs of the disorder include red mucous membranes, bleeding tendencies, the passing of large amounts of urine, excessive thirst, seizures or behavioral changes, lack of coordination, weakness, and blindness.

Diagnosis requires measuring the percentage of red blood cells in the blood, a simple test available in most veterinary clinics. Repeated tests should be normal in dogs with relative and transient erythrocytosis; however, Greyhounds and other Sighthounds have higher percentages of red blood cells compared to other breeds. Your veterinarian will use other blood and urine tests to distinguish between primary and secondary erythrocytosis. In some cases, x-rays and other tests may also be required.

Relative erythrocytosis may need to be treated with intravenous fluids and by treating the underlying condition. Treatment of polycythemia vera includes removing red blood cells by withdrawing blood through a catheter placed in a vein, then replacing the lost blood with fluids. Drugs such as hydroxyurea or clorambucil may also be added to the treatment. In animals with secondary erythrocytosis, the underlying disease must be treated.

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