Polycythemia is an increase in the amount of red blood cells in the bloodstream. It can be relative, transient, or absolute.

Relative polycythemia 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 polycythemia 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 polycythemia is a real increase in red blood cell numbers resulting from increased production. Diagnosis requires directly measuring the total number of red blood cells, a test that is not available in most veterinary clinics. Your veterinarian will use tests of blood and urine to help diagnose polycythemia. In some cases, x-rays and other tests may also be required. 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.

Treatment of polycythemia 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 some cases, the underlying disease must be treated.

Last full review/revision July 2011 by Peter H. Holmes, BVMS, PhD, Dr HC, FRCVS, FRSE, OBE; Nemi C. Jain, MVSc, PhD; David J. Waltisbuhl, BASc, MSc; Michael Bernstein, DVM, DACVIM; Karen L. Campbell, MS, DVM, DACVIM, DACVD; Timothy M. Fan, DVM, PhD, DACVIM; Wayne K. Jorgensen, BSc, PhD; Sarah E. Payne, DVM, DACVIM