Relative polycythemia is characterized by the presence of an increased hematocrit with a normal total red blood cell number. It can be the result of abnormal fluid balance resulting in decreased plasma volume as might be seen in dehydration, or it might arise secondary to splenic contraction.
Absolute polycythemia can be further characterized as being primary (polycythemia vera, polycythemia rubra vera) or secondary in nature. Secondary polycythemia results from excessive production of erythropoietin. The excessive erythropoietin might arise in response to systemic hypoxia resulting from cardiac disease, pulmonary disease, or a hemoglobinopathy. This is an appropriate physiologic response to tissue hypoxia. In other cases of polycythemia, there is increased erythropoietin secretion without systemic hypoxia. This might occur as a result of diseases in the kidney such as a renal carcinoma. In human beings, this form of secondary polycythemia has also been identified with other types of tumors and nonneoplastic renal disease.
Primary polycythemia is considered to be a myeloproliferative disorder, resulting from the clonal expansion of erythroid precursors, which do not require, or require little, erythropoietin for development. This is a rare condition in the dog; relative and secondary causes of polycythemia are diagnosed far more frequently. Historical signs reported for dogs diagnosed with polycythemia vera include red mucous membranes, congested scleral vessels, weakness, hemorrhage, and various neurologic signs. The latter three signs most commonly arise as a result of hyperviscosity syndrome.