Polycythemia is an increase of circulating erythrocytes, manifested as above the reference values increased packed cell volume (PCV/hematocrit), red cell count or hemoglobin concentration. Because the term “polycythemia” implies that all blood cells are increased, some literature uses the term erythrocytosis as more appropriate.

Note that certain dog breeds, such as sighthounds, have their normal PCV values above the reference range for the species (racing Greyhounds can have PCV as high as 70%).

Polycythemia is classified as relative or absolute.

RELATIVE POLYCYTHEMIA is associated with an elevated PCV but normal red blood cell mass which may occur due to:

  • decreased plasma volume caused by dehydration or body fluid shifts. Patients usually also have increased plasma protein concentration and clinical evidence of dehydration.
  • erythrocyte redistribution due to splenic contraction. This condition is transient and usually causes only a modest increase of PCV (no greater than 60%). It is more commonly seen in dogs with normally high PCV (greyhounds, some poodles, dachshunds) and is less pronounced in cats. Splenic contraction may occur secondary to exercise or as a response to epinephrine release in animals that are excited or in pain. Plasma protein is not increased, excitement leucogram may be present (mature neutrophilia, lymphocytosis).

ABSOLUTE (TRUE) POLYCYTHEMIA is caused by an actual increase in the red cell mass and may be primary or secondary, depending on the pathogenesis.

  • Secondary absolute polycythemia results from the overproduction of erythrocytes caused by increased erythropoietin production.

In case of physiologically appropriate polycythemia, increased erythropoietin production is triggered by generalized hypoxia or hypoxemia. Usually, the PaO2 must be less than 60 mm Hg to induce polycythemia. This may be seen in animals with severe chronic heart or pulmonary disease. A similar condition is seen in animals that live at high altitudes.

With physiologically inappropriate polycythemia, tissue oxygenation is normal and increased erythropoietin concentration is caused by renal lesions, usually tumors, or rarely some non-renal tumors that produce erythropoietin or erythropoietin-like substance.

  • Primary absolute polycythemia or polycythemia vera is a rare myeloproliferative disorder in which erythrocytes proliferate uncontrollably, independent of the erythropoietin levels – serum erythropoietin is usually low or even undetectable. Neoplastic erythroid cells appear normal and have a normal maturation sequence. Primary polycythemia is usually diagnosed by excluding other causes of polycythemia.

Clinical findings of polycythemia may be caused by underlying disease (signs of dehydration, heart or pulmonary disease, neoplasia) or may result secondary to increased blood volume and hyperviscosity due to an increased number of erythrocytes. Most animals do not exhibit clinical signs until the red blood cells have reached critical mass. Mucous membranes may appear bright red, sometimes slightly cyanotic. Increased blood viscosity may result in slower blood flow, decreased tissue perfusion, hemorrhage and thrombosis. Decreased oxygen transport may cause mild to severe central neurological signs, such as lethargy, ataxia, blindness, or seizures. A possible manifestation in dogs is paroxysmal sneezing, attributed to increased blood viscosity in the nasal mucosa. Polyuria and polydipsia are occasionally reported.

Diagnostic plan: when increased PCV is observed, a complete blood count should be repeated to confirm the finding. Dehydration and excitement should be ruled out first, after that, secondary absolute polycythemia due to hypoxemia from congenital heart disease or chronic pulmonary disease should be considered. Hypoxemia is diagnosed through arterial blood gas analysis to determine the PaO2 and oxygen saturation. If the PaO2 is less than 60 mm Hg, hypoxemia is likely the cause of polycythemia, thus thoracic radiographs and ultrasound examination should be performed to determine the underlying cause. If hypoxemia is excluded, consider physiologically inappropriate polycythemia and perform abdominal ultrasonography or CT to search for renal lesions or extrarenal neoplasms. When secondary polycythemia is also excluded, the likely diagnosis is polycythemia vera. Serum erythropoietin concentration is usually increased with secondary absolute polycythemia and decreased with primary polycythemia. Other laboratory findings are not very helpful, as even bone marrow aspirates are usually normal.

Treatment: to address secondary polycythemia, one should treat the underlying cause. The prognosis depends on the nature of the primary disease. Primary polycythemia can be treated by intermittent therapeutic phlebotomy to maintain the PCV in the normal range. Approximately 20 mL/kg of blood is collected from a central vein. Because a sudden decrease in blood volume can result in marked hypotension, the blood collection should be accompanied by a concurrent infusion of saline solution. Oral hydroxyurea is the most common chemotherapeutic treatment to decrease red cell production, starting with a higher dose, which is gradually reduced to address the patient’s needs. A complete blood count should be monitored because of the hydroxyurea’s myelosuppressive potential and to adjust the dose of the drug to reach the desired PCV. Most dogs and cats with primary polycythemia have long survival times (> 2 years) if treated with hydroxyurea, with or without phlebotomies.


Thrall M. A., Weiser G., Allison R. W., Campbell T. W.: Veterinary Hematology and Clinical Chemistry, 2nd Ed. Wiley-Blackwell, 2012

Nelson R. W., Couto C. G.: Small Animal Internal Medicine, 4th Ed. Mosby Elsevier, 2009