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Erythrocytosis (polycythemia) is defined by an increase in total red blood cell (RBC) number, packed cell volume (PCV), and hemoglobin (Hb) concentration above reference intervals[1].

The causes of erythrocytosis include:

  • Absolute erythrocytosis - ↑RBCs, ↑PCV & ↑Hb due to an increase in RBC production (↑Erythropoietin)
- Primary erythrocytosis - polycythemia vera - neoplastic myeloproliferative disorder (Erythropoietin independent)
- Secondary erythrocytosis - a physiological response due to hypoxia - (↑Erythropoietin)
- e.g. high altitude (> 1800 m), polycystic kidney disease, pyelonephritis, hydronephrosis, reverse patent ductus arteriosus, tetralogy of Fallot, severe chronic respiratory disease, hemoglobin function abnormalities, and severe obesity (Pickwickian syndrome)
- e.g. neoplasia - renal cystadenoma or cystadenocarcinoma[2] associated with dermatofibrosis in German Shepherds, renal lymphoma, renal adenocarcinoma[3], renal angiomyxoma, hepatoma, uterine leiomyoma and cerebellar hemangioblastoma.
  • Relative erythrocytosis - ↑RBCs, ↑PCV & ↑Hb due to a decrease in total plasma fluid volume (hemoconcentration)[4]

Clinical signs associated with erythrocytosis are directly related to an increase in blood viscosity and include diarrhea, vomiting, epistaxis, engorged retinal vessels, seizures, polyuria and polydipsia, dehydration and increased capillary refill time[5].

Blood tests usually reveal a PCV > 50%.

Auscultation of the heart may reveal a heart murmur and secondary cardiomyopathy.

Primary absolute erythrocytosis is initially treated by phlebotomy to reduce the circulating RBC mass, removing 5 - 20 ml/kg of blood, while monitoring PCV[6]. Administration of supplemental iron should be considered.

In polycythemia vera, phlebotomy is augmented with hydroxurea (myelosuppressive drug) at 50mg/kg orally once daily for seven days, followed by 15 mg/kg once daily until remission[7]. The prognosis for animals with primary erythrocytosis is guarded.


  1. Hasler AH & Giger U (2000) Polycythemia. In: Ettinger SJ, Feldman EC (eds): Textbook of Veterinary Internal Medicine, 5th ed. W.B. Saunders Company, Philadelphia. pp:203-206
  2. Zanatta M et al (2012) Nodular Dermatofibrosis in a Dog without a Renal Tumour or a Mutation in the Folliculin Gene. J Comp Pathol Aug 4
  3. Crow SE et al (1995) Concurrent renal adenocarcinoma and polycythemia in a dog. J Am Anim Hosp Assoc 31:29-33
  4. Watson AD (2000) Erythocytosis and polycythemia. In: Feldman BF, Zinkl JG, Jain NC (eds): Schlam’s Veterinary Hematology, 5th ed. Lippincott Williams and Wilkins, Philadelphia. pp:216-221
  5. Nitsche EK (2004) Erythrocytosis in dogs and cats: Diagnosis and management. Compend Cont Educ Pract Vet 26:104-118
  6. Raskin RE (2000) Erythrocytes, leukocytes, and platelets. In: Birchard SJ, Sherding RG (eds): Saunders Manual of Small Animal Practice, 2nd ed. W.B. Saunders Company. pp:164
  7. Moore KW & Stepien RL (2001) Hydroxurea for treatment of polycythemia secondary to right-to-left shunting patent ductus arteriosis in 4 dogs. J Vet Int Med 15:418-421