Polycythemia vera

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Limbal hyperemia associated with primary polycythemia in a dog[1]

Polycythemia vera (primary polycythemia) is a chronic immune-mediated myeloproliferative syndrome characterized by erythrocytosis and thrombocytosis.

Polycythemia usually results from clonal proliferation of erythroid precursors that require little or no erythropoietin for differentiation, and does not respond to the normal negative feedback controls of replication[2].

In humans, the presence of an acquired recurrent mutation within the JAK2 gene has recently been identified in 90% of the patients with polycythemia vera, and a similar mutation has been reported in dogs[3].

In these dogs, peripheral erythrocytes and erythroid precursors appear normal. Mild hyperplasia and a normal myeloid to erythroid ratio are seen when the bone marrow is observed.

Affected dogs have increased circulating erythrocytes, and clinically symptoms relate to elevated hematocrit, an in some cases circulatory hyperviscosity syndrome, as well as anterior uveitis[4], seizures, hypertension and secondary glomerulonephritis[5].

Compensatory polydipsia and polyuria may be present.

A diagnosis is made by exclusion of relative erythrocytosis and other causes of secondary polycythemia.

Hematological tests may assist distinguishing primary from secondary polycythemia. A high PCV and plasma protein levels suggest secondary polycythemia, whereas thrombocytosis suggests primary polycythemia[6].

Bone marrow biopsies are usually required to exclude other primary and secondary myelodysplastic syndromes such as lymphoma, myelofibrosis, immune-mediated thrombocytopenia, immune-mediated hemolytic anemia, multiple myeloma and pyometra[7].

A low to undetectable serum erythropoietin level is often confirmatory[8].

Remissions have been reported following phlebotomy and treatment with oral hydroxyurea (30 mg/kg orally once daily until PCV <55% and then titrated) or chlorambucil (0.2 mg/kg orally once daily until PCV is <55% and then titrated).

References

  1. Vetnext
  2. McGrath CJ (1974) Polycythemia in dogs. J Am Vet Med Assoc 164:1117-1122
  3. Beurlet S et al (2011) Identification of JAK2 mutations in canine primary polycythemia. Exp Hematol 39(5):542-545
  4. Gray HE et al (2003) Polycythemia vera in a dog presenting with uveitis. J Am Anim Hosp Assoc 39(4):355-360
  5. Quesnel AD & Kruth SA (1992) Polycythemia vera and glomerulonephritis in a dog. Can Vet J 33(10):671-672
  6. Merck Vet Manual
  7. Weiss DJ & Aird B (2001) Cytologic evaluation of primary and secondary myelodysplastic syndromes in the dog. Vet Clin Pathol 30(2):67-75
  8. McGrath CJ et al (1982) Canine polycythemia vera: a review of diagnostic features. Vet Med Small Anim Clin 77:611-613