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 25-28 September, 2018 | Singapore
with polycythemia vera (P. vera) without considering other differential diagnoses. Clinical experience, however, suggests that P. vera may occur rarely compared to
other forms of polycythemia. New insight into the causes of primary and secondary polycythemias has been gained in human and veterinary medicine. However, if the underlying condition cannot be corrected, lowering the PCV into a safe range in patients with absolute polycythemia may be successfully accomplished with repeated phlebotomy and, if needed, chemotherapy.
Primary polycythemia appears erythropoietin independent and has often been considered synonymous with P. vera in small animals, although additional forms of primary polycythemia need to be considered. In fact, animals with a presumptive diagnosis of primary polycythemia as well as cases with an early presentation in life that continues over a chronic course of many years, point strongly to the existence of other forms of primary polycythemia in dogs and cats. More research is needed in small animals to determine, if such processes are occurring in these polycythemic patients who do not clearly have P. vera.
Polycythemia vera is a myeloproliferative clonal disease that arises from a multipotent hematopoietic progenitor cell in the bone marrow. A single transformed stem cell gains a selective growth advantage and becomes the predominant source of marrow precursors, and the clonality of the bone marrow cells of human patients with P. vera has been documented. P. vera, therefore, results in the accumulation of morphologically normal red blood cells, and less commonly white cells and platelets, and their progenitor cells in the absence of a definable stimulus. Granulocyte and platelet counts in the blood would be expected to be increased, but are usually normal. The bone marrow aspirate is consistent with erythroid hyperplasia, but is not diagnostic for
a myeloproliferative disease in humans. There are
no cytologically characteristic features of the bone marrow cells in P. vera as hematopoietic cells appear to fully mature. Classically, human patients have serum erythropoietin levels in the low to normal range and erythroid progenitor cells that proliferate and mature independent of erythropoietin, but these culture assays are not robust and readily available. In humans with
P. vera, Jak-2 mutations have been documented and
in a couple of dogs Jak-2 mutations have been found but the test for Jak-2 mutation is not clinically available in veterinary medicine. Hence, the diagnosis of P.
vera is still based on the exclusion of other causes of erythrocytosis.
Secondary polycythemia refers to a group of diseases triggered by an exaggerated erythropoietin dependent
stimulation of red cell production. This may be considered an appropriate response in which the erythron is responding normally to generalized tissue hypoxia or inappropriate in which the erythropoiesis is being stimulated by an aberrant production of erythropoietin or due to local renal hypoxia.
Appropriate PCV rises are seen in high altitudes and with cardiopulmonary disease such as congenital heart defects with right to left shunts (ventricular septal defects, reversed PDA, Tetralogy of Fallot) and rarely chronic obstructive pulmonary diseases. Cats with cardiac shunting usually die before they can develop signs of polycythemia. Furthermore, an appropriate secondary polycythemia has been documented in several breeds
of dogs and domestic shorthair cats with hereditary methemoglobin reductase deficiency, which results
in the erythrocytes’ inability to carry oxygen. Similarly, chronic carbon monoxide intoxication can cause polycythemia with pink mucous membranes.
Inappropriate absolute polycythemia includes renal diseases, as well as tumors producing erythropoietin, and is typically associated with typically high serum erythropoietin levels. Various renal tumors, including nephroblastomas and carcinomas, may result in
renal hypoxia and thereby cause elevations in serum erythropoietin and, consequently, inappropriate secondary polycythemia; whereas erythropoietin- producing tumors in other tissues have rarely been documented. Inappropriate secondary polycythemia of renal origin may also be rarely caused by amyloidosis, polycystic kidney disease, glomeronephritis, and renal fibrosarcoma and lymphoma. In most cases increased serum erythropoietin concentrations were documented or an association was established based upon the resolution of the polycythemia following the resection of the mass in animals and humans.
The clinical signs of relative polycythemia are easily recognized and will not be further discussed here. Clinical signs of absolute polycythemia are characterized by manifestations of the underlying disease process and are associated with hyperviscosity and the expanded blood volume. They include hyperemic or cyanotic mucous membranes (due to cardiopulmonary disorders and methemoglobinemia), hemorrhage (epistaxis and hyphema), and neurologic disturbances such as lethargy and seizures. Neurologic signs are the most common presenting complaints, but with the advent of more frequent health screens including complete blood cell counts, the erythrocytosis may be discovered earlier as an incidental finding. Cyanosis or renal size abnormalities may suggest a particular organ failure as well as mechanism, and define the type of the polycythemia. However, secondary cardiac and renal changes due to erythrocytosis may also be observed and confound the

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