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U. Giger1
1 Philadelphia, USA
Urs Giger, Prof. Dr. med. vet. MS FVH
Dipl. ACVIM & ECVIM-CA (Internal Medicine) & Dipl. ECVCP (Clinical Pathology)
School of Veterinary Medicine, University of Pennsylvania, Philadelphia
Although strictly speaking polycythemia implies a rise
of all blood cell counts above normal, the occurrence
of leukocytosis and thrombocytosis along with erythrocytosis is exceptionally rare in companion animals. Hence, polycythemia would be more appropriately called erythrocytosis and clinically refers to an increase to an above normal red blood cell count, hematocrit or packed cell volume (PCV), and hemoglobin (Hb) concentration. Polycythemic animals will not show clinical signs until
the PCV reaches >60% with some of the highest PCVs exceeding 85%. Because of an underappreciation of the normal upper limit of the PCV in cats (48%) versus dogs (56%), polycythemia is generally underdiagnosed in cats. Based upon blood volume and red cell mass, polycythemia can be divided into relative and absolute polycythemias, which represent completely different conditions, both requiring immediate but opposing therapeutic interventions.
Relative polycythemia is characterized by an elevated PCV in the presence of a normal (or even decreased) total red blood cell mass. This is usually due to a decrease in plasma volume associated with severe dehydration or increased serum total proteins, e.g. profound vomiting and diarrhea, or severe burns. The hematocrit is generally only mildly increased, therefore relative polycythemia is rarely associated with signs of hyperviscosity, and the clinical features of the underlying disorder prevail. Because of the obvious signs of dehydration, relative polycythemia is usually easily recognized and simply corrected with aggressive fluid therapy.
Absolute or true polycythemia is characterized by an expanded red blood cell mass. Splenic contraction is an unlikely cause in dogs and cats, as it only marginally increases the PCV. The blood volume and red cell mass could be determined by labeling red cells radioactively
or with biotin. This is, however, rarely if ever needed in clinical practice, as dehydration can be readily excluded as cause of relative polycythemia. More difficult is the differentiation of absolute polycythemia into primary or secondary polycythemia, which depends on whether the condition is erythropoietin independent or dependent.
Some clinicians readily equate absolute erythrocytosis 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.
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