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An Urban Experience
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 interpretation. Polydipsia and -uria and splenomegaly has been seen in animals assumed to have P. vera and rarely with other polycythemias.
Diagnostic Tests
As relative polycythemias are readily recognized, confirmation of a normal blood volume may not be required in clinical practice. In fact, patients with absolute polycythemia often have an expanded blood volume. Diagnostic tests for absolute polycythemia include a complete blood cell count, absolute reticulocyte count, chemistry screen, urine analysis, blood gases and pulse oxymetry, chest and abdominal radiographs, cardiac examination and an ultrasound
to evaluate the kidneys and liver. Cyanosis may only
be noted caudily. Dark blood may be exposed to air to determine if it is deoxyhemoglobin or methemoglobin. Serum erythropoietin values, determined by a species- validated assay, may be elevated in cases of secondary polycythemia, but a normal to low erythropoietin
level does not rule out secondary polycythemia. An increased absolute reticulocyte count in light of a polycythemia supports the exaggerated hematopoietic response and documents the presence of an absolute polycythemia. However, a bone marrow aspirate for cytologic examination adds no new information, since it fails to differentiate between primary and secondary polycythemias: the myeloproliferative disease resulting in P. vera has no characteristic cytologic or pathologic features of malignancy
The treatment of relative and absolute polycythemia
is clearly different. In emergency situations, patients with relative polycythemia will respond to fluid therapy, whereas patients with absolute polycythemia may need to be treated by phlebotomy. If possible, renal and other tumors/masses should be removed or treated and cardiopulmonary distress should be corrected. Animals with methemoglobin reductase deficiency may not need to be treated except during severe stress situations with methylene blue (1 mg/kg IV once). In severely polycythemic patients repeated phlebotomies at no more 20 ml/kg (10 ml in cats) per session with
or without simultaneous fluid replacement is the initial approach to lower the PCV to <60%. Phlebotomies can be repeated on a daily basis until the target PCV has been reached. In animals with absolute polycythemia,

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