Animal Erythrocytosis and Polycythemia

• Definition: An increase in the number of circulating RBCs, resulting in increased PCV, hematocrit, RBC count, and hemoglobin concentration above reference intervals.
  
  

  • Polycythemia is often used synonymously, but may imply leukocytosis and thrombocytosis in addition to erythrocytosis.

• Types of Erythrocytosis:
  
  

  • Relative Erythrocytosis:

    • Increase in RBC numbers without an increase in total RBC mass.

    • Caused by loss of plasma water with resultant hemoconcentration due to dehydration.

    • Alternatively, a mild, transient form unassociated with clinical signs can occur due to splenic contraction.

      • Splenic contraction is due to fear, excitement, causing immediate release of sequestered RBCs.

      • Most often observed in horses.

    • Clinical Signs: Typically those of dehydration (tacky mucous membranes, prolonged skin tent) and the underlying disease.

    • Diagnosis:

      • Clinical findings (tacky mucous membranes, loss of skin turgor).

      • Laboratory variables (hyperproteinemia, prerenal azotemia).

      • Response to rehydration.

      • For splenic contraction, normal PCV on subsequent blood samples collected less stressfully.

    • Treatment: Consists of rehydration with IV fluids and addressing the underlying cause.

  • Absolute Erythrocytosis:

    • Increase in RBC numbers because of increased RBC mass.

    • Generally requires a PCV exceeding 60% to observe clinical signs.

    • Clinical Signs (associated with more severe absolute erythrocytosis):

      • Red mucous membranes.

      • Bleeding tendencies.

      • Neurologic disturbances (ataxia, weakness, seizures, blindness, behavioral change).

      • Dilated, tortuous vessels on retinal examination.

      • These signs are attributed to hyperviscosity, which slows blood flow and increases the likelihood of thrombosis and rupture of small blood vessels.

    • Types of Absolute Erythrocytosis:

      • Primary Erythrocytosis (Polycythemia Vera):

        • A myeloproliferative disease resulting from the autonomous clonal expansion of hematopoietic progenitor cells.

        • RBC production is dramatically increased.

        • Serum erythropoietin (EPO) activity typically is low or low-normal.

        • Has been reported in dogs, cats, horses, and ferrets.

        • Diagnosis is usually made by eliminating secondary causes.

        • Routine examination of bone marrow is NOT useful to distinguish from secondary causes.

        • Clinical signs are attributed to increased blood viscosity.

        • Treatment typically includes phlebotomy and hydroxyurea.

          • Phlebotomy: 10–20 mL/kg to decrease PCV to ~50%–60%, with simultaneous fluid replacement.

          • Hydroxyurea: Extra-label administration (initial 30 mg/kg daily, then titrated dose/frequency) with monitoring of blood counts.

      • Secondary Erythrocytosis:

        • Develops from excessive production of EPO.

        • Can be classified as appropriate or inappropriate.

        • Appropriate Secondary Erythrocytosis:

          • Resultant erythrocytosis is an appropriate compensatory response.

          • Occurs when EPO is secreted because of systemic hypoxia.

          • May occur with severe pulmonary disease or heart anomalies resulting in right-to-left shunting.

          • Diagnosis: Arterial blood pO < 80 mmHg and oxygen saturation < 90%–95%, indicating a hypoxic state. Examination of heart/lungs (auscultation, radiography, ECG, echocardiography) may reveal the underlying problem. Selective angiography/contrast echoaortography may be needed for shunting.

          • Treatment: The underlying problem should be addressed. If corrective treatment isn't feasible, signs from hyperviscosity may be alleviated by judicious phlebotomy (5–10 mL/kg) and hydroxyurea. The PCV should be maintained at values above normal reference intervals as it's a compensatory response.

        • Inappropriate Secondary Erythrocytosis:

          • EPO production increases without systemic hypoxia.

          • Typically a paraneoplastic finding attributed to EPO-secreting tumors of the kidneys or other organs.

          • Can also be due to non-neoplastic renal disorders resulting in local hypoxia.

          • Another type: Endocrinopathy-associated erythrocytosis from hormones other than EPO stimulating erythropoiesis (e.g., cortisol, testosterone, thyroxine, growth hormone). This is typically mild and insufficient to cause clinical signs.

          • Diagnosis: If systemic hypoxia is absent, locate potential source via physical/neurologic exams, abdominal ultrasonography, IV urography, CT or MRI.

          • Treatment: EPO-secreting tumors should be managed with surgery, chemotherapy, or radiation therapy. Phlebotomy helps decrease hyperviscosity.

• General Diagnosis: Clinical evaluation, CBC. Serum EPO determinations could potentially differentiate primary from secondary but have limitations in availability and overlap.
  
  

• General Treatment: Supportive care, Condition-dependent treatment.