Hydroxyurea-resistant primary erythrocytosis in a juvenile mixed-breed bitch

Primary erythrocytosis, also known as polycythemia vera, is a myoproliferative disease resulting in an increased red blood cell mass and is associated with increased hematocrit and hemoglobin concentrations. Patients have an augmented risk for thrombosis and bleeding. Typical clinical signs are weakness and central neurologic disturbances. An important clinical finding is the evidence of brick-red mucous membranes. Diagnosis of primary erythrocytosis is made by exclusion. Diagnostic work up includes measurement of erythropoietin levels as well as exclusion of common causes of secondary erythrocytosis (i.e. hypoxic disease, EPO-secreting tumors, kidney disorders). The treatment consists of initial phlebotomies, often combined with hydroxyurea as a chemotherapeutic agent. Patients may occasionally survive for years with long-term treatment. In this case report, a mixed-breed dog failed to show adequate response to chemotherapy with fever, diarrhoea and anorexia occurring during the hydroxyurea treatment period. Following the discontinuation of chemotherapy, clinical signs subsided. Due to rapid increases in hematocrit, phlebotomies were required very frequently (every 2 to 3 weeks) throughout the treatment. This case report demonstrates that control of clinical signs may be achieved with phlebotomy only and survival may exceed 30 months.