ABSTRACT. A two-year old male Welsh Corgi was referred for persistent thrombocytosis and occasional seizure. Hematological findings indicated marked thrombocytosis, eosinophilia, basophilia and moderate anemia. Bone marrow examination revealed marked megakaryocytic hyperplasia with morphologic abnormality. A diagnosis of essential thrombocythemia was made and the treatment was initiated with combination chemotherapy and maintained by prednisolone and busulfan. The dog successfully achieved complete remission on 100 days after initial presentation and has been good in health without chemotherapy since then. KEY WORDS: busulfan, essential thrombocythemia, seizure.J. Vet. Med. Sci. 68(11): 1203-1206, 2006 Essential thrombocythemia (ET) is one of myeloproliferative disorders characterized by the proliferation of a pluripotent hematopoietic stem cell, leading to expansion of the megakaryocytic compartment in bone marrow, and the excessive and presumably autonomous production of structurally and functionally abnormal platelets [10,16]. The clinical signs in humans with ET are occasionally related to microvascular thrombosis or bleeding [13,14]. Although a number of human patients have contracted ET, limited cases have been reported in dogs so far. Canine cases of ET rarely showed symptoms of thrombosis or hemorrhage and clinical sings of them are nonspecific [1,3,5,6,11,16,18]. Hematological findings in dogs with ET indicated persistent marked thrombocytosis with morphologic abnormality and hypercoagulation and bone marrow examination revealed marked megakaryocytic hyperplasia without blastocytosis [1,3,5,6,11,[16][17][18]. Diagnosis of ET involves ruling out other causes of thrombocytosis and treatment has not yet been established. This report describes clinicopathological features and therapy in a dog with ET.A two-year old intact male Welsh Corgi was referred for 3 months history of persistent thrombocytosis and seizure in case of exercising and exciting. Mixed vaccination and heartworm prophylaxis were performed every year. The dog had mild weight loss, pale mucous membranes and rectal temperature of 39.5°C. All other aspects of physical examination including auscultatory sound, pulse pressure and neurological examination were unremarkable. Hematological findings were characterized by nonregenerative anemia (packed cell volume [PCV], 22%), marked thrombocytosis (3,940 × 10 3 /µl) and leukocytosis (46,400/ µl) including eosinophilia (7,190/µl), basophilia (10,440/µl) and unclassified cells (6,730/µl) ( Table 1). Abnormal thrombocyte morphology including hypogranular platelets, giant platelets and micromegakaryocytes were observed on the peripheral blood smear (Fig. 1). The results of blood biochemistry showed increases in alkaline phosphatase (ALP, 804 U/l), potassium (K, 5.5 mmol/l), lactate dehydrogenase (LDH, 3,380 U/l) with dominant fraction of LDH5 and vitamin B 12 (VB 12 , 558 pg/ml) ( Table 1). Bone marrow examination was performed on day 1 and revealed normal cellularity and marked thrombocytosis. Megaka...
