Myelofibrosis is a proliferative response of the bone marrow fibroblasts. Myelofibrosis can be classified as primary or secondary depending on the underlying etiology. Primary myelofibrosis is a myeloproliferative disorder in humans in which there is a clonal proliferation of a pluripotent stem cell. Hemopathology includes finding nucleated red blood cells and immature granulocytes in the circulation, extramedullary hematopoiesis, and myelofibrosis. The proliferation of the bone marrow fibroblasts is not clonal in origin. To the best of this author's knowledge, this type of myelofibrosis has not been reported to occur naturally in the dog. Secondary myelofibrosis has been reported in the dog associated with neoplastic conditions, irradiation, congenital hemolytic anemias, and a variety of unknown etiologies. It has been shown in some cases of myelofibrosis that there is often concurrent bone marrow necrosis. Bone marrow necrosis has been documented in dogs with Ehrlichiosis and septicemia, and associated with drug treatment, including estrogens and cephalosporins. It is thought that this necrosis is due to the destruction of the bone marrow microvasculature and/or hematopoietic elements. Release of growth factors by inflammatory cells may lead to the subsequent fibroblast proliferation. Several cases of secondary myelofibrosis in female laboratory beagles have been recently observed. These dogs present with a severe nonregenerative anemia and often a mild neutropenia with varying degrees of myelofibrosis in the bone marrow. Some animals have had concurrent bone marrow necrosis. At this time, the exact etiology is unknown.
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