Nonregenerative Anemia

Many serious underlying illnesses that are not primary hemopoietic disorders can be associated with nonregenerative anemia. Neoplasia, renal disease, inflammatory disease, and endocrinopathies, such as hypothyroidism and hypoadrenocorticism, commonly affect erythropoiesis. The pathophysiology of the anemia is well-understood for some of these conditions, and not for others. For example, the anemia associated with renal failure is multifactorial, but lack of erythropoietin is particularly important. Mechanisms which also play roles are: decreased red cell lifespan, blood loss from decreased platelet function and from oral and intestinal uremic ulcers, and decreased response of erythroid precursors to erythropoietin (see Case 5).

Anemia of Inflammatory Disease (AID)

With severe inflammatory disease, the most common cause of nonregenerative anemia in veterinary medicine, anemia results from reduced availability of iron for erythropoiesis, decreased red cell lifespan, and decreased responsiveness of the erythroid lineage to erythropoietin (see Table 1.5). These effects are due to the presence of cytokines, particularly interleukin (IL)-1, tumor necrosis factor (TNF), transforming growth factor (TGF)-β, and interferon (IFN)-α.

Primary Bone Marrow Disease and Nonregenerative Anemia

Historical information, physical findings, and other test results are useful to differentiate primary bone marrow disease from secondary causes of nonregenerative anemia. Bone marrow examination is indicated when secondary causes are ruled out and the anemia is suspected to be due to bone marrow pathology. Bone marrow disorders to consider are: erythroid hypoplasia or aplasia due to certain drugs, toxins, chemicals, hormones, or irradiation; dyserythropoiesis resulting in defective maturation; marrow necrosis; infiltration of the marrow with neoplastic cells or fibroblasts; viral infections such as feline leukemia virus or feline immunodeficiency virus; and immune-mediated destruction of erythroid precursors. When erythroid hypoplasia or pure red cell aplasia is found on bone marrow examination, an etiology is often not identified. With many conditions, cell lines in addition to erythroid are affected.

Repeat evaluation of the bone marrow and monitoring of the peripheral blood response may be required to determine if the condition is reversible or irreversible. Blood transfusion and immunosuppressive therapy may be useful to provide time for the marrow to respond and to treat a possible underlying immune-mediated mechanism, respectively. Students should learn the proper technique for acquiring bone marrow (see video on bone marrow collection) and making good quality smears so that bone marrow evaluation can be offered to clients in those situations where peripheral blood findings require further investigation. Unexplained, persistent abnormalities on the erythrogram, as well as the remainder of the CBC, usually warrant examination of the bone marrow.