Effects of CAV on hemopoietic cells in bone marrow. The cells which are susceptible to CAV infection in the bone marrow are the hemocytoblasts, which are the specialised progenitor cells from which the erythroid and myeloid series are derived. Evidence for this comes from immunohistochemical studies of bone marrow
and lymphoid tissues from birds at various times following infection with CAV at 1 day old. Following infection of 1-day-old birds, the hemocytoblasts in the bone marrow are among the first cells in which CAV antigen is detected at around 3±4 days after infection. They are highly susceptible to infection and are an early target for the virus, and their destruction results in a severe depletion of erythroid and myeloid cells. Destruction and depletion of the hemocytoblasts in the bone marrow, which is evident by 8 days after infection , gives rise to the anemia, which is the characteristic feature of the disease. The hemocytoblasts also give rise to the thrombocytes, and the reduction in their numbers is probably related to the increased number of intramuscular hemorrhages. In the extrasinusoidal spaces, the hemocytoblasts give rise to the granulocyte series, and destruction of these cells, which is evident by 8 days after infection, is responsible for the fall in the numbers of circulating granulocytes which follows infection. These destructive changes are reflected in the blood, where hematocrit levels and numbers of circulating leukocytes decline from 8 days after infection, due mainly to decreases in the numbers of erythrocytes, lymphocytes and heterophils. Only after 16±18 days following
infection is granulopoiesis and erythropoiesis activity restored in the bone marrow .