Gross and histopathological lesions in EIAV-infected horses are variable and closely associated with levels of viral replication. In the acute stage of EIA, gross pathological lesions consist of swelling of the parenchymatous organs, and hemorrhages can be observed in most tissues. The most pronounced histopathological lesions are hepatic and lymphoid necrosis in association with large numbers of activated macrophages and Kupffer cells. Hepatic necrosis is most severe near the central vein, evidently resulting from degenerative changes in the parenchymal cells. Lesions in the spleen are characterized by degenerative erythrocytes, and small focal hemorrhages are found in the splenic capsule and adjacent tissue. Lymphocytic infiltrations can be observed in several organs, including liver, spleen, lymph node, kidney, heart and lung. The majority of these lesions are thought to be the combined effect of immune-mediated lysis of virus-infected cells and an immune complex-mediated inflammatory response.
The pathological changes in the chronic form of EIA include a developing immunological control of virus replication. Gross pathological lesions include splenomegaly, lymphadenopathy and hepatomegaly. Microscopic changes are characterized by infiltration of lymphoid cells in almost all organs and tissues. Anemia has long been considered the hallmark of EIA. The two major causes of anemia, hemolysis and bone marrow depression, are closely associated with replicating virus. Hemolysis is immunologically mediated. Erythrocytes are coated with the viral surface glycoprotein which, in the presence of specific antibodies and bound C3, induces erythrophago-cytosis and complement-mediated hemolysis. Bone marrow suppression is less well characterized but appears to be associated with iron deficiency. Thrombocytopenia is frequently the earliest pathology observed during chronic EIA and can precede the detection of virus-specific antibodies. The mechanism for the marked reduction in blood platelets is unknown but has recently been associated with cytokine dysregulation.
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