Guinea-pigs and non-human primates have traditionally been used as animal models for studying Ebola Zaire virus (EBO-Z) infections. The virus was also recently adapted to the stage of lethal virulence in BALB/c mice. This murine model is now in use for testing antiviral medications and vaccines. However, the pathological features of EBO-Z infection in mice have not yet been fully described. To identify sites of viral replication and characterize sequential morphological changes in BALB/c mice, adult female mice were infected with mouse-adapted EBO-Z and killed in groups each day for 5 days post-infection. Tissues were examined by light microscopy, immunohistochemistry, electron microscopy and in-situ hybridization. As in guinea-pigs and non-human primates, cells of the mononuclear phagocytic system were the earliest targets of infection. Viral replication was observed by day 2 in macrophages in lymph nodes and spleen. By the time of onset of illness and weight loss (day 3), the infection had spread to hepatocytes and adrenal cortical cells, and to macrophages and fibroblast-like cells in many organs. Severe lymphocytolysis was observed in the spleen, lymph nodes and thymus. There was minimal infection of endothelial cells. All of these changes resembled those observed in EBO-Z-infected guinea-pigs and non-human primates. In contrast to the other animal models, however, there was little fibrin deposition in the late stage of disease. The availability of immunodeficient, "gene-knockout" and transgenic mice will make the mouse model particularly useful for studying the early steps of Ebola pathogenesis.
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