FIG. 1.

Immunohistochemical analysis of mice infected with a mouse-pathogenic Hong Kong H5N1 virus. Mice were infected intranasally with 100 PFU of the mouse-pathogenic HK483 virus. Mice were sacrificed, and brains (day 5; A and B) and hearts (day 6; C) were processed for identification of influenza virus replication with a monoclonal antibody specific for nucleoprotein. (A and B) Nonsuppurative encephalitis showing intense nuclear and slightly less intense cytoplasmic staining (brown) of influenza virus nucleoprotein in neurons (arrows) and glial cells of the brain stem at the metencephalon. Magnifications, ×100 (A) and ×400 (B). (C) Staining (brown) of the nucleoprotein in the nucleus and the cytoplasm of necrotic cardiac myofibers. Magnification, ×200.