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Immunohistochemical demonstration of influenza A virus antigen (red, see arrows) in numerous splenic macrophages of a falcon after challenge with 106.0 50% egg infectious doses of the highly pathogenic avian influenza strain A/Cygnus cygnus/Germany/R65/06 (H5N1). Avidin-biotin-peroxidase complex method. Bar = 25 μm.
