Selected tissues of the respiratory and gastrointestinal tracts of Mallard following immunohistochemical staining to detect nucleoprotein of influenza A virus. Tissues from the respiratory tract did not show virus antigen expression, such as (A) the respiratory epithelium of the nasal cavity and (B) air sac epithelium. In contrast, some tissues from the gastrointestinal tract such as (C) the epithelium lining the jejunal villi of the gastrointestinal tract, and (D) surface epithelium of the cloacal bursa did show virus antigen expression. Virus antigen expression is visible as diffuse to granular red staining, which is usually darker in the nucleus than in the cytoplasm. The tissues are counterstained blue with hematoxylin. Arrows have been included to illustrate positive cells. All images were captured at 400× magnification.