Histopathology and antigen expression in central nervous system of gulls at 12 dpi infection. By immunohistochemistry, influenza virus antigen expression is visible as a red staining. (A) Cerebral cortex showing intense perivascular cuffing and diffusely necrotic neurons and satellitosis. (B) Consecutive section of the same tissue without viral antigen expression. HE stain (A) and immunoperoxidase counterstained with hematoxylin (B). Both the pictures are at 20× power fields.