) Hematoxylin and eosin (H&E) (left) and anti-IBV S2 MAb 48.4 immunohistochemistry staining (right) of a section of the trachea of a mock-infected (7 dpi) or M41-infected (3 dpi and 7 dpi) chicken. The trachea of the M41-infected chicken at 3 dpi shows an intact epithelial lining with minimal hyperemia