Figure 3. Histopathologic lesions in the trachea and lungs of control (A and C) or experimentally infected (B, D–F) dogs (A/canine/Korea/01/2007 [H3N2]) at different days postinoculation (dpi). A) Control dog at 9 dpi, showing normal pseudostratified columnar epithelium lining of the trachea; original magnification ×400. Hematoxylin and eosin (HE) stain. B) Influenza-infected dog at 9 dpi, showing necrotizing tracheitis characterized by necrosis (n), squamous metaplasia (s), and hyperplasia of the epithelium and nonsuppurative inflammation (c) in the connective tissue; original magnification ×400. HE stain. C) Control dog at 3 dpi, showing normal alveoli; original magnification ×200. HE stain. D) Influenza-infected dog at 3 dpi, showing severe diffuse necrotizing bronchitis and bronchiolitis with suppurative inflammation in the lumina; original magnification ×100. HE stain. E) Influenza-infected dog at 6 dpi, showing severe necrotizing bronchiolitis; original magnification ×200. HE stain. F) Influenza-infected dog at 6 dpi (serial section of E), showing large amounts of influenza A virus antigens (red stain; arrows) in the bronchiolar epithelium and lumen. Immunohistochemistry; Red Substrate (Dako, Carpinteria, CA, USA); Mayer’s hematoxylin counterstain. G) Influenza-infected dog at 9 dpi, showing severe necrotizing alveolitis with accumulation of necrotic cells in terminal bronchioles (tb) and alveoli (a); original magnification ×200. HE stain.