Pathological changes (a,c) and immunohistochemical detection of viral antigen.

a. Lung. In-contact animal (TC4), 11 dpc. Multiple dark red areas of cranioventral pulmonary consolidation in the cranial and middle lung lobes. b. Nasal mucosa, 2 dpi. Influenza A nucleoprotein (brown) in numerous respiratory epithelial cells 20x. c. Lung, 7 dpi. Marked attenuation of bronchiolar epithelium due to epithelial cell necrosis, bronchiolar plug and peribronchiolar lymphohistiocytic infiltration. Haematoxylin and eosin 40x. d. Lung, 4 dpi. Viral nucleoprotein in abundant bronchiolar epithelial cells and cellular debris in the bronchiolar lumen and in several pulmonary alveolar macrophages 20x.