Histopathological analyses of lungs from different influenza virus-infected or uninfected pikas by H & E staining. (A) The uninfected lung serves as the normal control (n = 5, control group; n = 5, infected group). (B) Slight alveolar capillary congestion, edema, and thickening of the interstitium of alveolar cells with local lymphocyte infiltration were observed in H1N1-infected lung; no exudation within the alveolar lumen was found in the H1N1-infected lung. (C) Alveolar capillary hyperemia, edema, and alveolar collapse with lymphocyte infiltration were found throughout of the H5N1-infected lung. (D) The alveolar structure was normal, with only a few small bronchi and terminal bronchioles with lymphocytic infiltration in the H3N2-infected lung (arrows). Bar, 100 μm.