Immunohistochemical analysis of infected cells. Nasal (A, a), tracheal (B, b), bronchial (C, c) and lung (D→F, d→f) explants at 48 hpi inoculated with Swine/Gent/7625/99 (H1N2) (A→F) and Duck/Belgium/06936/06 (H4N6) (a→f) were analyzed. In the nasal (A: black arrow) and tracheal (B: orange arrow) explants, single swine influenza virus positive cells were diffusely spread while no avian influenza virus positive cells were present (a, b). Swine influenza virus positive cells were also found as a continuous line in bronchial epithelium (C), as multiple foci in the bronchioles (D: red arrows, E) and as single alveolar cells (F: green arrows) in lung explants. Avian influenza viral antigen-positive cells were limited to bronchiolar epithelium in lung explants (d: red arrows, e). Symbols underneath the pictures give the results for the semi-quantitative analysis of influenza virus positive cells by IF. -: no virus positive epithelial cells, +/-: single positive cells covering <10% of the epithelium, +: between 11 and 40% of the epithelium is positive, ++: between 41 and 70% of the epithelium is positive, +++: between 71 and 100% of the epithelium is positive.