FIG 2.

Passaging of rH1N1 virus in pigs led to increasing pulmonary damage. Gross and histopathological examinations were performed on the respiratory tracts of infected pigs at 4 dpi. Representative gross (left) and corresponding histological (H&E staining) findings at low (middle) and high (right) magnification are presented. (A and G) P1 virus-infected lung showed mild bronchopneumonia with minor lymphocytic infiltration. (B and H) P3 virus-infected lung had moderate bronchopneumonia with inflammatory cell infiltrates in the alveoli and interstitium (boxed area magnified) and loss of epithelial lining (black arrow in H). (C and I) P6 virus-infected lung showed severe bronchopneumonia with hemorrhage, edema, and diffuse consolidation. (D and J) P9 virus-infected lung displayed extensive consolidation in all lobes and severe bronchopneumonia with diffuse infiltration of neutrophils and alveolar macrophages (the boxed area is magnified). (E and K) Virulent SD731 virus-infected lung showed severe bronchopneumonia similar to the changes found in the P9 lung. (F and L) Mock-infected lung showed normal morphology. Blue arrows (left) indicate pulmonary tissue consolidation. Results shown are typical of three independent examinations for each animal. Scale bar, 200 μm.