Pathological analysis at day 28 PI. (A) A representative picture of lung from group Mock/pH1N1. The lungs show moderate broncho-interstitial pneumonia (BIP)-compatible lesions. (B) Lung from a Mock/Mock control pig. Normal pulmonary parenchyma. Note the minimal thickness of interalveolar walls, characterised by pneumocytes and blood capillaries. Peribronchial lymphoid tissue is minimally developed. Haematoxylin and eosin stain. Bar = 200 μm. (C) Lung from a pH1N1 infected pig (day 7 post-pH1N1 inoculation). Marked presence of mononuclear inflammatory cells within alveoli, almost collapsing alveolar lumen. Lymphocyte perivascular cuffing together with moderate hyperplasia of the lymphoid tissue surrounding a bronchioli. Haematoxylin and eosin stain. Bar = 200 μm. (D) Pathological score for all the animals in the assay ( Mock/Mock, Mock/pH1N1, SwH1N1/Mock, SwH1N1/pH1N1). (A color version of this figure is available at www.vetres.org.)