Pulmonary pathology of pandemic influenza A/H1N1 virus (2009)-infected ferrets upon longitudinal evaluation by computed tomography

Abstract

We investigated the development of pulmonary lesions in ferrets by means of computed tomography (CT) following infection with the 2009 pandemic A/H1N1 influenza virus and compared the scans with gross pathology, histopathology and immunohistochemistry. Ground-glass opacities observed by CT scanning in all infected lungs corresponded to areas of alveolar oedema at necropsy. These areas were most pronounced on day 3 and gradually decreased from days 4 to 7 post-infection. This pilot study shows that the non-invasive imaging procedure allows quantification and characterization of influenza-induced pulmonary lesions in living animals under biosafety level 3 conditions and can thus be used in pre-clinical pharmaceutical efficacy studies.

Fig. 1.

Two rows of four consecutive 3D lung CT images of ferrets #1 and #6 recorded in vivo under BSL-3⁺ compared with their appearance at necropsy on the far right. At day 3 before infection, the lungs showed the clear aerated baseline condition, at day 3 p.i. with the new pandemic H1N1 influenza virus marked almost diffuse ground-glass opacities are present that show a gradual reduction towards 7 days p.i. The two photographs taken at necropsy on 7 days p.i. depict the ventral aspect of the lungs, within the centre the hearts still attached to the pulmonary hilus. Both lungs show multifocal reddish consolidated areas of acute inflammation that essentially match with the opacities on the CT images taken just before necropsy; non-affected aerated lung tissue is light pink in colour.

Fig. 2.

Matching CT scan to histopathology of influenza pH1N1-infected ferret #7 on day 4 p.i. (a) Axial CT-image recorded 10 mm caudally of the tracheal bifurcation depicts bilateral pulmonary ground-glass opacities with peribronchovascular pre-dominance. The rounded white heart shadow (H) is visible in the centre enclosed by the darker lung lobes. During scanning the ferret is placed in dorsal recumbency on a V-shaped tray. The red frame indicates the approximate location of micrograph (c). (b) Subgross histological image matching the location of the axial CT-scan. The red frame indicates the exact location of micrograph (c) (H&E-staining; bar, 10 mm). (c) Low magnification micrograph depicting the histological lesions of the right lung corresponding to consolidated ground-glass opacities, they are composed of extensively flooded alveoli and thickened alveolar septa adjacent to a bronchus containing a plug of mucus with few neutrophils and desquamated epithelial cell remnants (arrow). On top right and middle to bottom left, white non-staining still aerated alveoli are present. The blue frame indicates the exact location of micrograph (d) (H&E-staining; bar, 1 mm). (d) Micrograph depicting the damaged alveoli characterized by infiltrated thickened septa and luminal flooding with protein-rich oedema (pink areas) admixed with variable proportions of macrophages, neutrophils, fibrin, erythrocytes and cellular debris (H&E-staining; bar, 200 µm). Insert on top right: high magnification of serially cut immunohistochemical slide from the same area of micrograph (d), depicting thickened alveolar septa infiltrated by neutrophils (arrows) and adjacent dark-brown staining nuclei of influenza virus-infected pneumocytes lining the flooded alveolus (immunoperoxidase-staining for NP of influenza A virus counterstained with haematoxylin; bar, 40 µm).