Pulmonary pathology and neutrophil influx in calves following aerosol challenge with either BRSV-Snk or BRSV-Dk. Calves were experimentally infected as described in Figure 1. Calves were euthanized seven days after infection and the macroscopic extent of lung lesions were documented (panel A). From each calf, trachea tissue and four lung tissue samples were collected for sectioning, staining and histopathological description and scoring of severity of inflammation (1–3). Panels B:I-IV show representative HE-stained sections from: (B:I) trachea from calf A2; (B:II) trachea from calf B3; (B:III) lung from calf A2; and (B:IV) lung from calf B3. Horizontal bars indicate 50 μm in panels B:I and B:II, and 100 μm in panels B:III and B:IV. Mean histopathological severity of inflammation per calf, is shown on the x-axis in panel C, along with the proportion (%) of macroscopic lung lesions per calf on the y-axis. Bronchoalveolar lavage (BAL) was performed on PID −1 and PID 7, and cell types in BAL samples enumerated (Panel D). Stacks represent the mean total number of cells in BAL per ml, with associated standard deviation, as well as the number of neutrophils, macrophages and lymphocytes in BAL per ml. The proportion of eosinophils were <1% in all samples.