Virus detected in the airways of calves after aerosol challenge with either BRSV-Snk or BRSV-Dk. Calves were experimentally infected as described in Figure 1. Daily nasal secretion (NS) samples were collected for eight consecutive days, starting on PID 0. After euthanization on PID 7, bronchoalveolar lavage (BAL) was collected, along with tissue samples from the trachea and lung, for histopathology and immunohistochemistry (IHC) to demonstrate BRSV-antigen (brown stain). BRSV RNA in daily NS (panel A) and BAL (x-axis, panel B) was detected by RT-qPCR, and is expressed as log10 TCID50 equivalent unit. The accumulated virus shed in NS (y-axis, panel B) was calculated as the area under individual curves. Panels C:I-IV show representative IHC-stained sections of: (C:I) trachea from calf A2; (C:II) trachea from calf B3; (C:III) lung from calf A2; and (C:IV) lung from calf B2. Horizontal bars in panels C:I-IV indicate 50 μm.