BRSV-Snk challenge in calves with low or moderate passive immunity: clinical, virological and pathological outcomes. Experimental challenge by inhalation of aerosolized BRSV passaged in gnotobiotic calves (BRSV-Snk) was performed in three calves (A1-3; study 1) with low levels of BRSV-specific maternal antibodies (MDA), and later reproduced in five calves (C1-5; study 2) with moderate levels of MDA. Clinical scores (Panel A) and BRSV RNA detected by RT-qPCR in nasal swab samples (Panel B), from the day of challenge (post-infection day or PID) 0 to PID 7. Individual values are presented for calves C1-5, and mean values are presented for calves A1-3. For both groups of calves, panel C shows mean extent of macroscopic lung lesions on the y-axis, expressed as a percent of total lung area, and the mean severity of histopathological inflammation, scored from 0 to 3, on the x-axis. For mean values, vertical and horizontal lines indicate standard deviation.