Development and progression of respiratory pathology following PRCV ISU-1 and 135 inoculations. (A) Representative gross lung pathology from 135-infected pig and percent area of lung consolidation for pigs infected with ISU-1 (black bars) or 135 (red bars) at 5 DPI. Arrowheads indicate areas of consolidation. Images are dorsal and ventral views. (B) Histopathology and immunohistochemical scores for nasal turbinate, trachea, and lung for 1, 5, 11, and 20 DPI. (C) Representative histopathology from 1 and 5 DPI. (D) Representative viral IHC from 1 and 5 DPI. Virus antigens were first detected in the nasal turbinate, with particular widespread labelling in the olfactory component, at 1 DPI in both groups, and was similar at 5 DPI. In the lower respiratory tract, virus antigens were only detected in the trachea and lung of 135-infected animals at 5 DPI, and these areas were colocalized with tracheitis and bronchopneumonia, respectively. Images taken at 400× for nasal turbinate and trachea, 200× for lung. H&E and IHC were serial sections. Two-way ANOVA and Bonferroni multiple tests were performed. Asterisks represent significant changes (*P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001). (E) Spearman rank correlation between respiratory pathology and viral load at 5 DPI from ISU-1- and 135-infected animals. Asterisks represent significant changes (*P < 0.05, **P < 0.01).