Representative histopathological changes in HE-stained trachea from inoculated guinea pigs day 4 post-inoculation. (A) Negative control. No apparent histopathologic lesions. (B) Bacteria-infected group. No apparent histopathologic lesions. (C) Virus-infected group. Dropout of mucous epithelium and inflammatory cell infiltration (solid arrow). (D) Co-infected group. Dropout of mucous epithelium cells, with many neutrophils and erythrocytes present (solid arrow). Tissue sections were observed under a microscope (Nikon, Japan). Magnification: (A, B, and D) × 400; C × 200.