2.7.3  Histopathology
After inoculation with 105 EID50 dose of the rH120, rIBYZ, and rH120-S1/YZ strains, three infected chickens per group were randomly selected to be euthanized by bleeding, and the tissue pathology was examined in the different groups at 3, 5, 7, 10 dpi. The tracheas, lungs, kidneys, and bursas of three dead or other randomly-selected chickens from the experimental and control groups (inoculated with PBS) were collected at 5 or 7 dpi and further processed for histopathology. The samples were fixed in formalin, embedded in paraffin, cut into 5 μm sections, and stained with hematoxylin and eosin. The slides were examined under light microscopy for the presence of lesions.