Pathogenicity of the rIBV rH120, rIBYZ, and rH120-S1/YZ strains. (A) Percent survival of SPF chickens infected at 1-day-of age during the 14-day observation period. No mortality was observed in rH120, rH120-S1/YZ, or the control groups. (B) Gross lesions on the lungs and kidneys of the infected chickens at 5 or 7 dpi.