3.2.1  Pathogenicity of the recombinants in one-day-old SPF chickens
The earliest death was observed at 3 dpi in the QX-like strain rIBYZ group, which virus was highly pathogenic to one-day-old SPF chickens; the final mortality of this group was 63.3% (Fig. 3 A). The morbidity of the infected chickens in the rIBYZ group reached higher than 80%, and the diseased chickens exhibited respiratory symptoms (e.g., sneezing, coughing, as well as tracheal and bronchiolar rales), and the severe cases presented with additional signs of listlessness, huddling, and ruffled feathers. At necropsy, lesions were detected both in the respiratory and urinary system, including mucus, hyperemia, and hemorrhage in the trachea, as well as swelling and urate deposition in the kidney. Edema and congestion were observed in the lungs in 20% of cases (Fig. 3B). However, following inoculation with the rH120 or rH120-S1/YZ strains, the morbidity rates of the chickens was less than 20%, with moderate respiratory signs of coughing in some of the chickens. All of the infected chickens presented no visible lesions in the kidney and the survival rate was 100% through 14 dpi in both the rH120 and rH120-S1/YZ groups (Fig. 3A and B). No obvious clinical signs, gross lesions, or death attributable to IBV was observed in the control group.
Fig. 3 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.