2.7.1  Assessment of pathogenicity in one-day-old SPF chickens
The chickens were separately housed in isolators under consistent conditions, and food and water was provided ad libitum. A total of 120 one-day-old SPF chickens were randomly divided into four groups (n = 30 per group). The chickens in the experimental groups were intranasally inoculated with 200 μL allantoic fluid per chick containing 105EID50 of the rH120, rIBYZ, and rH120-S1/YZ strains. The control group (n = 30) was inoculated with 200 μL sterile PBS via the same route. The morbidity and mortality was followed-up for 14 days. All experimental groups were monitored daily for clinical signs related to IB infection for 14 days post-infection (dpi), including coughing, sneezing, and tracheal rales. Dead chickens were examined for gross tracheal, lung, and kidney lesions.