The Efficacy of a Live Attenuated TW I-Type Infectious Bronchitis Virus Vaccine Candidate

Infectious bronchitis (IB) is a highly contagious avian disease caused by infection with infectious bronchitis virus (IBV), which seriously affects the development of the global poultry industry. The distribution of TW I-type IBV in China has increased in recent years, becoming a widespread genotype. We previously isolated a TW I-type IBV strain termed CK/CH/GD/GZ14 in 2014, but its pathogenicity and possibility for vaccine development were not explored. Therefore, this research aimed to develop a live-attenuated virus vaccine based on the CK/CH/GD/GZ14 strain. The wild type IBV CK/CH/GD/GZ14 strain was serially passaged in SPF embryos for 145 generations. The morbidity and mortality rate of wild-type strain in 14 day-old chickens is 100% and 80% respectively, while the morbidity rate in the attenuated strain was 20% in the 95th and 105th generations and there was no death. Histopathological observations showed that the pathogenicity of the 95th and 105th generations in chickens was significantly weakened. Further challenge experiments confirmed that the attenuated CK/CH/GD/GZ14 strain in the 95th and 105th generations could resist CK/CH/GD/GZ14 (5th generation) infection and the protection rate was 80%. Tracheal cilia stagnation, virus shedding, and viral load experiments confirmed that the 95th and 105th generations provide good immune protection in chickens, and the immunogenicity of the 105th generation is better than that of the 95th generation. These data suggest that the attenuated CK/CH/GD/GZ14 strain in the 105th generation may be applied as a vaccine candidate against TW I-type IBV.