Genetic and pathogenic characterization of new infectious bronchitis virus strains in the GVI-1 and GI-19 lineages isolated in central China

Avian infectious bronchitis (IB) is a highly contagious infectious disease caused by infectious bronchitis virus (IBV), which is prevalent in many countries worldwide and causes serious harm to the poultry industry. At present, many commercial IBV vaccines have been used for the prevention and control of IB; however, IB outbreaks occur frequently. In this study, two new strains of IBV, SX/2106 and SX/2204, were isolated from two flocks which were immunized with IBV H120 vaccine in central China. Phylogenetic and recombination analysis indicated that SX/2106, which was clustered into the GI-19 lineage, may be derived from recombination events of the GI-19 and GI-7 strains and the LDT3-A vaccine. Genetic analysis showed that SX/2204 belongs to the GVI-1 lineage, which may have originated from the recombination of the GI-13 and GVI-1 strains and the H120 vaccine. The virus cross-neutralization test showed that the antigenicity of SX/2106 and SX/2204 was different from H120. Animal experiments found that both SX/2106 and SX/2204 could replicate effectively in the lungs and kidneys of chickens and cause disease and death, and H120 immunization could not provide effective protection against the two IBV isolates. It is noteworthy that the pathogenicity of SX/2204 has significantly increased compared to the GVI-1 strains isolated previously, with a mortality rate up to 60%. Considering the continuous mutation and recombination of the IBV genome to produce new variant strains, it is important to continuously monitor epidemic strains and develop new vaccines for the prevention and control of IBV epidemics.