Epidemiological investigation of avian gamma-coronavirus infectious bronchitis virus isolates in South Korea between 2018 and 2025: An eight-year study.

Infectious bronchitis is a highly contagious disease in chickens caused by the infectious bronchitis virus. IBV is spread worldwide and can cause upper respiratory tract and nephropathogenic diseases in chickens. Despite routine vaccination, IBV remains a persistent problem in South Korea due to the emergence of new variants. 111 IBV strains isolated from South Korea between 2018 and 2025 were analyzed and sequenced to provide an update on the evolutionary trajectory of circulating IBVs. Most strains were identified as belonging to the GI-19 lineage, most of which were in the K-IId QX-like group, covering nearly 77% of the strains isolated during the 8-year study, with newly emerging QX-like strains forming a distinctive K-IIe group. Unlike previous epidemiological investigations in South Korea, strains from the GI-15 lineage were greatly reduced. Additionally, recombinant events in the S1 region of two IBV isolates were detected, mainly in the proximal and distal ends, with a combination of a K-IId QX-like strain and the KM91 strain, and the Mass type (H120) and a Korean GI-15 strain, respectively. Overall, the Chinese-origin QX-like strains appeared to be the most prevalent IBV strains circulating in the field.