Emergence of infectious bronchitis virus genotype GI-24 and GI-16 along with predominant circulation of genotype GI-1 in commercial poultry in Bangladesh.

Persistent Infectious bronchitis virus (IBV) outbreaks in Bangladesh, indicate inadequate vaccination strategy, low vaccination quality or inconsistent coverage of all the poultry production in terms of IBV. This study investigated the prevalence and genetic diversity of IBV in commercial poultry farms in Bangladesh. A total of 390 samples from commercial broiler and layer flocks (July 2022-June 2025) were screened by RT-qPCR, with selected positives subjected to in ovo isolation and S1 gene sequencing for phylogenetic analysis. IBV was the most prevalent respiratory pathogen, detected in 26.67% of samples and often co-circulating with others. Phylogenetic analysis of the S1 gene revealed the co-circulation of three distinct IBV genotypes: GI-1, GI-16, and GI-24. The GI-1 (Mass-type) genotype was predominant and exhibited high nucleotide identity (99.8-100%) with commonly used vaccine strains, including Ma5, H120, and B-48. Additionally, two other genotypes were identified for the first time in Bangladesh: GI-24, closely related to nephropathogenic Indian strains, and GI-16 (Q1-like), a highly transmissible variant widely reported in Europe and Asia. Pairwise genetic distance analysis demonstrated measurable divergence between vaccine strains and circulating field isolates, potentially explaining continued viral circulation and disease occurrence in vaccinated flocks. Notable genetic variation was observed within S1 hypervariable regions, including mutations affecting predicted neutralizing B-cell epitopes, suggesting antigenic divergence that may contribute to reduced vaccine-induced protection against emerging genotypes. These findings suggest IBV introduction events in Bangladesh and support continuous molecular surveillance in order to test the cross-protection capability of the commercially available vaccine strains.