Abstract:
Infectious bronchitis (IB), caused by infectious bronchitis virus (IBV), is a highly contagious respiratory disease in chickens, causing significant economic losses in poultry industry worldwide. IBV has a high frequency of genetic mutation and recombination in its genome, particularly in the S1 gene, resulting in the emergence of several new IBV variants. This potentially affects the effectiveness of currently used IBV vaccine. Therefore, the continuous monitoring of the currently circulating IBV strains is essential. In this study, the genetic characteristic of IBV strains circulating in Thai chicken flocks during 2014-2016 was investigated by analysis of the complete S1 gene. A total of 629 (37.7%) out of 1,668 samples, collected from all chicken-raising regions of Thailand, was positive for IBV by S1-specific RT-PCR. The results showed that IBV was widely distributed in all chicken-raising regions of Thailand. Among the IBV positive samples collected in this study, 150 IBVs were randomly selected for complete S1 gene sequencing and analysis. The results based on the phylogenetic analysis of the complete S1 gene revealed that the 2014-2016 Thai IBVs characterized in this study were clustered into five genotypes, including QX-like IBV (n=129), Massachusetts (n=12), 4/91 (n=5), Connecticut (n=2) and a novel IBV genotype (n=2). The results demonstrated that QX-like IBV has become the predominant genotype currently circulating in Thailand. Recombination analysis of the S1 gene of the 2014-2016 Thai IBVs showed the emergence of a novel recombinant IBV genotype originating from Thai QX-like IBV and 4/91 vaccine strain, which was first identified in this study. Overall, the results from this study indicate that the genetic characteristic of Thai IBVs has changed when compared to the characteristic described in previous reports, suggesting the continuing evolution of IBV in Thailand. Therefore, this study highlights the importance of continuous IBV surveillance in chickens for effective control and prevention of IB.
