Phylogenetic distribution and predominant genotype of the avian infectious bronchitis virus. Part 2
It was documented that nephropathogenic type IB has become more and more prevalent in China. The unprecedented economic losses caused by the nephropathogenic IB suggested that selecting the appropriate vaccine strain against the IB outbreaks is of great importance. However, the integrated natures of novel circulating IBV strains in mainland China were not well-learned.
The previous study by other researchers has been revealed that the variation in S1 sequences was closely confirmed relative to the emergence of novel strains, and S1 gene sequence was a good predictor of challenge of immunity in chickens. This study was conducted to identify the IBV strains that have escaped immune defenses conferred by vaccination in China. The genetic characterization of recent IBV field isolates in China was performed by sequencing the whole S1 genes, sequence alignment and phylogenetic analysis compared with other reference strains.
Results
Eighty IBV strains isolated during 2008-2009 in China
From unhealthy birds suspected of IBV infection in the vaccinated chicken flocks from Guangdong, Guangxi, Fujian, Hainan, Jiangsu, Zhejiang, Chongqing, Hubei, Sichuan and Jiangxi province of China, 80 filed IBV strains were isolated during 2008-2009. The isolation rates in the two years were season-dependent to some extent, 30 strains were isolated in October, while only seven strains were isolated in summer (from June to August). The ages of flocks at the time of the outbreak varied between 4 and 69 days. Most of the strains were isolated from the chickens between 10 to 30 days of age.
After three passage propagation, IBVs of all isolates induced peripheric lesions and growth retardation of embryo at 72 h post-inoculation. Since the fourth day post-inoculation, most of the chicks were listless and huddled together, showed ruffled feathers. The results of virus recovery in chicks indicated 87.5% (70/80) isolates caused serious kidney lesions, which were presented with swollen specked kidney and distended ureters filled with uric acid were nephropathogenic type, and the other ten isolates in the study caused respiratory system signs, which were consistent with the clinical record of each strain.
Homologies among S1 nucleotide and deduced amino acid sequences
The obtained strains were characterized phylogenetically by nucleotide sequence analysis of the hyper-variable S1 gene of IBV. The nucleotide and amino acid sequence similarities between the eighty IB strains were ranging from 75.4% (strain CQ8 and HY) to 100% (strain PT1 and PT3) and 73.9% to 100%, respectively. Compared to the 28 reference strains published in the GenBank, the identity of the nucleotide and amino acid sequence among the 108 isolates (including the 80 isolates in this study plus the 28 reference strains) were 75.1 to 99.8% and 73.1 to 99.8%, respectively, indicating low homology and high variation among the isolated and reference strains.