Molecular Epidemiology Of Avian Infectious Bronchitis Viruses Isolated In Guangxi

Infectious bronchitis is currently one of the highly contagious respiratory diseases in chickens caused by infectious bronchitis virus (IBV) which is a member of coronavirus. IBV mainly causes damages in respiratory tissue, reproductive system and kidney, and results in decreased both egg production and quality and poor performance. IBV is known to have high potential of point mutation, gene recombination, gene deficiency and gene insert which induce the virus variation. There are a number of IBV serorypes and there is poor cross-protection between them, so different serotypes and genotypes emerge frequently and could result in immunization failure. Recently, IB has occurred frequently in lots of poultry flocks in Guangxi in spite of vaccination which is economically devasting to the poultry industry. Therefore, study and know the phylogeny and variation on the isolates of IBV from the commercial chickens in Guangxi is the key for effective control of the disease.Samples were collected from the chicken flocks with typical clinical symptoms of IB from Nanning, Yulin, Liuzhou. Virus isolations were carried out in 9～11 day embryonated eggs by inoculating via the allantoic cavity. The samples of embryos were detected routinely for bacteria, Newcastle disease virus (NDV) and Avian influenza virus (AIV) by culture and hemagglutination (HA) test. The negative samples were further investigated by dwarfing chicken embryo test, growth interference with NDV and nucleic acid detection by RT-PCR. The data indicated that 20 isolates didn't hemagglutinate chicken red blood cells directly but interfere with NDV. Gross lesions were observed in chicken embryo after four passages in chicken embryo. IBV S1 gene hypervariable region I (HVR I) of all isolates were successfully amplified by RT-PCR.S1 gene hypervariable region I (HVR I) of 22 IBV strains isolated in Guangxi between 1985 and 2007 were sequenced and compared to that of other IBV reference strains and pigeon coronavirus isolates. A phylogenetic tree based on amino acid sequences of HVR I of all the viruses showed that they were classified into 5 distinct clusters. 16 out of IBV isolates were grouped into cluster I , and have higher homology with pigeon coronavirus isolates but lower homology with the Masschusetts (Mass) type vaccine strains. There were 4 and 3 amino-acid residues inserted at the sites of 33-34 and 34-35 respectively in 15 of the isolates, except that of isolate GX-NN6 was 4 amino-acid residues inserted at the both sites; Isolates GX-YL1 and GX-NN2 had close relationship with Mass type vaccine strains, and they shared Cluster II; Isolates GX-G and GX-XD of cluster III had close relationship with the Japanese strain JP Miyazaki 89 which was isolated at the same period; Isolates GX-YL6 and GX-NN7 of cluster V had close relationship with the European strain 4/91. The results showed that there were hight phylogenetic diversity among the IBVs prevailled in the field in Guangxi, resulting from the commonly occurred mutation or insertion within the S1 gene HVR I of the viruses, and the majority of the isolates has lower homology with the commonly used Mass type vaccine strains, shared much higher homology between the viruses which were isolated in the same period, but with no distinct difference in geological origins.Chickens were challenged with 6 representative isolates of different genotypes. The results showed that 6 isolates had strong pathogenicity to chickens with the morbidity of 60%～100%, GX-NN4, GX-NN6 and GX-YL5 even caused the mortalities of 30%, 20% and 60%, respectively. Pathological examination showed that besides severe lesions of respiratory tract and nephropathy, isolates can also caused obviously lesions in digestive system and immune system. The order of viral antigens appeared in the organs was respiratory system, urinary system, digestive system, immune system. The results showed that the prevalent IBV strains(GX-NN4,GX-NN6,GX-YL5) isolated in Gunagxi were the virulent strains which caused the clinical problems of respiratory and urinary system in commercial chicken flocks.The results of the study demonstrated that IBV infections have been popular in the commercial chicken flocks. The recently prevalent IBVs isolated in Guangxi have a high variability and have strong pathogenicity to chickens. These data will be helpful to the applications of the vaccines in the field and the development of new vaccine in future.