Veterinary Science - Theses
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ItemInvestigations into the molecular basis of pathogenicity of the Australian infectious bronchitis viruses( 2012)Infectious bronchitis virus (IBV) is a highly infectious pathogen of poultry and has been detected in commercial flocks worldwide. Due to the labile nature of its large single-stranded RNA genome, numerous strains with widely varying properties have been detected and characterised so far. Australian IBV strains have been shown, using molecular techniques, to be highly divergent from international strains. The work described in this thesis aimed to identify and characterise a number of Australian IBV strains and to relate their pathogenicity to the genomic structure of the 3' end of their genome. Initially, high resolution melt (HRM) curve analysis was used to detect novel strains of IBV in field submissions. One such strain, N1/08, was found to have emerged as a result of recombination between subgroup 2 and subgroup 3 strains. An in vivo study determined that the pathogenicity of N1/08 was more similar to subgroup 2 strains. HRM curve analysis also detected a variant strain in the commercial preparation of the IBV vaccine, VicS. Two subpopulations were isolated from the VicS vaccine, namely VicS-v and VicS-del. It was determined that VicS-v and VicS-del grew similarly in embryonated chicken eggs, but that the pathogenicity of VicS-del was significantly lower than that of VicS-v in vivo. Remarkably, an equal ratio of VicS-v and VicS-del appeared more pathogenic in vivo than VicS-v alone. This suggested that there may be a synergistic relationship between VicS-v and VicS-del in vivo. Genetic characterisation of the VicS-del structural protein gene region found a number of differences compared to VicS-v, including a 40 bp deletion in the 3' UTR, providing an explanation for the distinct HRM curve pattern of the VicS vaccine. In order to investigate whether differences in the nucleotide sequence of the structural protein genes were reflected in differential transcription of the novel strains studied, the transcriptomes of N1/08, VicS-v and VicS-del and other reference Australian IBVs were examined. No difference in quantity of RNA transcripts was observed, however differences were observed in the number and lengths of transcripts between strains, including the lack of a gene 3 transcript for subgroup 2 strains. This investigation also found that the substantial gene rearrangements and mutations present in subgroup 2 strains may have abolished transcription of gene 3, which encodes a structural envelope protein.