As an outcome of very active arbovirus monitoring programs that began in Australia in the 1950s, some of the most diverse and unusual rhabdoviruses in the world have been isolated from this continent. These novel rhabdoviruses represent an important and valuable pool of highly diverse viruses; however, most of them have remained poorly characterised. In light of the significant disease potential of numerous rhabdoviruses, the characterisation of novel rhabdoviruses is indispensable for threat assessment to livestock, wildlife and humans and preparedness for outbreaks. The genetic characterisation of novel viruses is also an essential step for the development of molecular detection assays for improved monitoring and investigations into unidentified disease cases. In this study, the complete genomes of four novel rhabdoviruses have been sequenced and a fifth is close to completion. The substantial new data generated has significantly extended the understanding of the biology and evolution of the Rhabdoviridae. Wongabel virus (WONV), isolated from the biting midge Culicoides austropalpalis, was found to contain a unique genome structure encoding ten genes, including five novel genes (Chapter 2). Analysis by western blotting suggested that four out of the five novel genes were expressed in infected cell cultures. Ngaingan virus (NGAV), isolated from Culicoides brevitarsis, was found to have the largest genome of any rhabdovirus sequenced to date, and with thirteen genes has the largest number of genes of any (-) ssRNA virus sequenced to date (Chapter 3). Seven of the thirteen genes are novel. Similar to viruses in the genus Ephemerovirus (bovine ephemeral fever virus and Adelaide River virus), NGAV contains a second glycoprotein with an unknown function. Phylogenetic analysis places this virus alongside WONV and the north-American bird and mosquito-associated Flanders virus within the Hart Park group that remains to be classified by the ICTV. Screening of various wildlife and livestock sera collected in northern Australia indicated a strong association of NGAV with macropods. Tibrogargan virus (TIBV) and Coastal Plains virus (CPV) were isolated from cattle and Culicoides brevitarsis (TIBV). Past serological surveys reported both viruses to be highly prevalent in cattle in northern Australia and demonstrated that the two viruses share a relatively close relationship at the antigenic level. The genomic analyses revealed that these two viruses have a unique genome organization, with three additional genes (Chapter 4). These additional genes are highly diverged at the sequence level but the encoded putative proteins share a significant conservation of secondary structure elements. The sequencing of these two related viruses has provided a unique opportunity to gain insights into the characteristics and evolution of novel proteins in two different rhabdoviruses. Phylogenetic analyses showed that TIBV and CPV form an independent cluster which does not appear to belong to any of the current genera, but which is most closely related to the genus Ephemerovirus based on N protein analysis. Although neither virus has been associated with disease, a serological survey of various animal sera collected in northern Australia showed that these viruses are currently highly prevalent in sentinel cattle and buffalo. Oak Vale virus (OVRV) was isolated from mosquitoes, Culex edwardsi and Ochlerotatus vigilax, from two geographically diverse regions of Australia located approximately 3000 km apart. The genome of OVRV was found to contain only one novel gene (Chapter 5). Comparatively, the genome of this virus is much less complex than the others in this study, but this virus displays considerable divergence from all other rhabdoviruses. A high seroprevalence for this virus was found in the feral pig population in northern Australia. The data generated from this study represents a considerable increase in the quantity of genetic data available for this viral family, and has revealed the existence of a large number of previously unidentified genes, highlighting that that the potential for complexity within the prototype genomic model of a rhabdovirus is much greater than previously thought. The novel nature of the additional genes provides grounds for further research into rhabdovirus evolution. Analysis of this new data suggests that these viruses cannot be classified into existing genera under the current criteria and it is clear that the taxonomy of the Rhabdoviridae requires revision. The observation that these viruses are currently circulating in livestock and wildlife in northern Australia accentuates the need for closer monitoring of animals and the need for further study of this diverse and fascinating group of viruses.
| Identifer | oai:union.ndltd.org:ADTP/279339 |
| Creators | Aneta Gubala |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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