Arbovirus infections in Australia cause significant economic impacts on livestock production and trade. Bluetongue virus (BTV), Akabane virus (AKAV) and bovine ephemeral fever virus (BEFV) are enzootic and seasonally epizootic in cattle in northern and eastern Australia. BTV is an orbivirus that is transmitted by biting midges. It occurs here as 11 serotypes distributed in two episystems, one in the far North and another in the East. AKAV, a bunyavirus also transmitted by biting midges, causes abortions in cattle, primarily at the southern extent of its distribution in NSW. BEFV is a rhabdovirus which occurs as a single serotype. Most likely transmitted by mosquitoes, BEFV causes sweeping epizootics and sporadic outbreaks across its geographic range.
We have applied a combination of mass genome sequencing of virus isolates, vector population genetics and modelling of wind-borne vector displacement to analyse the structure, dynamics and evolution of these episystems. The northern BTV episystem is highly dynamic with regular introductions of new genome segment lineages from the Indonesian Archipelago and frequent segment reassortment within and between the serotypes. In contrast, the eastern BTV episystem displays limited genetic diversity, comprising until recently only two serotypes and a single lineage of most segments. Since 2010, two additional BTV serotypes and new segment lineages have been introduced to the East. AKAV appears to show a similar pattern of dynamic reassortment with related bunyaviruses in the North and limited diversity in the East. Interestingly, although spatial modelling supports regular aerial displacement of midges from Timor, the principal vector of BTV and AKAV (Culicoides brevitarsis) appears to form a single population across the continent. In contrast to BTV, BEFV forms a single evolving lineage from the North through to its southern extent in the East. BEFV appears to have been introduced to Australia on only a single occasion, dating back to 1956. Potential reasons for differences in the spatial dynamics of these midge-borne and mosquito-borne viruses will be discussed.