Geographic distribution and evolution of Sindbis virus in Australia.

The molecular epidemiology and evolution of Sindbis (SIN) virus in Australia was examined. Several SIN virus strains isolated from other countries were also included in the analysis. Two regions of the virus genome were sequenced including a 418 bp region of the E2 gene and a 484 bp region containing part of the junction region and the 5' end of the C gene. Analysis of the nucleotide and deduced amino acid sequence data from 40 SIN virus isolates clearly separated the Paleoarctic/Ethiopian and Oriental/Australian genetic types of SIN virus. Examination of the Australian strains showed a temporal rather than geographic relationship. This is consistent with the virus having migratory birds as the major vertebrate host, as it allows for movement of virus over vast areas of the continent over a relatively short period of time. The results suggest that the virus is being periodically redistributed over the continent from an enzootic focus of evolving SIN virus. However, SIN virus strains isolated from mosquitoes collected in the south-west of Australia appear to represent a new SIN virus lineage, which is distinct from the Paleoarctic/Ethiopian and Oriental/Australian lineages. Given the widespread geographic dispersal of the Paleoarctic/Ethiopian and Oriental/Australian lineages, it is surprising that the South-west genetic type is so restricted in its area of circulation. Nucleotide sequence data from the C gene of the prototype strain of the alphavirus Whataroa were also determined. This virus was found to be genetically distinct from the SIN virus isolates included in the present study; however, it is clearly SIN-like and appears to have evolved from a SIN-like ancestral virus.