GENETIC-HETEROGENEITY AMONG ISOLATES OF ROSS RIVER VIRUS FROM DIFFERENT GEOGRAPHICAL REGIONS

The RNase T1 maps of 80 isolates of Ross River virus from different re gions of mainland Australia and the Pacific Islands were compared. Fou r different clusters of isolates with greater than an estimated 5 to 6 % diversity at the nucleotide level were found. There was a pattern of differences between eastern and western Australian strains; however, the pattern was disturbed by overlaps and incursants. Pacific Islands isolates belonged to the eastern Australian topotype. Our findings sug gest that certain genetic types of Ross River virus predominate in dif ferent geographical regions. In contrast, populations of other importa nt Australian arboviruses (Murray Valley encephalitis, Kunjin, and Sin dbis viruses) are distributed across the Australian continent as minor variants of one strain. Our data also show that in one region, strain s of Ross River virus with identical RNase T1 maps circulate during bo th years when epidemics occur and years when they do not. This finding suggests that Ross River virus epidemics are not dependent on the int roduction or evolution of new strains of the virus. Two strains, belon ging to the eastern Australian topotype, were isolated in Western Aust ralia. It is likely that viremic humans or possibly domestic livestock travelling by aircraft were responsible for this movement.