Ross River virus genetic variants in Australia and the Pacific Islands

HaeIII and TaqI restriction digest profiles of cDNA to infected cell RNA or virion RNA were used as a guide to genetic relationships between fourteen isolates of Ross River virus (RRV) obtained from mosquitoes collected in various localities in eastern Australia where the virus is endemic. RRV isolates from Fiji, American Samoa, the Cook Islands and the Wallis Islands where major outbreaks of epidemic polyarthritis took place in 1979-1980 were also examined. Among these RRV isolates we have identified three genetic types (I-III) on the basis of differences between their restriction digest profiles. We estimate that 1.5-5% nucleotide sequence diversity exists between genetic types. Within each genetic type strain differentiation gave rise to small but significant differences in restriction digest profiles. No clear pattern of geographic distribution of RRV genetic types could be established from the limited number of RRV isolates examined. Genetic types I, II and III, respectively, were isolated from three, three and one different mosquito species, indicating there is no strong association between genetic type and the species of mosquito vector. HaeIII restriction digest analysis did not detect any genetic difference between the four Pacific Island isolates, suggesting that a single RRV variant was involved in the epidemics. Genetically, this variant was closely related to isolates of genetic type II. Virtually identical HaeIII restriction digest profiles were observed for isolates obtained at various stages of the Pacific Island epidemics, suggesting that extensive sequence evolution did not accompany Ross River virus spread.