Isolation of Murray Valley encephalitis virus and other arboviruses in the Ord River Valley 1972-1976.

This paper summarizes the isolation of arboviruses from mosquitoes collected in the Ord Valley between 1972 and 1976. A total of one hundred and ninety five strains of at least fifteen antigenically distinct viruses have been isolated. Seven of these isolates appear to be "new' antigenic types, and several are undergoing further testing. These are three new rhabdoviruses (Kununurra [OR194], a virus provisionally named Kimberley [OR250] and OR189 [provisionally named Parry's Creek]), three ungrouped, non-haemagglutinating viruses (OR379, OR512, OR869) and a virus (OR540) which reacts to Poly Anopheles A world grouping fluid. The remaining viruses have been previously identified in Australia. These include Murray Valley encephalitis (MVE), Kunjin, Kokobera, Sindbis, Koongol, Wongal, Wongorr and a virus in the Corriparta serological group. The most important finding of these studies is that MVE displays as annually recurrent pattern of activity with a peak seasonal transmission rate at the end of the wet monsoon. This is the first definition of a probable endemic focus of MVE activity in Australia. The major vector for the majority of the viruses isolated was, by inference, Culex annulirostris. However, Aedeomyia catasticta was implicated as a major vector of the Corriparta group virus.

A preliminary investigation of the ecology of arboviruses in the Derby area of the Kimberley region, Western Australia.

A survey of mosquito populations in the Derby area of the Kimberley region, Western Australia, in March/April of 1977 yielded a total of 3,318 adult female mosquitoes. Fifteen taxa were represented, seven being new locality records for this area. Culex annulirostris was the dominant species, comprising 85.41% of the total catch. All mosquitoes collected were processed for virus isolation and thirteen strains of four (and possibly five) distinct arboviruses were obtained, all from pools of Culex annulirostris. These viruses include Murray Valley encephalitis, Ross River, Wongal, an untyped non-haemagglutinating member of the Koongol group and a virus which reacts to polyvalent antisera against the Anopheles A and B groups. With the exception of Ross River, all these viruses had been previously isolated from the Ord River Valley, some 500 km to the north-east. Comparison of virus isolations in the Ord Valley and Derby supports the suggestion that both sites share a common viral flora. Both also display an exceedingly high overall isolation rate (approximately 20% for Cx. annulirostris pools). Such comparisons suggest that a number of arboviruses transmitted by Cx. annulirostris are active throughout the Kimberley region and have peak isolation rates at the end of the wet season. Further studies are needed to fully define these viral cycles.