Virus isolations Mosquito collections obtained during most field trips to the north-west of Western Australia have been processed for virus isolation. Until 1985, virus isolation was undertaken by intracerebral inoculation of suckling mice, but this was then replaced by cell culture using C6/36 mosquito, PSEK, BHK and Vero cells. The use of cell culture has significantly reduced the overall virus isolation rate by largely excluding arboviruses, rhabdoviruses and most bunyaviruses, but is as effective as suckling mice for the isolation of flaviviruses and alphaviruses. MVE virus has been isolated every year that significant numbers of adult mosquitoes have been processed except 1983 (Broom et al. 1989; Broom et al. 1992; Mackenzie et al. 1994c). Isolations of MVE, Kunjin and other flaviviruses are shown in Table 8.2. There was a strong correlation between the number of virus isolates in any given year and the prevailing environmental conditions. Thus those years with a heavy, above average wet season rainfall and subsequent widespread flooding yielded large numbers of virus isolates (1981, 1991, 1993) compared with years with average or below average rainfall and with only localized flooding. Although most MVE virus isolates were obtained from Culex annulirostris mosquitoes, occasional isolates were also obtained from a variety of other species, including Culex quinquefasciatus, Culex palpalis, Aedes normanensis, Aedes pseudonormanensis, Aedes eidvoldensis, Aedes tremulus, Anopheles annulipes, Anopheles bancroftii, Anopheles amictus and Mansonia uniformis (cited in Mackenzie et al. 1994b; Mackenzie and Broom 1995), although the role of these species in natural transmission cycles has still to be determined. Virus carriage rates in Culex annulirostris mosquitoes are shown in Table 8.3 for the Ord River area (Kununurra–Wyndham) and Balgo and Billiluna in south-east Kimberley. Very high mosquito infection rates were observed in those years with above average rainfall. Virus spread and persistence Stanley (1979) suggested that viraemic waterbirds, which are often nomadic, may generate epidemic activity of MVE in south-east Australia and in the Pilbara region. In an attempt to understand the genesis of epidemic activity better, our laboratory initiated a long-term study in the arid south-east Kimberley area at Billiluna and Balgo, two Aboriginal communities on the northern edge of the Great Sandy Desert. Occasional cases of Australian encephalitis had occurred in both communities (1978, 1981). The studies have clearly shown that MVE virus activity only occurs following very heavy, widespread rainfall both locally and in the catchment area of the nearby watercourse, Sturt Creek, which results in extensive flooding across its floodplain (Broom et al. 1992). Localized flooding is insufficient to generate virus activity. Two possible explanations can be proposed to account for the reappearance of MVE virus activity when environmental conditions are suitable: either virus can be reintroduced into the area by viraemic waterbirds arriving from enzootic areas further north; or virus may