Objectives:To determine the current immune status of high-risk populations of New South Wales and Victoria to the arboviral pathogens, Murray Valley encephalitis (MVE) and Kunjin (KUN) viruses, which are associated with Australian encephalitis (AE), and Ross River (RR) and Kokobera (KOK) viruses which are associated with polyarthritis. Further, to estimate seroconversion rates to these viruses in high-risk populations over the 10-year period 1981-1991. Design and study population: Blood was taken from 2873 permanent residents, children and adults from previously identified high-risk areas in western NSW and northern Victoria. Samples were tested by the haemagglutination-inhibition (HI) test for antibodies to the four viruses. All sera were also tested for MVE and KUN antibodies by the more specific neutralisation test (NT). Ninety-five of the subjects had been seronegative when sampled 10 years previously.Results: Age standardised prevalence rates for f1avivirus HI antibodies (MVE, KUN, KOK) ranged from 66% (Bourke) to 15% (Forbes), and were similar to those observed 10 years previously. However, specific NT antibodies to MVE and KUN were uncommon in all districts except Bourke, indicating a very high level of susceptibility to Australian encephalitis, should a fresh epidemic occur. Whereas KUN virus seems enzootic in NSW and Victoria, MVE did not appear to have been present since the last outbreak in 1974, even in Bourke. Flavivirus antibody rates (as detected by the broadly reactive HI test) greatly exceeded those specifically attributable to MVE and KUN (NT test) or KOK, leading to the speculation that unidentified flaviviruses are responsible for most human infections. Ross River virus antibody prevalence rates exceeded those of flaviviruses in all districts, ranging from 72% (Bourke) to 25% (Cohuna), and were uniformly higher than those observed in 1981. Ten-year seroconversion rates in seronegative panels were 8.5% for flaviviruses and 24.2% for RR virus, and are broadly consistent with the cross-sectional study. Conclusions: Although f1avivirus and

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