ARBOVIRUS INFECTIONS OF HUMANS IN HIGH-RISK AREAS OF SOUTH-EASTERN AUSTRALIA - A CONTINUING STUDY

Objectives: To determine the current immune status of high-risk popula tions of New South Wales and Victoria to the arboviral pathogens, Murr ay Valley encephalitis (MVE) and Kunjin (KUN) viruses, which are assoc iated with Australian encephalitis (AE), and Ross River (RR) and Kokob era (KOK) viruses which are associated with polyarthritis. Further, to estimate seroconversion rates to these viruses in high-risk populatio ns over the 10-year period 1981-1991. Design and study population: Blo od was taken from 2873 permanent residents, children and adults from p reviously identified high-risk areas in western NSW and northern Victo ria. 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). Ni nety-five of the subjects had been seronegative when sampled 10 years previously. Results: Age standardised prevalence rates for flavivirus HI antibodies (MVE, KUN, KOK) ranged from 66% (Bourke) to 15% (Forbes) , and were similar to those observed 10 years previously. However, spe cific NT antibodies to MVE and KUN were uncommon in all districts exce pt Bourke, indicating a very high level of susceptibility to Australia n 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 t hose specifically attributable to MVE and KUN (NT test) or KOK, leadin g to the speculation that unidentified flaviviruses are responsible fo r most human infections. Ross River virus antibody prevalence rates ex ceeded those of flaviviruses in all districts, ranging from 72% (Bourk e) to 25% (Cohuna), and were uniformly higher than those observed in 1 981. Ten-year seroconversion rates in seronegative panels were 8.5% fo r flaviviruses and 24.2% for RR virus, and are broadly consistent with the cross-sectional study. Conclusions: Although flavivirus and alpha virus infections have occurred at a ''steady rate'' in western NSW and northern Victoria, there is a general lack of immunity to the agents of Australian encephalitis in all centres except Bourke. This needs to be considered in public health policy in these areas.