We undertook annual surveys of flavivirus virus activity in the community of Billiluna of Western Australia in the southeast Kimberley region between 1989 and 2001. Culex annulirostris was the dominant mosquito species, particularly in years of above average rains and flooding. Murray Valley encephalitis (MVE) virus was isolated in 8 of the 13 years of the study from seven mosquito species, but more than 90% of the isolates were from Cx. annulirostris. The results suggest that MVE virus is epizootic in the region, with activity only apparent in years with average or above average rainfall and increased numbers of Cx. annulirostris. High levels of MVE virus activity and associated human cases were detected only once (in 1993) during the survey period. Activity of MVE virus could only be partially correlated with wet season rainfall and flooding, suggesting that a number of other factors must also be considered to accurately predict MVE virus activity at such communities.
Previously existing sources of data regarding mosquitoes in inland areas of south-west Western Australia are few and qualitative in nature. This is the first attempt to quantitatively investigate mosquito fauna in this region. The existing data are reviewed and compared with the results of the quantitative surveys presented in this paper. Temporal comparisons appear to indicate that mosquito community structure in the region may have changed since the initial surveys in the 1950s from a combination of freshwater-breeding species towards a strong dominance of Aedes camptorhynchus (Thomson), a major vector of Ross River virus in southern Australia. It is speculated that this shift may have been brought about by the increasing area and severity of dryland salinity in the region over the last century, and also may increase the potential for Ross River virus disease transmission.
Abstract. Murray Valley encephalitis (MVE) virus is a mosquito-borne flavivirus causing severe encephalitis with a resultant high morbidity and mortality. In the period 1989-1993, we undertook a cross-sectional and longitudinal study by annually screening members of a small remote Aboriginal community in northwestern Australia for MVE virus antibodies. Of the estimated 250-300 people in the community, 249 were tested, and 52.6% had positive serology to MVE. The proportion testing positive increased with increasing age group, and males were slightly more likely to be positive than females. During the study period, a high proportion of the population seroconverted to MVE; the clinical/subclinical ratio seems to be lower than previously reported. Although MVE is mostly asymptomatic, the devastating consequences of clinical illness indicate that advice should be provided regarding the avoidance of mosquito bites. Our longitudinal study showed that the risk of seroconversion was similar for each age group, not just the young.
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