Investigation of the southern limits of Murray Valley encephalitis activity in Western Australia during the 2000 wet season.

Western Australia experienced its worst-ever outbreak of the mosquito-borne Murray Valley encephalitis (MVE) virus during the 2000 wet season. Highest-on-record rainfall throughout much of the state during the 2000 wet season gave rise to extensive mosquito breeding and increased MVE virus transmission, resulting in nine cases of encephalitis. Activity of MVE virus in Western Australia is monitored by detecting MVE virus-specific antibodies in serum from sentinel chickens, located at towns and communities throughout the north of the state. However, during 2000, all 28 flocks of chickens seroconverted to MVE virus, including a flock located >600 km further south than MVE virus activity had ever previously been recorded. Furthermore, the majority of the nine cases of encephalitis occurred outside the enzootic Kimberley region. We therefore undertook a major serosurvey of domestic chicken flocks both south and east of the previously defined regions of virus activity. The results suggest that MVE virus activity extended as far south as the Midwest and northern Goldfields during 2000. A new southern limit of activity of MVE virus is therefore proposed. The results have implications for managing outbreaks of MVE virus in Western Australia and have enabled us to locate additional sentinel flocks as part of the MVE surveillance program for future years.