The human and animal health impacts of introduction and spread of an exotic strain of West Nile virus in Australia

The overall probability of a WNVoutbreak (WNV released into Australia, susceptible hosts exposed and the virus spread)occurring in the human and the horse population during this time period is estimated tobe 7.0 × 10'6 and 3.9 × 10'6, respectively. These values are largely influenced by the presenceof mosquitoes in aircrafts and whether the introduced infected mosquito contactswild birds. Results of this study suggest there is a low risk of introduction and spread ofan exotic strain of WNV from the U.S via aircraft, and provides an insight into the magnitudeand impact of the spread among human and horse populations. The generic frameworkpresented could be applied to assess the potential introduction of other mosquito-borne diseases(which involve a wild bird transmission cycle) via international aircraft movements.Vector-borne diseases can have substantial impacts on human and animal health, includingmajor epidemics. West Nile virus (WNV) is of particular international importance due to itsrecent emergence and impact in the Western Hemisphere. Despite the presence of a subtypeof WNV (Kunjin virus, KUN) in Australia, a potential ecological niche could be occupiedby an exotic strain of WNV of the North American type. This study assesses the probabilityan exotic strain of WNV enters Australia via an infected mosquito in an aircraft fromthe United States (U.S.) landing at Sydney airport, the probability it spreads to susceptiblespecies and the impact of the resulting outbreak on human and animal health. A release,exposure and consequence assessment were conducted using expert opinion and scientificliterature to parameterise the inputs for the models (OIE, 2009). Following establishment ofWNV in Australia, the spatio-temporal spread of WNV was predicted over a six year periodbased on the Australian human and equine populations at-risk, the known distribution ofother mosquito-borne flaviviruses in Australia, climatic factors, and the spread of WNV inthe U.S. following it's incursion in New York City in 1999. The impact of this spread was measuredas a multiplier of human and equine demographics using the U.S. incidence and casefatality rates as a reference. For an 8 month period from September to April (consideringseasonal impact on mosquito activity during the coldest months in Australia and the U.S.),and assuming WNV is endemic in the U.S., the median probability an infected mosquitois introduced is 0.17, and the median number of infected mosquitoes introduced is predictedto be zero, with a 95th percentile range of one.