Characterization of mosquito-adapted West Nile virus

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Abstract
West Nile virus (WNV), a mosquito-borne flavivirus, has significantly expanded its geographical and host range since its 1999 introduction into North America. The underlying mechanisms of evolution of WNV and other arboviruses are still poorly understood. Studies evaluating virus adaptation and fitness in relevantin vivosystems are largely lacking. In order to evaluate the capacity for host-specific adaptation and the genetic correlates of adaptationin vivo, this study measured phenotypic and genotypic changes in WNV resulting from passage inCulex pipiensmosquitoes. An increase in replicative ability of WNV inC. pipienswas attained for the two lineages of WNV tested. This adaptation for replication in mosquitoes did not result in a replicative cost in chickens, but did decrease cell-to-cell spread of virus in vertebrate cell culture. Genetic analyses of one mosquito-adapted lineage revealed a total of nine consensus nucleotide substitutions with no accumulation of a significant mutant spectrum. These results differed significantly from previousin vitrostudies. When St Louis encephalitis virus (SLEV), a closely related flavivirus, was passaged inC. pipiens, moderately attenuated growth inC. pipienswas observed for two lineages tested. These results suggest that significant differences in the capacity for mosquito adaptation may exist between WNV and SLEV, and demonstrate that further comparative studies in relevantin vivosystems will help elucidate the still largely unknown mechanisms of arboviral adaptation in ecologically relevant hosts.