Faculty Opinions recommendation of A newly emergent genotype of West Nile virus is transmitted earlier and more efficiently by Culex mosquitoes.

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Abstract
While not specifically mentioned, to me, this article provides another example of how detailed studies of RNA viruses and their hosts are elucidating the processes and outcomes of selective pressures driving the evolution of viral genotypes over short time periods. This experimental study develops a model for the complete displacement of West Nile virus (WNV) genotypes involved in the initial invasion of North America (NY99) in 1999 and wildtype virus currently circulating (WN02). Both the experimental in vitro and in vivo data support a model, whereby, selection for WNV genotypes is determined by increased vectoral capacity of WN02-infected mosquitoes, mediated by shortening the extrinsic incubation period (EIP) (the time between a mosquito imbibing an infectious bloodmeal and the mosquito’s ability to transmit the virus) by up to four days. The authors determined parameters of the in vitro and in vivo growth of currently (WN02) and previously (NY99) circulating WNV genotypes to develop an evolutionary hypothesis regarding genotype replacement. While in vitro studies in cell culture revealed no difference, replication and systemic dissemination of WN02 virus occurred more rapidly in Culex tarsalis mosquitoes after peroral infection. Bypassing the midgut by intrathoracic inoculation of virus into Culex pipiens suggests that rapid and increased replication of WN02 virus within cells of the midgut mediate selection for this genotype by decreasing the EIP. A decrease in EIP increases the likelihood that a given mosquito will survive long enough to infect an additional vertebrate host, thereby increasing the relative vectoral capacity of WN02-infected mosquitoes. Of particular interest to me, are the disparate findings of recent studies on the evolution of different RNA viruses in North America. Whereas Moudy et al. document the rapid genotypic displacement of West Nile virus (WNV) strain NY99 by strain WN02 in the United States between 1999 and 2002, the findings of another study by Biek et al. {1} indicate the long-term persistence of rabies virus genotypes, associated with raccoons, after accidental introduction and initial geographic dissemination. The data from this article by Moudy et al. suggest genotypic displacement was driven by selection favoring increased relative vectoral-capacity of WN02-infected mosquitoes; however, corresponding studies of the kinetics of viral growth in vertebrate hosts are needed. The Biek et al. study supported a ‘surfing mutation’ model, whereby, specific rabies virus lineages first invading an area have persisted for 15-25 years post-invasion. These findings raise interesting issues about the time-scales and processes that mediate the evolution of different RNA viruses with different modes of transmission (i.e. vector-borne versus directly-transmitted). Reference: {1} Biek et al., Proc Natl Acad Sci US. 2007, 104(19):7993-7998 [PMID:17470818].

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