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. 2015 Nov:485:79-85.
doi: 10.1016/j.virol.2015.07.004. Epub 2015 Jul 25.

Genotype-specific variation in West Nile virus dispersal in California

Nisha K Duggal  1 , William K Reisen  2 , Ying Fang  2 , Ruchi M Newman  3 , Xiao Yang  3 , Gregory D Ebel  4 , Aaron C Brault  5
Affiliations

Genotype-specific variation in West Nile virus dispersal in California

Nisha K Duggal et al. Virology. 2015 Nov.

Abstract

West Nile virus (WNV) is an arbovirus that was first reported in North America in New York in 1999 and, by 2003, had spread more than 4000 km to California. However, variation in viral genetics associated with spread is not well understood. Herein, we report sequences for more than 100 WNV isolates made from mosquito pools that were collected from 2003 to 2011 as part of routine surveillance by the California Mosquito-borne Virus Surveillance System. We performed phylogeographic analyses and demonstrated that 5 independent introductions of WNV (1 WN02 genotype strain and 4 SW03 genotype strains) occurred in California. The SW03 genotype of WNV was constrained to the southwestern U.S. and had a more rapid rate of spread. In addition, geographic constraint of WNV strains within a single region for up to 6 years suggest viral maintenance has been driven by resident, rather than migratory, birds and overwintering in mosquitoes.

Keywords: Arbovirus; Evolution; Phylogenetics; West Nile virus.

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Figures

Figure 1
Figure 1
Temporal distribution of WNV isolates sequenced in this study. (A) The number of WNV positive mosquito pools identified in California (dashed line) and number of human WNV disease cases per year in California (solid line). (B) The number of sequenced WN02 (filled bars) and SW03 isolates (open bars) that were collected per year. (C) The number of sequenced WN02 (filled bars) and SW03 (open bars) isolates that were collected each month. (D) The number of WN02 (filled bars) and SW03 (open bars) isolates stratified by mosquito species. (E) The number of WN02 (filled bars) and SW03 (open bars) isolates collected in northern or southern CA.
Figure 2
Figure 2
Spatial distribution of WNV isolates sequenced in this study. (A) A map showing the location of WN02 (filled circles) and SW03 (open circles) isolates sampled in this study, with locations of mosquito control districts indicated. (B) A map showing the location of isolates sampled in this study stratified by mosquito species.
Figure 3
Figure 3
Phylogenetic relationship between WNV isolates. A MCC phylogeny constructed from the open reading frame of 196 WNV isolates is shown. Nodes with >0.9 posterior probability are indicated by filled circles. Isolates from California are highlighted in gray. The two emergent genotypes of WNV are indicated on the phylogeny by color: WN02 in blue lines and SW03 in orange lines.
Figure 4
Figure 4
Dispersal of WNV isolates in California from 2002 – 2010 (A through E). Solid lines represent the MCC tree of WNV isolates, with 80% HPD regions for the location of uncertain nodes shown by clouds. The two emergent genotypes of WNV are indicated by color: WN02 in blue and SW03 in orange.
Figure 5
Figure 5
Overwintering of WNV in southern CA. (A) The MCC phylogeny of WNV, with the areas of interest highlighted. (B) A close-up view of one branch from the MCC phylogeny, with winter isolate COAV179_2011 highlighted. (B) A close-up view of another branch from the MCC phylogeny, with winter isolate GRLA6143_2007 highlighted. Isolate names include location of collection, isolate number, and date of collection in decimal format. WNV was introduced into California at least 5 times. The SW03 genotype of WNV was constrained to southern California. The SW03 genotype spread with a more rapid dispersal rate than the WN02 genotype. The overall dispersal rate of WNV has decreased dramatically since its emergence. WNV overwintered in California in Culex mosquitoes.
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