Diversification of West Nile virus in a subtropical region

Daniel M Chisenhall,Christopher N Mores

doi:10.1186/1743-422x-6-106

Abstract

BackgroundWest Nile virus (WNV) has spread across North, Central, and South America since its introduction in 1999. At the start of this spread, Florida was considered a potentially important area with regards to transmission due to its geographic, climatological, and demographic conditions. Curiously, the anticipated high levels of transmission or disease outbreaks have not been observed. As other studies have predicted that the lack of intense WNV transmission is not due to vector incompetence, we sought to evaluate the role of viral strain diversity in WNV transmission in Florida. Therefore, a phylogentic analysis was carried out on several isolates collected from three distinct locations in Florida.ResultsContrasting with a positive control collected in Indian River County, Florida during 2003 that contains the original NY99 genotype with valanine at amino acid 159 of the envelope region, all of the isolates collected in 2005 contain the WN02 genotype composed of a substation with alanine at that position indicating the window of introduction of the WN02 genotype occurred between 2003 and 2005. From the eight isolates collected in Duval, Indian River, and Manatee Counties; there is also a silent nucleotide substitution that differentiates the isolates collected on the Atlantic side of the state compared to the isolate collected on the Gulf side, which groups closer to isolates from other locations near the Gulf.ConclusionAs a whole, the Florida isolates contained numerous variable nucleotide and amino acid sites from the reference sequences, as well as each other; indicating greater nucleotide diversity within the Florida 2005 isolates than within other regions. Finally, a series of three amino acid substitutions surrounding a set of histidines located in the envelope coding region that hypothesized to play a role in conformational changes was found in the isolate collected in Indian River County, perhaps changing the antigenicity of the homodimer. Taken together, these findings expand our understanding of the temporal and spatial compartmentalization of West Nile virus subtypes within North America.

Highlights

West Nile virus (WNV) has spread across North, Central, and South America since its introduction in 1999
It has been suggested that the reason for this shift was due to the ability of the WN02 strain to be transmitted after two fewer days of extrinsic incubation compared to the NY99 strain, thereby giving it a competitive edge [6]
Our phylogenetic analysis of the envelope sequence along with the corresponding sequences from several other strains obtained from GenBank showed that the isolates from Florida were clustered mostly together with the exception of isolate #967. This isolate was one of six collected over a 58 day period beginning August the 23rd and ending September the 30th. This isolate appears to be part of the 2002 North American clade, as defined by Ebel et al [5], yet it has two additional substitutions at nucleotide positions 2209 and 2233 that lead to two translated amino acid substitutions at 415 (Ala to Thr) and 423 (Asp to Asn) respectively

Summary

Introduction

West Nile virus (WNV) has spread across North, Central, and South America since its introduction in 1999. West Nile virus (WNV) is a member of the family Flaviviridae and in particular, part of the Japanese encephalitis serocomplex It consists of a single-stranded positivesense RNA genome that is contained in a virion that is approximately 50 nm in diameter [1]. It has been suggested that the reason for this shift was due to the ability of the WN02 strain to be transmitted after two fewer days of extrinsic incubation compared to the NY99 strain, thereby giving it a competitive edge [6] This shift occurred during 2002 and 2003, which coincided with a peak in human cases of WNV infections [7], suggesting the importance of viral variant emergence

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