Declining growth rate of West Nile virus in North America

Abstract

To determine the demographic history of West Nile virus (WNV) in North America, we employed a coalescent method to envelope coding region data sets for the NY99 and WN02 genotypes. Although the observed genetic diversities in both genotypes were of approximately the same age, the mean rate of epidemiological growth of the WN02 population was approximately three times that of the NY99 population, a finding compatible with the recent dominance of the former genotype. However, there has also been a marked decrease in the recent growth rate of WN02, suggesting that WNV has reached its peak prevalence in North America.

FIG. 1.

(a) Maximum a posteriori phylogenetic tree of 110 WN02 genotype viruses from samples obtained during the period from 2001 to 2005. For all branches, the times assigned to each tip correspond to the dates of sampling. (b) Bayesian skyline plot for the WN02 genotype. The bold line represents the median estimate of the effective number of infections through time, with the 95% HPD values shown in the shaded area. The effective number of infections, a measure of relative genetic diversity, is given as N_eτ, where N_e is the effective population size and τ is the generation time. (c) Relative proportions of the NY99 and WN02 genotypes from 1999 to 2005 among the virus isolates analyzed in this study.