How West Nile Virus Became Dangerous

West Nile virus is a mosquito-borne virus that is transmitted between avian hosts and mosquitoes, and until the mid 1990s was associated with only mild infections of humans in Africa and the Middle East. More severe outbreaks of encephalitis were reported in Romania in 1996, and subsequently in Israel, Tunisia, Russia, and North America.

Aaron Brault and colleagues sequenced the genomes of West Nile virus strains that have been sampled globally in recent years, and found that a single mutation in a gene encoding an enzyme called a helicase arose independently on at least three separate occasions in strains associated with outbreaks of disease. When a poorly virulent strain from Kenya was engineered to contain the helicase mutation, it was found to replicate more rapidly and to cause death of inoculated American crows at a much higher rate than that of the original Kenyan virus. The authors also provide evidence that the helicase mutation was positively selected, which they say highlights the potential for viruses like West Nile to adapt rapidly to changing environments, with unpredictable consequences for human health.

Author contact:

Aaron Brault (University of California, Davis, CA, USA)
E-mail: acbrault@ucdavis.edu

Abstract available online.

(C) Nature Genetics press release.

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