West Nile virus (WNV) is transmitted through an infected mosquito's bite and one may not experience any signs or symptoms except a skin rash and headache. However, some people who become infected develop a life-threatening illness that includes inflammation of the brain.
The virus which mainly infects birds, was not a serious threat to humans until the mid-1990s when deadly outbreaks occurred in Israel, Romania, Russia and eventually North America.
Aaron Brault and colleagues discovered that new strains of the virus have a mutation in the gene for helicase, a protein involved in viral replication.
Researchers suggested that the virus had a selective advantage as the mutation occurred independently in each strain.
As part of the study, researchers injected American crows with a weak strain of virus engineered to have the helicase mutation to confirm their suspicion.
The experiment found that the death rate was increased from 25 per cent to nearly 100 per cent and the rate of virus replication also increased, resulting in amount of circulating viral particles rising by a factor of at least 10,000.
'Mosquitoes feeding on these birds would take in more than 1 million viral particles in a meal. This would greatly enhance the efficiency of infection,' NewScientist.com quoted Brault, as saying.