Scientists have documented a new way in which the West Nile virus can propagate. The findings indicate that the virus can pass from one mosquito to another while they feed on previously uninfected animals. This mode of transmission is significantly faster than the assumed cycle of infection for the disease, which may help to explain why West Nile has spread so rapidly across North America.

Prior to this discovery West Nile virus was thought to pass via mosquitoes in the following way: an infected mosquito dining on a bird's blood would pass the West Nile virus into the bird's bloodstream and after a few days high levels of the virus would be detectable in the bird. When a second mosquito then bit the bird, the virus would be passed along to it. The new results, reported by Stephen Higgs of the University of Texas and his colleagues this week in the Proceedings of the National Academy of Sciences, suggest that this days-long waiting period between mosquito feedings is not required for transmission. The fast-track transmission they observed occurs between 2 and 6 percent of the time. "We were amazed to see that it could happen," Higgs remarks. "It is basically a brand new component of the virus' life cycle."

This method of spreading a disease, dubbed nonviremic transmission, was known to take place in cases of diseases carried by other vectors, such as ticks and black flies, but not mosquitoes. "Direct transfer of virus from the infected mosquitoes that initially feed on them to others that feed on them afterwards could significantly accelerate the spread of the disease," Higgs says. "Instead of only birds infecting mosquitoes, all sorts of animals may be involved, and transmission could be happening much faster because you don't have to wait for a high viremia [high levels of the virus in the bloodstream]."