Ever since West Nile virus appeared in New York in 1999, scientists have been working on ways to stop the spread of the disease, which claimed 218 lives in the U.S. in 2003. Surveillance programs--to track both the mosquitoes that carry the virus and birds that often succumb to it before people do--have been implemented in 49 states. New findings should help researchers better model potential outbreaks and plan to avoid them. In particular, the results suggest that controlling mosquito levels is key to prevention, whereas removing birds may actually increase the chance of a virus outbreak.

Marjorie Wonham of the Center for Mathematical Biology at the University of Alberta and her colleagues used data from previously published papers on mosquito biology, crow biology and West Nile biology to develop a computer model to predict the risk of an outbreak. "This virus is endemic, and we're probably never going to get rid of it completely unless we say that we're going to kill all the mosquitoes in the world--that's not going to happen," Wonham remarks. "What this work does is tell you just what percentage of mosquitoes it is necessary to kill to keep the virus below an outbreak. This is a first step towards effective management." By changing data values related to climate, such as mosquito life span and biting rate, the program can be tailored for specific regions, the team reports in the latest issue of the Royal Society of London's journal Proceedings B.

Current methods aimed at avoiding West Nile outbreaks include applying chemical larvicides, removing potential mosquito habitats and spraying pesticides to kill adult mosquitoes. "Since applying chemicals and filling in wetlands costs money and causes environmental damage, one would ideally use the minimum amount of control that would still be effective in preventing outbreak," Wonham says. "Our model lets you calculate the threshold mosquito population for West Nile outbreak--you just have to keep them below the threshold level. This means, we would hope, minimal economic cost and environmental damage while still preventing outbreak."