Mosquitoes that spread West Nile virus in North America are a blend of two strains that remain essentially distinct in the Old World, researchers report. Interbreeding may explain why the insects often bite both birds and people in North America but generally only one or the other in Europe.
West Nile virus has caused fatal encephalitis in people during recent U.S. outbreaks but only rarely has it been lethal in Northern European cases. If the hybrid mosquitoes become widespread in Europe, West Nile disease could become a serious problem there, says Dina M. Fonseca of the Smithsonian Institution in Washington, D.C.
In Europe, Culex pipiens mosquitoes come in two varieties. One