West Nile virus (WNV) spread rapidly across the western United States between the years 2001 and 2004. This westward movement is generally believed to have been mediated by random dispersive movements of resident birds. Little attention has been focused on the possible role of mosquito vectors in dispersing the virus across North America. The mosquito vector largely responsible for WNV amplification and transmission in the western United States is Culex tarsalis. Venkatesan and Rasgon (2010) studied the genetic population structure of C. tarsalis across its range in the United States and used these data to infer patterns of gene flow and dispersal. Gene flow appeared to be relatively unobstructed over large portions of the West Coast and within the Great Plains region. The pattern of genetic clustering in C. tarsalis is congruent with the pattern of invasion of WNV across the western United States, suggesting that movement of this mosquito may be involved in dispersal of WNV. Mosquito movements may occur not only by normal flight, but also mediated by humans and weather patterns. Along with C. tarsalis, C. quinquefasciatus is a major vector of WNV in the western United States, especially in residential areas. Studies in California have shown that C. quinquefasciatus may move several kilometres and travel between riparian and urban habitats, at times moving even farther than C. tarsalis. Together, these and other mosquitos may have played an important role in moving WNV across North America. (Venkatesan and Rasgon 2010)
