Migratory birds modeled as critical transport agents for West Nile Virus in North America

West Nile Virus has spread more rapidly than expected in the Western Hemisphere. We tested Culex mosquitoes and long-distance migratory birds as potential agents of spread for the virus, using a series of techniques, as follows. (1) Mosquito vector distributions were modeled using an ecological niche modeling approach (GARP) to produce a map of suitability of the landscape for mosquito transmission of the virus. (2) Simulations of spread were developed with an algorithm originally developed for modeling the spread of wildfires (EMBYR), seeding an initial presence of the virus in the New York City area. (3) Alternative spread scenarios were developed as (a) just mosquitoes as movement agents (spread simulation seeded once at New York City and allowed to spread across the mosquito suitability surface), versus (b) spread via mosquitoes on local scales in tandem with long-distance colonization with migratory birds as movement agents (spread simulation seeded once at New York City, and again at sites sampled from the winter destinations of birds breeding in southeastern New York State). The first scenario (mosquitoes only) did not coincide with observed patterns of spread, whereas the second (mosquitoes and migratory birds) coincided closely, suggesting that observed patterns of spread are best explained with migratory birds as critical long-distance transport agents; the virus, in regions to which it is transported by migratory birds, then is transmitted enzootically via mosquitoes. Similar simulations of spread were used to predict extensions of the virus in the Western Hemisphere in coming years.