Fine-scale genetic variation and evolution of West Nile Virus in a transmission "hot spot" in suburban Chicago, USA.

Mosquitoes and birds were sampled for West Nile virus (WNV) in suburban Chicago, USA, in a "hot spot" of arboviral transmission. Viral genetic diversity within this area was similar to that within Illinois and the United States. Diversity was higher among viruses from mosquitoes than from birds, higher among viruses from birds in urban "green spaces" than from birds in residential areas, but lower among viruses from mosquitoes in green spaces than from mosquitoes in residential areas. Viral transmission was distance-limited, as evidenced by decreasing autocorrelation of WNV sequences with increasing geographic separation. The evolutionary rate of WNV within the study area between 21 July and 4 October 2005 was ten times higher than that for WNV across North America between 2002 and 2005. These results indicate that WNV transmission and evolutionary dynamics can vary seasonally and in response to fine-scale environmental conditions and landscape characteristics related to urbanization.