Risk Assessment of West Nile Virus Vectors biting mosquitoes (Table). In contrast, while some of the species previously suggested as important bridge vectors have high infection rates (Cx. salinarius), are abundant (Aedes vexans, Oc. canadensis), or are extremely efficient vectors in the laboratory (Oc. japonicus), none make up >5% of the total Risk for human WNV infections. Discussion Integrating 4 important aspects of disease transmission into a single measure of Risk suggests that 2 mosquito species that were previously overlooked as vectors for transmission to humans may in fact be the most important. Current WNV management guidelines (3) call for broadly controlling mosquitoes by using both insecticides and water flow management. Our results argue for focusing mosquito control efforts on Cx. pipiens and Cx. restuans, which primarily breed in a small subset of habitats (tires, gutters, catch basins, polluted surface pools) that are dif- ferent from those of many other vectors (20). This focus could substantially reduce the detrimental effects of mos- quito control on nontarget species, especially in wetlands. In addition, focusing control on these habitats and species should improve the effectiveness of control measures and reduce the number of human WNV infections. Finally, reducing the densities of these mosquito species should also decrease transmission of WNV between birds. This management strategy has the dual benefit of decreasing the severity of WNV epidemics in birds and the subsequent spillover to mammals. These results should be placed within their proper spa- tial and geographic context. The most important vectors for transmitting WNV to humans in other regions of the United States are likely to be different. Cx. quinquefascia- tus and Cx. nigripalpus are the predominant vectors of WNV between birds in the southeastern United States (17,21), and Cx. tarsalis and Cx. quinquefasciatus play this role in much of the western United States (19). Broad feeding habits, host switching from birds to mammals in the fall, or both (22–24) make these 3 species likely to also be the dominant vectors in transmitting WNV to humans in these regions. Similarly, while our analysis of vectors in the northeastern United States determined the most impor- tant vectors for human WNV infections by averaging over several years and a multicounty scale, other vectors may be more important on a local scale (e.g., Cx. salinarius near a salt marsh [25]) or during portions of the transmis- sion season. Our results should be verified at smaller tem- poral and spatial scales because averaging over data in which abundance and infection rates negatively covary can produce biased results (26). The validity of our conclusions rests on the assump- tions we have made and the data on which they are based. Of primary importance is the relative number of feedings of Cx. pipiens and Cx. restuans on humans, which is based on abundance estimates generated by using CO2-baited CDC light traps and mosquito blood meals identified from mammalian hosts (as well as a small number from humans). Recent work by Gingrich and Casillas (25) strengthens our results and suggests that feedings by Cx. pipiens on humans may be more common than was previ- ously thought. These researchers compared the landing rates of mosquitoes on humans (which were then captured with an aspirator and identified) with the abundance of mosquitoes trapped with CO2-baited CDC light traps at 4 sites in Delaware. When data from their Table 1 were used, the ratio of mosquitoes caught after landing on a human to those caught by using CO2-baited CDC light traps is 0.36 for Cx. pipiens, 0.40 for Cx. salinarius, and 0.07 for Ae. vexans (25, Table). This finding suggests that in terms of feeding on humans, Cx. pipiens are relatively underrepresented by CO2-baited CDC light traps com- pared to Ae. vexans, which implies that Ae. vexans may be less important and Cx. pipiens more important than our analyses have shown. Of the species considered in both Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 11, No. 3, March 2005 427