West Nile Virus Risk Assessment and the Bridge Vector Paradigm A. Marm Kilpatrick,* Laura D. Kramer,† Scott R. Campbell,‡ E. Oscar Alleyne,§ Andrew P. Dobson,¶ and Peter Daszak* In the northeast United States, control of West Nile virus (WNV) vectors has been unfocused because of a lack of accurate knowledge about the roles different mosquitoes play in WNV transmission. We analyzed the risk posed by 10 species of mosquitoes for transmitting WNV to humans by using a novel risk-assessment measure that combines information on the abundance, infection prevalence, vector competence, and biting behavior of vectors. This analysis suggests that 2 species (Culex pipiens L. and Cx. restuans Theobald [Diptera: Cilicidae]) not previously considered important in transmitting WNV to humans may be respon- sible for up to 80% of human WNV infections in this region. This finding suggests that control efforts should be focused on these species which may reduce effects on nontarget wetland organisms. Our risk measure has broad applica- bility to other regions and diseases and can be adapted for use as a predictive tool of future human WNV infections. SWest Nile virus (WNV) has spread across the continent ince its first appearance in North America in 1999, and into Central America. It has infected >17,000 persons and caused >670 deaths (1,2). Reducing the number of human cases of WNV through vector control depends on efficiently using limited resources (3), which requires understanding which vectors are most important in trans- mitting WNV to humans. Previous research has suggested that different mosqui- to species play different roles in spreading WNV. Culex pipiens L. and Cx. restuans Theobald mosquitoes are thought to be the primary amplification vectors of WNV in *Consortium for Conservation Medicine, Wildlife Trust, Palisades, New York, USA; †New York State Department of Health, Albany, New York, USA; ‡Suffolk County Department of Health Services, Yaphank, New York, USA; §Rockland County Health Department, Pomona, New York, USA; and ¶Princeton University, Princeton, New Jersey, USA birds in the northeastern and north-central United States (4) because they are primarily ornithophilic, or bird-biting, are abundant, and have the highest prevalences for WNV in this region (5). These species have not been considered important in transmitting WNV to humans because of their feeding habits (3,4). Instead, other mosquitoes, which take a larger fraction of their blood meals from mammals, are thought to be bridge vectors in transmitting WNV to humans (3,4). Species that have been proposed as bridge vectors include members of the genera Aedes and Ochleratus and other Culex species (3,4). However, classi- fication of mosquito species as enzootic or bridge vectors was previously based primarily on qualitative categories and did not incorporate other data that are critical to deter- mining the risk for human infection from each species. In this study, we integrate quantitative information on the abundance, WNV infection prevalence, vector compe- tence, and biting behavior of the most important vectors in the northeast and north-central United States to predict the risk for human infection from each species. Materials and Methods The probability or risk that a species of mosquito will infect a human with WNV can be estimated as Risk = A × Fm × P × Cv where A is the abundance, Fm is the fraction of blood meals taken from mammals, P is the WNV infection prevalence, and Cv is an index of vector competence (the fraction of WNV-infected mosquitoes that will transmit virus in a sub- sequent bite). Our equation for Risk (capitalized to denote our calculated expression) is an estimate of the relative number of WNV-infectious bites on mammals by each mosquito species. We discuss the data we used for each variable in turn. We used abundance data from 2 counties near the orig- inal 1999 outbreak (Suffolk and Rockland) in New York Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 11, No. 3, March 2005 425