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Host Selection by Culex pipiens Mosquitoes and West Nile Virus Amplification

Recent field studies have suggested that the dynamics of West Nile virus (WNV) transmission are influenced strongly by a few key super spreader bird species that function both as primary blood hosts of the vector mosquitoes (in particular Culex pipiens) and as reservoir-competent virus hosts. It has been hypothesized that human cases result from a shift in mosquito feeding from these key bird species to humans after abundance of the key birds species decreases. To test this paradigm, we performed a mosquito blood meal analysis integrating host-feeding patterns of Cx. pipiens, the principal vector of WNV in the eastern United States north of the latitude 36°N and other mosquito species with robust measures of host availability, to determine host selection in a WNV-endemic area of suburban Chicago, Illinois, during 2005–2007. Results showed that Cx. pipiens fed predominantly (83%) on birds with a high diversity of species used as hosts (25 species). American robins (Turdus migratorius) were marginally overused and several species were underused on the basis of relative abundance measures, including the common grackle (Quiscalus quiscula), house sparrow (Passer domesticus), and European starling (Sturnus vulgaris). Culex pipiens also fed substantially on mammals (19%; 7 species with humans representing 16%). West Nile virus transmission intensified in July of both years at times when American robins were heavily fed upon, and then decreased when robin abundance decreased, after which other birds species were selected as hosts. There was no shift in feeding from birds to mammals coincident with emergence of human cases. Rather, bird feeding predominated when the onset of the human cases occurred. Measures of host abundance and competence and Cx. pipiens feeding preference were combined to estimate the amplification fractions of the different bird species. Predictions were that approximately 66% of WNV-infectious Cx. pipiens became infected from feeding on just a few species of birds, including American robins (35%), blue jays (17%, Cyanocitta cristata), and house finches (15%, Carpodacus mexicanus).

Received June 10, 2008. Accepted for publication October 6, 2008.

Acknowledgments: We thank the Village of Oak Lawn for providing field laboratory facilities and the other municipalities (Evergreen Park, Palos Hills, Burbank, Alsip, Blue Island, Orland Park, Indian Head Park, Western Springs, Dolton, Harvey, Evanston, Holland, and the City of Chicago) and private homeowners for allowing us to conduct this research; Mike Goshorn, Beth Pultorak, Mike Neville, Seth Dallmann, Eric Secker, Timothy Thompson, Diane Gohde, Jonathon McClain, and Sarah Hamer for providing assistance in the field; Blair Bullard, Lisa Abernathy, Amy Wechsler, Jonathon McClain, Rachael Atkins, and Jennifer Sidge for assisting with processing samples in the laboratory; Goudarz Molaei for assisting with establishing the blood meal analysis protocol; and Marm Kilpatrick for consultation during the analysis. We also thank two anonymous reviewers for helpful comments.

Financial support: This study was supported by the National Science Foundation Ecology of Infectious Diseases Program award no. EF-0429124, a George J. Wallace Scholarship award, and the Illinois Department of Public Health.

^* Address correspondence to Gabriel L. Hamer, Department of Fisheries and Wildlife, Michigan State University, 13 Natural Resources, East Lansing, MI 48910. E-mail: ghamer{at}msu.edu

Authors’ addresses: Gabriel L. Hamer and Daniel B. Hayes, Department of Fisheries and Wildlife, Michigan State University, 13 Natural Resources, East Lansing, MI 48910, E-mails: ghamer{at}msu.edu and hayesdan{at}msu.edu. Uriel D. Kitron, Department of Environmental Studies, Emory University, 400 Dowman Drive, Math and Science Center, Suite E511, Atlanta, GA, 30322, E-mail: ukitron{at}emory.edu. Tony L. Goldberg, Department of Pathobiological Sciences, University of Wisconsin, 2015 Linden Drive, Madison, WI 53706, E-mail: tgoldberg{at}vetmed.wisc.edu. Jeffrey D. Brawn, Department of Natural Resources and Environmental Sciences, Program in Ecology, Evolution, and Conservation Biology, University of Illinois, 606 East Healey Street, Champaign, IL 61820, E-mail: jbrawn{at}uiuc.edu. Scott R. Loss, Conservation Biology Graduate Program, University of Minnesota, 1980 Folwell Avenue, St. Paul, MN 55108, E-mail: lossx004{at}umn.edu. Marilyn O. Ruiz, Department of Pathobiology, University of Illinois, 2001 South Lincoln Avenue, Urbana, IL 61802, E-mail: moruiz{at}uiuc.edu. Edward D. Walker, Department of Microbiology and Molecular Genetics, Michigan State University, 2215 Biomedical Physical Sciences Building, East Lansing, MI 48824, E-mail: walker{at}msu.edu.

Reprint requests: Gabriel L. Hamer, Department of Fisheries and Wildlife, 13 Natural Resources, Michigan State University, East Lansing, MI 48910, E-mail: ghamer{at}msu.edu.