West Nile virus re-emerges annually in urban settings across the United States in a predictable cycle of accelerating mosquito-bird transmission known as amplification. When the virus amplifies sufficiently in birds and mosquitoes, it spills over into humans. This project will focus on a persistent hot spot of viral amplification and human disease in suburban Chicago. The project will determine how fine-scale ecological processes spark local transmission foci, and how these foci ultimately coalesce into larger patterns of amplification. By combining field studies of birds and mosquitoes with molecular studies of the virus and geographic analyses of fine-scale landscape characteristics, the study will elucidate the fundamental ecological drivers of West Nile virus transmission, amplification, and evolution in urban settings.
West Nile virus is responsible for recurring illness in humans and wildlife. By determining how ecological factors in the urban environment facilitate viral transmission, this study will provide information that will improve both human public health and wildlife conservation. Moreover, new and re-emerging mosquito-borne diseases threaten urban populations throughout the USA and the world. Information on West Nile virus from this study will help predict and prevent the emergence of similar diseases in urban environments across the USA and elsewhere.
West Nile virus emerged in the United States in 1999 and has subsequently spread across North America and beyond. Since 2002, the western Chicago suburbs have been a "hot spot" for viral transmission, as measured by a high concentration of human cases and consistently high infection rates in mosquitoes and birds. This project examined mosquitoes, birds, the virus, and the landscape of the west Chicago suburbs to determine what factors combine to make this area a persistent focus of West Nile virus transmission. The research used DNA analysis of blood from mosquitoes to show that American robins, a common bird in the area, are the primary drivers of West Nile virus transmission. The research then used radio telemetry to track American robins across the landscape, and in so doing identified very large nighttime roosts of robins and other birds. Follow-up studies determined that these "mega-roosts" actually decrease the risk of West Nile virus to individual birds, due to a "dilution" of exposure to infectious mosquitoes as a result of group living. This result was unanticipated but filled an important gap in our understanding of West Nile virus ecology. The research also used DNA sequencing to understand how the virus is moving and changing in the Chicago area. Results indicate that certain genetic lineages of the virus are persisting across years in the area, but that the virus’s genetic sequence is steadily changing and diversifying, indicating active viral evolution. In addition, the same methods have detected a surprising number of co-circulating viruses, bacteria, and parasites, some of which may influence how West Nile virus is transmitted locally. Geographic analyses of the physical landscape and weather patterns showed that wet Springs followed by hot and dry Summers enhance the local transmission of West Nile virus, due to the fact that such conditions create ideal mosquito breeding habitats in catch basins throughout the suburban environment. These results have led to a mathematical model that can predict West Nile virus outbreaks in the area, which is being used by public health agencies. These same results indicate critical control points for disrupting West Nile virus transmission, such as focusing insecticide treatments on certain particularly productive mosquito breeding sites at key times of year. The intellectual merit of this project was thus to elucidate key components of the urban ecology of West Nile virus that have led directly to enhanced surveillance and entry points for public health intervention in Chicago and beyond. The project has also provided training opportunities to students ranging from high school to graduate school, as well as engaging local municipalities and stakeholder groups in the research. Thus, the broader impacts of the project include training the next generation of infectious disease researchers while raising awareness about the importance of disease ecology for public health.
