West Nile virus in the greater Chicago area: A geographic examination of human illness and risk from 2002 to 2006

The state of Illinois experienced a large outbreak of illness from the West Nile virus in 2002, with the majority of human infections occurring in the greater Chicago area. Although an outbreak as large as the first has not occurred since then, transmission of the virus to humans has persisted, and relatively large outbreaks of human illness occurred again in 2005 and 2006. During the larger outbreaks, some neighborhoods exhibited significantly higher rates of infection than did others. This study first examines the changing spatial distribution of West Nile virus outbreaks in this area from 2002 to 2006. Multivariate statistical analysis with a spatial dependence term then is used to explore the relationship between rates of human WNV infection and potential explanatory environmental and socioeconomic factors and to compare the risk of WNV across years. Several environmental and socioeconomic characteristics were found to be associated with increased risk for human West Nile virus infection, but differences were found in different years. Overall, predominantly white neighborhoods with lower housing density and a greater amount of post-World War II housing were particularly at risk. This research provides a useful example of how aggregated disease data may be mapped and spatial patterns characterized, as well as how these data may be combined with sociodemographic and environmental variables to analyze risk factors in a spatially explicit manner.