Environmental and social determinants of human risk during a West Nile virus outbreak in the greater Chicago area, 2002

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Abstract
The outbreak of West Nile Virus (WNV) in and around Chicago in 2002 included over 680 cases of human illness caused by the virus within this region. The notable clustering of the cases in two well-defined areas suggests the existence of specific environmental and social factors that increase the risk for WNV infection and/or illness in these locations. This investigation sought to create an empirically based model to account for these factors and to assess their importance in explaining the possible processes that may have led to this pattern. The cluster pattern of high incidence of cases was statistically significant. The risk factors that were found to be important included the presence of vegetation, age, income, and race of the human population, distance to a WNV positive dead bird specimen, age of housing, mosquito abatement and geological factors. The effect of different mosquito abatement efforts was particularly notable. About 53 percent of the variation of the location of WNV clusters was explained by these factors. The models developed indicate that differential mosquito abatement efforts are especially important risk factors, even when controlling for key environmental factors. Human population characteristics play a role in risk that is measurable in this ecological study but would require further research to associate causality with risk. The analysis of spatial clusters of case incidence indicates that this approach provides more insight into the focal nature of differential risk factors that tend to be associated with WNV than an analysis of all individual cases.

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