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We used epidemiologic data for human West Nile virus (WNV) disease in Colorado from 2003 and 2007 to determine 1) the degree to which estimates of vector-borne disease occurrence is influenced by spatial scale of data aggregation (county versus census tract), and 2) the extent of concordance between spatial risk patterns based on case counts versus incidence. Statistical analyses showed that county, compared with census tract, accounted for approximately 50% of the overall variance in WNV disease incidence, and approximately 33% for the subset of cases classified as West Nile neuroinvasive disease. These findings indicate that sub-county scale presentation provides valuable risk information for stakeholders. There was high concordance between spatial patterns of WNV disease incidence and case counts for census tract (83%) but not for county (50%) or zip code (31%). We discuss how these findings impact on practices to develop spatial epidemiologic data for vector-borne diseases and present data to stakeholders.

In the absence of a vaccine for use in humans against West Nile virus (WNV), mosquito control and personal protection against mosquito bites are the only measures available to prevent disease. Improved spatial targeting is desirable for costly mosquito and WNV surveillance and control schemes. We used a multivariate regression modeling approach to develop spatial models predicting high risk of exposure to WNV in western and eastern Colorado based on associations between Geographic Information System-derived environmental data and zip code of residence for 3,659 human WNV disease cases from 2002 to 2006. Models were robust, with user accuracies for correct classification of high risk areas of 67-80%. The importance of selecting a suitable model development area in an ecologically and climatically diverse environment was shown by models based on data from the eastern plains landscape performing poorly in the mountainous western part of Colorado and vice versa.

Assessments of spatial risk of exposure to vector-borne pathogens that combine vector and human disease data are needed for areas encompassing large tracts of public land with low population bases. We addressed this need for West Nile virus (WNV) disease in the northern Colorado Front Range by developing not only a spatial model for entomological risk of exposure to Culex tarsalis WNV vectors and an epidemiological risk map for WNV disease but also a novel risk-classification index combining data for these independently derived measures of entomological and epidemiological risk. Risk of vector exposure was high in the densely populated eastern plains portion of the Front Range but low in cooler montane areas to the west that are sparsely populated but used heavily for recreation in the summer. The entomological risk model performed well when applied to the western, mountainous part of Colorado and validated against epidemiologic data.