Irrigated agriculture is an important risk factor for West Nile virus disease in the hyperendemic Larimer-Boulder-Weld area of north central Colorado.

This study focused on two West Nile virus (WNV) disease outbreak years, 2003 and 2007, and included a three-county area (Larimer, Boulder, and Weld) in North Central Colorado that is hyperendemic for WNV disease. We used epidemiological data for reported WNV disease cases at the census tract scale to: (1) elucidate whether WNV disease incidence differs between census tracts classified as having high versus lower human population density (based on a threshold value of 580 persons/km2) and (2) determine associations between WNV disease incidence and habitat types suitable as development sites for the larval stage of Culex mosquito vectors. WNV disease incidence was significantly elevated in census tracts with lower human population density, compared with those with high density of human population, in both 2003 (median per census tract of 223 and 143 cases per 100,000 population, respectively) and 2007 (median per census tract of 46 and 19 cases per 100,000 population). This is most likely related, in large part, to greater percentages of coverage in less densely populated census tracts by habitats suitable as development sites for Culex larvae (open water, developed open space, pasture/hay, cultivated crops, woody wetlands, and emergent herbaceous wetlands) and, especially, for the subset of these habitats made up by irrigated agricultural land (pasture/hay and cultivated crops) that presumably serve as major producers of the locally most important vector of WNV to humans: Culex tarsalis. A series of analyses produced significant positive associations between greater coverage of or shorter distance to irrigated agricultural land and elevated WNV disease incidence. As an exercise to produce data with potential to inform spatial implementation schemes for prevention and control measures within the study area, we mapped the spatial patterns, by census tract, of WNV disease incidence in 2003 and 2007 as well as the locations of census tracts that had either low (<25th percentile) or high (>75th percentile) WNV disease incidence in both outbreak years (relative to the incidence for each year). This revealed substantial changes from 2003 to 2007 in the spatial pattern for census tracts within the study area with high WNV disease incidence and suggests a dynamic and evolving scenario of WNV transmission to humans that needs to be taken into account for prevention and control measures to stay current and represent the most effective use of available resources.