West Nile virus (WNV) is the most widespread arbovirus in the world, occurring on every continent except Antarctica. Since its introduction to New York City in 1999, over 29,000 human WNV cases have been reported in the United States; over 1,100 of which have resulted in death. Accordingly, management and control of vector populations has become an urgent public health activity in many areas. As vector populations are sensitive to environmental fluctuations and heterogeneity, factors such as temperature, rainfall, land use, and human population density could be used to identify areas of increased risk of WNV infection in humans. Numerous studies have reported correlations between different land cover types and WNV incidence in humans, and seroprevalence in mosquitoes and various mammals. However, the strength and sign of these correlations has varied considerably. We sought to clarify this relationship by analyzing the association between human WNV incidence and land cover at two spatial extents (national and regional) in the continental U.S. We calculated the correlations between the percent of land cover types and average WNV incidence from 2000-2008. We used Pearson's and Spearman's correlation coefficients as alternative measures of the association of human WNV incidence with different land cover types. 

Results/Conclusions

We found regional differences in the associations between the proportion of specific land cover types and human WNV incidence. In the Northeast we found the proportion of developed land cover to be positively associated with human WNV incidence, while in the western U.S. human WNV incidence was positively associated with the proportion of grassland and crop land cover. Analyses incorporating peak year of human infection as well as WNV meningitis and encephalitis incidence supported these associations. In addition to findings concerning the correlation between WNV incidence and land covers, many studies also report an abundance of Culex pipiens (in eastern regions) or Culex tarsalis (in western regions). West of the Mississippi River the main mosquito vector of WNV is Culex tarsalis, which breeds in standing water that receives ample sunlight. The prominent WNV vector on the east coast is Culex pipiens, which is found more commonly in suburban to urban areas, mostly because it breeds in artificial containers that are often polluted or eutrophic. Due to the similarities between vector habitat and WNV-promoting land covers in these regions, we speculate that regional differences in species composition is the determining factor behind the associations we found.