Statistical and visual analysis of human West Nile virus infection in the United States, 1999–2008

Human cases of West Nile virus (WNV) infection have spread across the continental United States since the disease’s first appearance in the United States in 1999. However, most WNV spatial studies to date have focused on relatively small scale urban areas. This study examines spatial autocorrelation and clustering of WNV cases from 1999 to 2008 throughout the continental United States. The data, collected by the Centers for Disease Control and Prevention (CDC) at the county level, were normalized by population, then a global Moran’s I test for spatial autocorrelation was performed on both the non-normalized and normalized datasets for each year during the study period. Both datasets exhibited strong positive spatial autocorrelation for every year (p < 0.01). There was also a geographic pattern of high-value clustering in the northern Midwest that was unexpected. These results indicate significant clustering of human WNV cases throughout the United States, as well as an interesting unexplained regional pattern in the northern Midwest that deserves further investigation.

► Spatial extent and pattern of human WNV infections in US from 1999 to 2008 demonstrated visually.
► Strong positive Moran’s I scores in both normalized and non-normalized datasets found.
► Unexpected and unexplained pattern of high infection rates discovered in Northern Great Plains.