Abstract
BackgroundThe incidence of West Nile virus (WNv) has remained high in the northern Great Plains compared to the rest of the United States. However, the reasons for the sustained high risk of WNv transmission in this region have not been determined. To assess the environmental drivers of WNv in the northern Great Plains, we analyzed the county-level spatial pattern of human cases during the 2003 epidemic across a seven-state region.Methodology/Principal FindingsCounty-level data on WNv cases were examined using spatial cluster analysis, and were used to fit statistical models with weather, climate, and land use variables as predictors. In 2003 there was a single large cluster of elevated WNv risk encompassing North Dakota, South Dakota, and Nebraska along with portions of eastern Montana and Wyoming. The relative risk of WNv remained high within the boundaries of this cluster from 2004–2007. WNv incidence during the 2003 epidemic was found to have a stronger relationship with long-term climate patterns than with annual weather in either 2002 or 2003. WNv incidence increased with mean May–July temperature and had a unimodal relationship with total May–July precipitation. WNv incidence also increased with the percentage of irrigated cropland and with the percentage of the human population living in rural areas.Conclusions/SignificanceThe spatial pattern of WNv cases during the 2003 epidemic in the northern Great Plains was associated with both climatic gradients and land use patterns. These results were interpreted as evidence that environmental conditions across much of the northern Great Plains create a favorable ecological niche for Culex tarsalis, a particularly efficient vector of WNv. Further research is needed to determine the proximal causes of sustained WNv transmission and to enhance strategies for disease prevention.
Highlights
West Nile virus (WNv) is indigenous to Africa, Asia, Europe, and Australia, and was first identified in North America in the New York City metropolitan area during the summer of 1999 [1]
Clustering Cluster analysis of the 2003 WNv epidemic identified a single elliptical cluster encompassing most of North Dakota, South Dakota, and Nebraska along with portions of eastern Montana and Wyoming
The relative risk and 95% confidence interval for populations inside the 2003 WNv cluster compared with those outside the cluster was 7.3 (5.8±9.1) in 2002, 24.2 (22.4±26.1) in 2003, 7.4 (5.5±9.8) in 2004, 13.4 (11.3, 16.0) in 2005, 9.5 (8.2± 11.1) in 2006, and 7.6 (6.8±8.5) in 2007. These statistics, combined with overlay maps of the 2003 WNv cluster boundary with incidence rates from 2004±2007 (Figure 1), indicated that the broad zone of elevated WNv morbidity in the northern Great Plains remained relatively stable over this time period
Summary
West Nile virus (WNv) is indigenous to Africa, Asia, Europe, and Australia, and was first identified in North America in the New York City metropolitan area during the summer of 1999 [1]. WNv incidence rates, expressed in cases per 100,000 people, were 97 in North Dakota, 135 in South Dakota, and 112 in Nebraska during the 2003 epidemic. WNv incidence in the northern Great Plains has been lower than in 2003 but has remained substantially higher than in other parts of the United States [2]. To improve our knowledge of the environmental determinants of WNv, we analyzed the influences of climate, weather, land cover, and land use on the spatial pattern of the 2003 epidemic in the northern Great Plains. The incidence of West Nile virus (WNv) has remained high in the northern Great Plains compared to the rest of the United States. To assess the environmental drivers of WNv in the northern Great Plains, we analyzed the county-level spatial pattern of human cases during the 2003 epidemic across a seven-state region
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