West Nile Virus
West Nile virus (WNV) is indigenous to Africa, Asia, Europe, and Australia, and was first identified in the North America in the metropolitan area of New York City during the fall of 1999. In 2002, widespread WNV cases were reported in the Midwest and south-central states. In 2003 the focus of the West Nile epidemic shifted to the Great Plains and the Front Range of the Rocky Mountains. Since that time, WNV has spread across the Rocky Mountains to the west coast and has become seasonally endemic across most of the United States. As of September 26, 2007, a total of 28,943 cases have been reported in the United States, including 16,463 cases of West Nile fever, 11,740 cases of more severe neuroinvasive disease (encephalitis and meningitis), and 1,130 fatalities.
Our case study area encompasses the states of Montana, Wyoming, North Dakota, South Dakota, Nebraska, Minnesota, and Iowa. South Dakota has been documented as having the highest cumulative incidence of WNV neuroinvasive disease in the United States from 2002-2006 (Lindsey et al. 2008). Although overall WNV case numbers have declined since 2003, incidence rates in the Northern Great Plains have remained consistently high compared to the rest of the United States. Several other states in the northern Great Plains, including Nebraska, North Dakota, and Wyoming, also rank as having among the highest levels of WNV neuroinvasive disease in the country. Compared to the other parts of the country, there has been relatively little research on WNV in the northern Great Plains where it continues to pose a significant threat the public health.
The environmental characteristics of the Northern Great Plains appear to provide conditions that are particularly suitable for the continued enzootic transmission of WNv, and there is a need to develop long-term strategies for disease surveillance and prevention. An environmental analysis of the 2003 epidemic found that both climate and land use influenced the patterns of human WNV cases (Wimberly et al., 2008). WNV incidence increase with mean spring/summer temperatures and had a unimodal relationship with total spring/summer precipitation. WNV incidence also increased with the percentage of irrigated cropland and the percentage of the human population living in rural areas. Following the 2003 epidemic, WNV incidence has exhibited considerable spatial and temporal variability in the northern Great Plains. Our current research focuses on understanding the environmental factors that drive the amplification of WNV in spring and early summer, and using this information to forecast risk of transmission to humans during the mid to late summer.
