Epidemiology of West Nile Virus in the Highly Epidemic State of North Dakota, 2002-2007

Objectives. West Nile virus (WNV) continues to cause seasonal epidemics of neuroinvasive disease and febrile illness, which have been most dramatic in the central plains states. We studied the epidemiology of WNV disease in North Dakota (ND), a highly epidemic state, six years following its first appearance in the state. Methods. We analyzed information from cases of WNV disease reported to the ND Department of Health during August 2002 through December 2007. Results. A total of 1,246 cases of WNV disease were reported in ND; 183 cases experienced neuroinvasive disease, including meningitis, encephalitis, or acute flaccid paralysis. Risk factors associated with developing neuroinvasive disease rather than West Nile fever included older age, male gender, and residence in a rural area. Annual cumulative incidence of neuroinvasive disease in ND ranged from 0.3 per 100,000 population to 14.6 per 100,000 population. Conclusions. Annual cumulative incidence rates of West Nile neuroinvasive disease suggest that this region offers favorable conditions for its continued enzootic transmission, which highlights the need for improved targeted prevention measures, particularly for rural areas of the state.