West Nile virus activity - United States, 2009.

West Nile virus (WNV) was first detected in the Western Hemisphere in 1999 in New York City and has since caused seasonal epidemics of febrile illness and neurologic disease across the United States, where it is now the leading cause of arboviral encephalitis. This report updates a previous report and summarizes WNV activity in the United States reported to CDC in 2009. A total of 38 states and the District of Columbia (DC) reported 720 cases of WNV disease. Of these, 33 states and DC reported 386 cases of WNV neuroinvasive disease, for an incidence of 0.13 per 100,000 population. The five states with the highest incidence of WNV neuroinvasive disease were Mississippi (1.05 per 100,000), South Dakota (0.74), Wyoming (0.73), Colorado (0.72), and Nebraska (0.61). Neuroinvasive disease incidence increased with increasing age, with the highest incidence among persons aged >or=70 years. A total of 33 WNV deaths were reported, 32 from neuroinvasive disease. Calculating from the number of neuroinvasive disease cases and projections from 1999 serosurvey data, CDC estimated that 54,000 persons were infected with WNV in 2009, of whom 10,000 developed nonneuroinvasive WNV disease. The continuing disease burden caused by WNV affirms the need for ongoing surveillance, mosquito control, promotion of personal protection from mosquito bites, and research into additional prevention strategies.