Problem/Condition: West Nile virus (WNV) is an arthropod-borne virus (arbovirus) in the family Flaviviridae and is the leading cause of arboviral disease in the United States. An estimated 80% of WNV infections are asymptomatic. Most symptomatic persons develop an acute systemic febrile illness that often includes headache, myalgia, arthralgia, rash, or gastrointestinal symptoms. Less than 1% of infected persons develop neuroinvasive disease, which typically presents as encephalitis, meningitis, or acute flaccid paralysis.

Reporting Period: 1999--2008.

Description of System: WNV disease is a nationally notifiable disease with standardized case definitions. State and metropolitan heath departments report cases to CDC through ArboNET, an electronic passive surveillance system. Variables collected include patient age, sex, race, county and state of residence, date of illness onset, clinical syndrome, and outcome of illness.

Results: During 1999--2008, a total of 28,961 confirmed and probable cases of WNV disease, including 11,822 (41%) WNV neuroinvasive disease cases, were reported to CDC from 47 states and the District of Columbia. No cases were reported from Alaska, Hawaii, Maine, or any U.S. territories. A total of 93% of all WNV patients had illness onset during July--September. The national incidence of WNV neuroinvasive disease peaked in 2002 (1.02 cases per 100,000 population) and was stable during 2004--2007 (mean annual incidence: 0.44; range: 0.39--0.50). In 2008, the incidence was 0.23 per 100,000 population, compared with 0.41 in 2007 and 0.50 in 2006. During 1999--2008, the highest incidence of neuroinvasive disease occurred in West North Central and Mountain states. Neuroinvasive disease incidence increased with increasing age, with the highest incidence (1.35 cases per 100,000 population) occurring among persons aged =70 years. The hospitalization rate and case-fatality ratio increased with increasing age among persons with neuroinvasive disease.

Interpretation: The stability in reported incidence of neuroinvasive disease during 2004--2007 might represent an endemic level of WNV transmission. Whether the incidence reported in 2008 represents a decrease that will continue is unknown; variations in vectors, avian amplifying hosts, human activity, and environmental factors make predicting future WNV transmission levels difficult.

Public Health Action: Surveillance of WNV disease is important for detecting and monitoring seasonal epidemics and targeting prevention and control activities. Public health education programs should focus on older persons, who are at increased risk for neurologic disease and poor clinical outcomes. In the absence of an effective human vaccine, WNV disease prevention depends on community-level mosquito control and household and personal protection measures.