This summary includes surveillance data for 2000 that were collected from 20 states (Alabama, Connecticut, Delaware, Florida, Georgia, Louisiana, Maine, Maryland, Massachusetts, Mississippi, New Hampshire, New Jersey, New York, North Carolina, Pennsylvania, Rhode Island, South Carolina, Texas, Vermont, and Virginia), New York City, and the District of Columbia. All states began to submit surveillance data for May 2000 except New York (started with January 2000 data), Vermont (started with June 2000 data), and New Hampshire (started with July 2000 data). Except for Louisiana, New Hampshire, and Maine, which stopped submitting data in October 2000, all other states collected data at least through mid-November 2000.

Data about surveillance activities were gathered by counties in these 20 states and forwarded to a state WN virus surveillance coordinator. At the state level, data aggregated by county and by week of bird report, specimen collection, or illness onset were entered into a standardized database and electronically reported to CDC weekly. Types of data included the numbers of dead crows and dead birds of other species reported by county residents; crows and birds of other species that were tested for evidence of WN virus infection; mosquitoes of a specific species that had been collected; wild birds that were trapped and bled to determine the prevalence of recently developed antibody against WN virus; sentinel chickens that had been bled to identify seroconversion following recent WN virus infection; and ill or dead humans, horses, and other mammals from which a tissue or serum sample had been submitted to determine if illness or death was attributable to WN virus infection.

In addition, humans, nonhuman vertebrates, and mosquitoes with documented WN virus infections were reported continuously to CDC by telephone, facsimile, or e-mail from the 20 states, New York City, and the District of Columbia. Reports were submitted either directly from the state public health laboratory or the WN virus surveillance group. The methods used to document infection differed by state, species, and the type of tissue tested (5). In mosquitoes and nonhuman vertebrates, testing included combinations of reverse-transcription polymerase chain reaction or real-time (TaqMan) polymerase chain reaction to identify WN virus genome in tissue or cerebrospinal fluid (CSF); immunofluorescent or immunohistochemistry studies to demonstrate WN virus antigen in tissue; virus culture from tissue or serum; or serology testing using immunoglobulin (Ig) M-capture enzyme-linked immunosorbent assay (MAC-ELISA) or plaque-reduction neutralization test (PRNT) to identify WN virus-specific antibodies that demonstrate recent infection. In ill humans, WN virus infections were confirmed by isolating WN virus from or demonstrating WN viral antigen or genomic sequences in tissue, blood, CSF, or other body fluid; demonstrating IgM antibody to WN virus in CSF by MAC-ELISA; demonstrating a fourfold serial change in PRNT antibody titer to WN virus in paired, appropriately timed serum samples; or demonstrating both WN virus-specific IgM by MAC-ELISA and IgG antibody in a single serum specimen by various methods. The county, state, specific species, and the week of bird report, specimen collection, or illness onset that corresponded to each reported WN virus-infected human, nonhuman vertebrates, or mosquito were also collected.

Although WN virus was first identified in metropolitan New York City in 1999, surveillance data submitted to the ArboNET WN virus surveillance system have shown a widespread geographic range of virus activity in 2000. Epizootic activity in birds was reported from nine jurisdictions without recognized WN virus activity in 1999 (District of Columbia, Delaware, Massachusetts, New Hampshire, North Carolina, Pennsylvania, Rhode Island, Vermont, and Virginia), as well as the four states that reported activity in 1999 (Connecticut, Maryland, New Jersey, and New York). Similarly, human illnesses attributable to WN virus infection in 2000 were reported from seven counties without identified human illnesses in 1999, as well as three of the six counties that reported human illnesses in 1999.

Despite the widespread virus activity and regional intensification of surveillance activities, 21 acute human illnesses attributable to WN virus infection were identified in 2000, compared with 62 in 1999. Although some decrease in severe human illness may be attributable to vector control and other prevention activities, experience in Europe shows that incidence of human illness can be variable and outbreaks sporadic. Because widespread WN virus epizootic activity probably will persist and expand in the United States, large outbreaks of illness attributable to WN virus infection are possible if adequate surveillance, prevention activities, and mosquito control are not established and maintained.

The large number of avian deaths, particularly among highly recognizable and common birds such as the American Crow, has provided a unique view of a widespread and possibly expanding epizootic from a newly introduced flavivirus. However, a more important question is to what extent avian deaths and mosquito surveillance can serve as early warning sentinels of epizootic activity, so that increased prevention and intervention activities can be implemented before human infections occur. In 2000, all 21 patients had illness onsets at least 15 days after WN virus-infected birds were first collected in the county of residence, suggesting that avian data may be a sensitive indicator of the level of activity associated with subsequent human disease. However, the occurrence of an infected bird in a county was a relatively poor predictor of human illness. Of 136 counties reporting WN virus-infected birds in 2000, 10 (7%) reported humans with illness due to WN virus infection. Further research to identify threshold levels with greater positive predictive value should be undertaken.

The presence of WN virus-infected mosquito pools may be a less sensitive indicator of epizootic activity associated with subsequent human disease. In 2000, 14 of the 21 patients had illness onsets at least 15 days after WN virus-infected mosquito pools were first collected in their county of residence. However, 8 (21%) of the 38 counties with positive mosquito pools reported at least one ill person. Further analysis of 2000 surveillance data, including an assessment of the timing, number, and geographic location of WN virus-infected birds, and an assessment of mosquito-trapping activities, infection rates, and species identified are required to further interpret these data and refine their use.

The avian deaths and mosquito-based surveillance data from the northeastern United States in 2000 indicate that these surveillance modalities may have greater utility as an early warning system for human infections than surveillance among horses and other nonhuman mammal species. Although documented infections among crows occurred as early as April, most reported WN virus illnesses in horses and small mammals occurred relatively late compared with human illnesses. The horse epizootic peaked 6 weeks later and persisted 5 weeks longer than the human epidemic. Similarly, although few infected small mammals were reported, these also occurred relatively later than human illnesses. More data are needed to determine the reasons for this relative delay in horses and small mammals, and, as the epizootic expands, further evaluation of these surveillance modalities in other regions of North America will be required.

The persistence of widespread WN virus activity in 2000 indicates the need for expanded surveillance and prevention activities. In 2001, enhanced ecologic surveillance should be a high priority for states that have been affected or at high risk for being affected by WN virus (6). States with potential for WN virus activity should establish the following: 1) surveillance systems to receive reports of dead and ill crows and other corvids and to collect and test these reported specimens; 2) rapid mosquito surveillance in response to reports of dead WN virus-infected birds to identify potential mosquito vectors, especially those with a propensity to feed on mammals, and to monitor the population densities of those vectors; and 3) enhanced passive surveillance for neurologic disease in horses and other animals to monitor the degree of WN virus transmission outside the bird-mosquito cycle.

Depending on the geographic location of the state, this surveillance should be implemented in the spring and continued until late fall (for states where mosquito activity will cease because of cold weather) or through the winter (for southern states where mosquito activity may be continuous throughout the year).

Even before the recognition of WN virus activity, prevention activities in these states should include programs to 1) eliminate mosquito-breeding habitats in public areas; 2) control mosquito larvae where these habitats cannot be eliminated; 3) promote the increased use of personal protection and reduce peridomestic conditions that support mosquito breeding; and 4) implement adult mosquito control when indicated by increasing WN virus activity or the occurrence of human disease. In addition, because arbovirus infections are endemic in the United States, jurisdictions should have a comprehensive plan and a functional arbovirus surveillance and response capacity that includes trained personnel with suitable laboratory support for identifying arbovirus activity, including WN virus.

In summary, WN virus activity was widespread and possibly expanding in 2000. Although the coordinated, multistate surveillance effort may have led to a wider recognition of epizootic activity in 2000, reports of equine cases from counties that were not affected in 1999 and the large number of reported and WN virus-infected birds strongly suggest that a true expansion occurred. Because of the success of this system in accomplishing its goals, this coordinated, multistate surveillance effort will be expanded in 2001 to include all the continental United States.