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West Nile Virus Activity --- Eastern United States, 2001

In 2000, ArboNET, an enhanced human and animal surveillance system designed to monitor the geographic spread of West Nile virus (WNV) in the United States and to identify areas at increased risk for human infections with WNV, detected WNV activity in the District of Columbia and 12 states (1). This system, first implemented in the District of Columbia and 20 states along the Atlantic and Gulf coasts, was later expanded throughout the continental United States. This report summarizes ArboNET data from January 1 through July 25, 2001, which documents epizootic WNV activity in the southeast and indicates the need for widespread implementation of WNV prevention activities.

The first human infection in 2001 was identified in a 73-year-old man from Madison County, Florida, with illness onset on approximately July 15. He remains hospitalized with encephalitis. Equine surveillance identified three horses with neurologic disease attributed to WNV infection in Jefferson County, Florida, with illness onsets beginning on June 24. Avian mortality surveillance identified 142 WNV-infected birds from the District of Columbia (one bird) and 34 counties in nine states (Connecticut [four], Florida [21], Georgia [two], Maryland [51], Massachusetts [six], New Jersey [37], New York [16], Rhode Island [three], and Virginia [one]) (Figure 1). Crows accounted for 126 (89%) of the reported birds. In New York City, one live hatch-year house sparrow had antibody to WNV. One sentinel chicken from Duval County, Florida, seroconverted to WNV in a serum specimen drawn on July 10.

WNV also was detected in 38 mosquito pools collected in 10 counties in four states, representing at least nine species, including a pool of six Culex salinarius collected in Baltimore, Maryland, on July 11, a mixed pool of Cx. pipiens/Cx. restuans collected in Queens, New York, on July 3, a pool of Ochlerotatus canadensis collected on July 5 and a pool of Cx. pipiens collected on July 16 in Fairfield County, Connecticut, and 34 pools collected in seven New Jersey counties as early as May 31. The New Jersey mosquito pools included Cx. pipiens (17 pools), Cx. restuans (nine), Culiseta melanura (three), unidentified Aedes/Oc. species (two), Ae. vexans (one), Oc. canadensis (one), and Oc. triseriatus (one).

Reported by: ArboNET surveillance group in local and state health depts. National Wildlife Health Center, US Geologic Survey, Madison, Wisconsin. National Veterinary Svcs Laboratories, Veterinary Svcs, Animal and Plant Health Inspection Svc, US Dept of Agriculture, Ames, Iowa. Walter Reed Army Institute of Research, District of Columbia. US Air Force, Frederick, Maryland. Arbovirus Diseases Br, Div of Vector-Borne Infectious Diseases, National Center for Infectious Diseases, CDC.

Editorial Note:

The findings in this report demonstrate multifocal epizootic WNV activity across the eastern United States, including new areas in the southeast. In 2000, avian mortality surveillance indicated northward spread of epizootic activity from the New York City metropolitan area in late spring and early summer, and southward spread as far as North Carolina in late summer and early fall (1), a finding consistent with viral spread by migrating birds (2). The detection of WNV in Florida and southern Georgia in 2001, extends substantially the known distribution of this virus. Although first detected in these areas in 2001, WNV may have been introduced into these states earlier, but epizootic viral activity remained below the detection threshold of surveillance.

In 2000, avian mortality surveillance identified 4305 WNV-infected birds, 77 of which were identified by August 1 (1,3). The finding of 142 WNV-positive birds as of July 25, 2001, is twice the 2000 surveillance figures, and the geographic distribution of these birds differs between the 2 years. All of the birds identified in the early summer of 2000 were from four states (Connecticut, Massachusetts, New Jersey, and New York), compared with 44% of those identified as of July 25, 2001.

Illness onset on approximately July 15 in the patient from Florida was the earliest of any person reported since the 1999 recognition of WNV in the United States. The extensive epizootic WNV activity and continued geographic expansion of the virus highlight the need for widespread implementation and intensification of surveillance, prevention, and control measures to minimize the risk for human and equine disease. Prevention activities have included the development and maintenance of long-term sustained mosquito-control programs using integrated pest management strategies and public education programs, emphasizing residential mosquito larval control and personal prevention measures to reduce mosquito exposure (4).

WNV detection in Cx. salinarius and Ae. vexans is of particular concern because these species more readily feed on mammals (including humans) than do Cx. pipiens, Cx. restuans, or Cs. melanura, which have a strong feeding preference for birds (5). However, mosquito-control programs in urban areas should continue to emphasize reduction of Cx. pipiens populations in the north and Cx. quinquefasciatus populations in the south. Although the role of these species in the direct transmission of WNV to humans is unclear, their role in the amplification of this virus and the closely related St. Louis encephalitis (SLE) virus in urban ecosystems is well established (6,7). The occurrence of WNV in Florida raises the possibility of transmission of this virus by other mosquito species such as Cx. nigripalpus, a primary vector of SLE virus in that state (8), and the possibility of a longer transmission season than is typical in regions with a more temperate climate.

The U.S. Geological Survey, CDC, and other federal, state, and local government agencies have collaborated to establish World-Wide Web-based maps to track the spread of WNV. These maps are available at Additional information also is available from sites maintained by local and state health agencies. A partial listing of these sites is available at


  1. Marfin AA, Petersen LR, Eidson M, et al. Widespread West Nile virus activity, eastern United States, 2000. Emerg Infect Dis 2001;7:730--5.
  2. Rappole JH, Derrickson SR, Hubálek Z. Migratory birds and spread of West Nile virus in the Western Hemisphere. Emerg Infect Dis 2000;6:319--28.
  3. CDC. Update: West Nile virus activity---Northeastern United States, January--August 7, 2000. MMWR 2000;49:714--7.
  4. CDC. Epidemic/epizootic West Nile virus in the United States: revised guidelines for surveillance, prevention, and control, 2001. Available at Accessed July 25, 2001.
  5. CDC. Guidelines for arbovirus surveillance in the United States. Fort Collins, Colorado: US Department of Health and Human Services, Public Health Service, CDC, 1993. Available at Accessed July 25, 2001.
  6. Nasci RN, White DJ, Stirling H, et al. West Nile virus isolates from mosquitoes in New York and New Jersey, 1999. Emerg Infect Dis 2001;7:626--30.
  7. Savage HM, Ceianu C, Nicolescu G, et al. Entomologic and avian investigations of an epidemic of West Nile fever in Romania in 1996, with serologic and molecular characterization of a virus isolate from mosquitoes. Am J Trop Med Hyg 1999;61:600--11.
  8. Day JF. Predicting St. Louis encephalitis virus epidemics: lessons from recent, and not so recent, outbreaks. Annu Rev Entomol 2001;46:111--38.

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