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Update: West Nile Virus Activity --- Northeastern United States, January--August 7, 2000

Surveillance programs initiated in response to the 1999 West Nile virus (WNV) outbreak have detected increased transmission in the northeastern United States (1). Seventeen states along the Atlantic and gulf coasts, New York City (NYC), and Washington, D.C., have conducted WNV surveillance and are reporting to CDC (1). Surveillance for WNV infection includes monitoring of mosquitoes, sentinel chicken flocks, wild birds, and potentially susceptible mammals (e.g., horses and humans) (2). This report summarizes findings of this surveillance system through August 7, 2000.

Avian morbidity and mortality surveillance has identified 188 WNV-infected birds from 34 counties in four northeastern states; 111 (59%) have been reported since August 1. These include 128 birds from New York (Albany, Broome, Columbia, Dutchess, Erie, Franklin, Nassau, New York, Niagara, Onondaga, Orange, Putnam, Queens, Rensselaer, Richmond, Rockland, Schenectady, Suffolk, Ulster, Warren, Washington, and Westchester counties), 54 from New Jersey (Bergen, Essex, Hudson, Middlesex, Monmouth, Passaic, and Union counties), four from Massachusetts (Middlesex, Norfolk, and Suffolk counties), and two from Connecticut (Fairfield and Tolland counties). Infected species reported include 147 American crows (78%) and 23 blue jays (12%). Infections also have been reported in the red-tailed hawk, fish crow, house sparrow, American robin, merlin, song sparrow, Canada goose, great blue heron, northern mockingbird, eastern bluebird, cockatiel, mute swan, and yellow-rumped warbler. WNV has not been reported in sentinel chickens.

WNV also has been detected by reverse-transcriptase polymerase chain reaction and/or virus isolation in 38 mosquito pools collected in New York (New York, Orange, Richmond, Rockland, Suffolk, and Westchester counties) and one from Connecticut (Fairfield County). Thirty-five of the WNV-positive mosquito pools from New York were Culex pipiens/restuans, two were Aedes japonicus, and one was C. pipiens. The positive pool from Connecticut was C. restuans.

On August 4, 2000, the New York City Department of Health (NYCDOH) reported that a person aged 78 years from south Richmond County, hospitalized with viral encephalitis, tested positive for WNV. Cerebrospinal fluid and serum samples were positive for IgM antibody by enzyme-linked immunosorbent assay. Confirmatory testing by plaque reduction neutralization for IgG antibody from convalescent serum is pending. The patient's symptoms began on July 20, indicating that infection had started 3 to 15 days earlier (3); therefore, the infection occurred before the initial mosquito adulticide spraying in Richmond County on July 19 and 20. The patient resides in an area where WNV-infected mosquitoes and birds have been found during 2000; no WNV activity was detected in this part of the city in 1999. The patient had not traveled for more than 50 years to areas where other flaviviruses had been endemic and had not left New York during the incubation period.

Reported by: A Novello, MD, D White, PhD, L Kramer, PhD, C Trimarchi, MS, M Eidson, DVM, D Morse, MD, B Wallace, PhD, P Smith, MD, State Epidemiologist, New York State Dept of Health; W Stone, Dept of Environmental Conservation, Albany; Rockland County Health Dept, Pomona; Suffolk County Health Dept, Hauppague; Westchester County Health Dept, New Rochelle; V Kulasekera, PhD, L Mill, MD, A Fine, MD, J Miller, MD, M Layton, MD, New York City Dept of Health. Bergen County Health Dept, Paramus; W Crans, PhD, Rutgers Univ, New Brunswick; F Sorhage, DVM, E Bresnitz, MD, State Epidemiologist, New Jersey Dept of Health and Senior Svcs. R French, DVM, A Garmendia, DVM, Univ of Connecticut, Storrs; T Andreadis, J Anderson, Connecticut Agricultural Experiment Station, New Haven; R Nelson, D Mayo, PhD, M Cartter, MD, J Hadler, MD, State Epidemiologist, Connecticut Dept of Public Health. B Werner, PhD, R Timperi, MPH, A DeMaria, Jr, MD, State Epidemiologist, Massachusetts Dept of Public Health. National Wildlife Health Center, US Geologic Survey, Madison, Wisconsin. P Kelley, MD, Walter Reed Army Institute of Research, M Bunning, DVM, US Air Force. Arbovirus Diseases Br, Div of Vectorborne Infectious Diseases, National Center for Infectious Diseases; and EIS officers, CDC.

Editorial Note:

These data suggest an expanding zone of epizootic transmission in four northeastern states with viral activity most intense in the NYC area. The finding of WNV-positive birds in upstate New York and in Massachusetts indicates transmission foci that had not been identified during the 1999 outbreak. American crows typically do not travel long distances during the early summer, and some are permanent, nonmigratory residents of an area (4). WNV-infected mosquitoes also have been identified in areas where WNV-positive crows have been found. These facts suggest that WNV-infected crows signify local epizootic transmission; therefore, the presence of WNV-infected crows over a wide geographic area indicates that epizootic transmission is widespread in the northeast and that a potential risk for human infection exists in a wider area in 2000 compared with 1999.

In temperate regions, human WNV infections usually occur in late summer and early fall, a finding consistent with the peak incidence in mid-August during the 1999 outbreak (5). A serosurvey conducted after the 1999 outbreak in Queens indicated that most human infections were asymptomatic; <1% of persons developed severe neurologic dis ease, most frequently the elderly. Health-care providers, especially in the northeastern United States, should consider WNV as the etiology of disease for persons with signs or symptoms suggesting viral encephalitis (all ages, especially if associated with muscle weakness) or meningitis (especially in persons aged >17 years). Laboratory diagnosis that is based on serum IgM test results should be interpreted in the context of the clinical and epidemiologic findings. In areas where WNV activity occurred in 1999, health-care providers should ensure that IgM reactivity in human serum samples represents recent infection as opposed to persistent antibody from last year. NYCDOH, in collaboration with CDC, has been following the WNV cases diagnosed in 1999 for both long-term clinical sequelae and persistence of antibody. Among the 22 persons participating, approximately 55% had detectable serum IgM antibody 6 months after illness onset. These results indicate that laboratory confirmation by serologic testing of suspected WNV cases occurring in 2000 in regions where epidemic WNV transmission occurred in 1999 should include acute and convalescent serum samples to demonstrate a four-fold increase or decrease in WNV-specific neutralizing antibody. Convalescent serum specimens should be collected 14--21 days after acute serum specimens.

Evidence of intensifying epizootic transmission in NYC and surrounding counties and of epizootic transmission in distant locations suggests a need for broadening previous recommendations for prevention and control (6). The following actions may be necessary in affected or potentially affected regions:

  1. Continue surveillance to define the geographic spread and intensity of WNV transmission and to assist in targeting and evaluating control efforts. The apparent high sensitivity of wild bird surveillance indicates that WNV surveillance should include an avian morbidity and mortality component. The significance of finding a WNV-positive bird in an area will depend on that species' flight range and other behavioral characteristics and the bird's age.
  2. Implement or enhance public education programs that emphasize individual awareness of risk factors for WNV infection and describe risk-reduction actions, such as mosquito avoidance, personal protection (i.e., behavior modification, appropriate clothing, and use of repellents), use or repair of window screens, and residential mosquito source reduction. These measures are particularly important for the elderly, who are at increased risk for severe complications if they contract the illness. The involvement of Aedes japonicus, a daytime feeder, as a potential vector indicates a need for persons to be attentive to personal protective measures during outdoor activities regardless of the time of day.
  3. Intensify Culex mosquito larval mapping and control measures to prevent the emergence of adult mosquitoes that would feed on birds and potentially contribute to viral amplification in or near populated areas.
  4. Implement or continue adult mosquito control to reduce vector density in response to surveillance data that reveal one or more of the following: 1) human cases, 2) cases in equine or other mammal species, 3) continued, multiple positive surveillance events (birds or mosquito isolates), and/or 4) in densely populated urban/suburban centers in proximity to areas identified in 1, 2 and 3. In some instances, large-scale aerial applications may be needed to provide adequate coverage in affected areas. Areas with evidence of WNV activity but without the preceding criteria should implement, if necessary, the recommended focal adult mosquito control (6). Retreatment 3--4 days after initial application will be needed to appreciably reduce Culex populations (7--9). Surveillance should be maintained to determine whether further adulticide is required.

A cooperative effort between the U.S. Geological Survey, CDC, and federal, state, and local government agencies in the 19 surveillance jurisdictions has resulted in the production of interactive, World-Wide Web-based maps to track the spread of WNV. These maps with data from the WNV surveillance system, updated weekly, can be viewed at the U.S. Geological Survey's National Atlas Web site at*


  1. CDC. Epidemic/epizootic West Nile virus in the United States: guidelines for surveillance, prevention, and control: a workshop cosponsored by US Department of Health and Human Services, CDC, and US Department of Agriculture, Fort Collins, Colorado, November 8--9, 1999. Available at Accessed August 2000.
  2. CDC. National West Nile Virus Surveillance System, 2000: final plan, May 26, 2000. Available at Accessed August 2000.
  3. Hannoun C, Panthier R, Mouchet J, Eouzan JP. Isolement en France du virus West-Nile a partir de malades et du vecteur Culex modestus Ficalbi. CR Acad Sc Paris 1964;259: 4170--2.
  4. Caceamise DF, Reed LM, Romanowski J, Stauffer PC. Roosting behavior and group territoriality in American crows. Auk 1997;114:628--37.
  5. CDC. Update: West Nile virus encephalitis---New York, 1999. MMWR 1999;48:944--6,955.
  6. CDC. Update: West Nile virus activity---New York and New Jersey, 2000. MMWR 2000; 49:640--2.
  7. Andis MD, Sackett SR, Carroll MK, Bordes ES. Strategies for the emergency control of arboviral epidemics in New Orleans. J Am Mosq Control Assoc 1987;2:125--50.
  8. Reiter P, Eliason DA, Francy DB, Moore CG, Campos EG. Apparent influence of the stage of blood meal digestion on the efficacy of ground applied ULV aerosols for the control of urban Culex mosquitoes. I. Field evidence. J Am Mosq Control Assoc 1990;6:366--70.
  9. Moore CG, Reiter P, Eliason DA, Bailey RE, Campos EG. Apparent influence of the stage of blood meal digestion on the efficacy of ground applied ULV aerosols for the control of urban Culex mosquitoes. III. Results of a computer simulation. J Am Mosq Control Assoc 1990;6:376--83.

* References to sites of non-CDC organizations on the World-Wide Web are provided as a service to MMWR readers and do not constitute or imply endorsement of these organizations or their programs by CDC or the U.S. Department of Health and Human Services. CDC is not responsible for the content of pages found at these sites.

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