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West Nile Virus Activity --- New York and New Jersey, 2000

In late August 1999, an outbreak of encephalitis caused by West Nile virus (WNV) was detected in New York City and subsequently identified in neighboring counties (1). In response, an extensive mosquito-control and risk-reduction campaign was initiated, including aerial and ground applications of mosquito adulticides throughout the affected areas. No human WNV infections were found in New York City with an onset date after the campaign was completed. Cases continued to occur among humans in surrounding counties that did not undertake mosquito-control efforts until later, suggesting that the campaign may have reduced human risk. In May 2000, CDC issued guidelines to direct national surveillance, prevention, and control efforts (2) and provided funds to support these efforts in 19 state and local health departments where WNV transmission had occurred or where transmission would probably occur based on known bird migration patterns. This report presents the findings of surveillance activities.

From May 6 through July 8, 2000, state and local health departments confirmed WNV infections in 26 birds from five counties in New York and New Jersey. Twenty-one infections have been confirmed in American crows in New York and New Jersey, four in blue jays, and one in a red-tailed hawk. The first infected crow was found May 22 in Rockland County, and the most recently infected crows were found July 6 and 8 in the same county. Fourteen infected crows identified in New York were found in Rockland (eight crows), Suffolk (three), Westchester (one), and Richmond (Staten Island) (two) counties. Seven infected crows were found in Bergen County, New Jersey. Rockland County also identified four blue jays with WNV infection, and one infected hawk was found in Westchester County. WNV has been detected by polymerase chain reaction molecular methods in mosquito pools collected in Westchester County (Aedes japonicus) and in Suffolk County (mixed Culex species). No cases of human or equine infection have been reported in the region or in surrounding states.

Reported by: A Novello, MD, D White, PhD, L Kramer, PhD, C Trimarchi, MS, M Eidson, DVM, D Morse, MD, P Smith, MD, State Epidemiologist, New York State Dept of Health; W Stone, PhD, Dept of Environmental Conservation, Albany; Rockland County Health Dept, Pomona; Suffolk County Health Dept, Hauppague; Westchester County Health Dept, New Rochelle; 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. National Wildlife Health Center, US Geologic Survey, Madison, Wisconsin. Arbovirus Diseases Br, Div of Vectorborne Infectious Diseases, National Center for Infectious Diseases, CDC.

Editorial Note:

WNV is transmitted readily by mosquitoes. Culex species were the primary vectors of WNV during previous outbreaks and epizootics; however, WNV also has been isolated from many species of Aedes and Anopheles (3). In New York, WNV was isolated primarily from Culex species mosquitoes during the 1999 outbreak; WNV also was detected in overwintering Culex species in New York City. These findings suggest an important role for these species in the transmission of WNV in the United States. Aedes japonicus was detected recently in the United States, and research is needed to determine the flight range and feeding behavior of mosquitoes and to better understand the risk for transmission to humans.

The susceptibility of crows to infection and death is a sensitive surveillance tool that is unique to the United States (4). No data exist from which to infer the mosquito WNV infection rate associated with a small number of dead crows in an area, or to infer the risk to humans. Data also are lacking to infer where and how the dead crows acquired infection. Time of year and reproductive status of the crow population may be used to indicate whether transmission occurred locally. On the basis of the known nesting habits of crows, the finding of infected crows in early summer suggest local transmission in Rockland, Westchester, Suffolk, and Bergen counties. Data from the U.S. Geologic Survey's National Wildlife Health Center indicate that crows infected with WNV are likely to have high viremias and also are likely to be sedentary approximately 4 days before death, suggesting that they can be a source of WNV for mosquitoes in areas where they are found (National Wildlife Health Center, unpublished data, 2000).

On the basis of the surveillance indicators described in this report and the phased response plan (2), CDC recommends the following for those areas where evidence suggests local transmission of WNV:

  1. Intensify local and regional Culex mosquito larval control to prevent the emergence of adult mosquitoes that feed on birds and may contribute to the virus amplification/transmission cycle.
  2. Expand and intensify surveillance activities in and around areas where WNV-infected birds are found. Additional surveillance data about the species population densities, virus infection rates in mosquito vectors, seroprevalence in resident wild birds (e.g., house sparrows), and seroconversion rates in sentinel chickens will permit a more accurate interpretation of dead bird surveillance data and the relative risk for human disease.
  3. Continue active WNV surveillance to determine the presence of new or expanding WNV transmission foci.
  4. Reinforce public education and outreach programs to reduce mosquito breeding sites around the home and use personal protective measures.
  5. Implement, if necessary, focal adult mosquito control to reduce the number of virus-infected mosquitoes, thus reducing the immediate risk to humans. Mosquito species that feed on birds probably are driving enzootic transmission in 2000 and probably are the vector for human cases (5). Adult mosquitoes should be controlled within approximately a 2-mile radius around the area where a WNV positive dead bird or infected mosquitoes are found. This radius depends on the length of time between transmission of the virus and the execution of control; as the time period increases, larger areas must be treated.
  6. Consider aerial spraying of adulticides in areas where WNV transmission is sustained and further amplification is evident despite intensive local mosquito control efforts.
  7. Monitor adult and larval mosquito control efforts to ensure that the control programs are effectively reducing vector mosquito densities and virus infection rates.

Counties where WNV transmission occurred in 1999, but has not been identified in 2000, should maintain active surveillance for WNV and continue larval mosquito-control, such as controlling larval mosquito habitats, particularly around homes in suburban and urban areas and monitoring Culex larval habitats regularly for mosquito breeding.


  1. CDC. Update: West Nile virus encephalitis---New York, 1999. MMWR 1999;48:944--6,955.
  2. 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
  3. Hayes CG. West Nile fever. In: Monath TR, ed. The arboviruses: epidemiology and ecology. Boca Raton, Florida: CRC Press, Inc., 1989.
  4. Komar N. West Nile viral encephalitis. Rev Sci Tech OIE 2000;19:166--76.
  5. CDC. Update: West Nilelike viral encephalitis---New York, 1999. MMWR 1999;48:890--2.

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