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Update: West Nile-Like Viral Encephalitis -- New York, 1999

The outbreak of human arboviral encephalitis attributable to a mosquito-transmitted West Nile-like virus (WNLV) continues to wane in the Northeast (Figure 1). As of October 5, the number of laboratory-positive cases had increased to 50 (27 confirmed and 23 probable), including five deaths. The increase in cases is mainly a result of completed retesting with West Nile virus antigen of specimens previously tested with the related St. Louis encephalitis virus antigen and to intensive retrospective case finding in the ongoing epidemiologic investigations (1,2).

Of the 50 case-patients, none had onset of illness after September 17. Thirty-eight resided within boroughs of New York City (NYC): 26 from Queens, nine from the Bronx, two from Manhattan, and one from Brooklyn. An additional 12 cases were reported from the adjacent counties of Westchester (eight) and Nassau (four). In NYC, the earliest laboratory-positive case occurred in a patient on August 4, followed by 11 cases in patients from Queens with onset dates ranging from August 5 to August 18.

The association of WNLV with deaths in crows and domestic and exotic birds was confirmed during September. As a result, CDC, state wildlife veterinarians, and an expanding group of federal agencies are using deaths in crows as sentinel events to define the current geographic distribution of mosquitoes and birds infected with WNLV (1). As of October 5, results from selected bird tissue samples tested indicate that WNLV has been identified from 41 avian tissue specimens collected in NYC; Nassau, Suffolk, Rockland, and Westchester counties in New York; Fairfield County, Connecticut; and Bergen, Union, Middlesex, and Essex counties in New Jersey. No human cases of encephalitis attributable to WNLV have been reported from either Connecticut or New Jersey. Pools of Culex mosquitoes collected in localized areas of Queens, Brooklyn, and the Bronx in mid-September and a pool of Culex pipiens collected from Nassau County in late September have been positive for WNLV by reverse transcriptase polymerase chain reaction testing. One pool of Culex pipiens and one pool of Aedes vexans mosquitoes collected from a single trap in Greenwich, Connecticut, on September 13 yielded isolates of WNLV.

Reported by: A Fine, MD, M Layton, MD, J Miller, MD, D Cimini, MPH, MC Vargas, DVM, A Inglesby, MD, A Labowitz, K Bornschlegel, MPH, B Maldin, E Samoff, MPH, D Haddow, the New York City Outbreak Investigation Team, S Mullin, MSW, J Gadd, MPP, E Giebelhaus, MPP, L Mascuch, MSW, A Sher, M Foggin, BJ Mojica, N Cohen, MD, I Weisfuse, MD, R Bhalla, MD, E Lee, MD, D Malebranche, MD, G Sacajiu, MD, A Sharma, MD, M Eisenberg, A Ramon, MD, I Poshni, PhD, H Stirling, MPH, A Goldberg, New York City Dept of Health; J Hauer, MHS, Mayor's Office of Emergency Management, New York City; A Huang, MD, A Rosenberg, MD, P Yang-Lewis, MPH, HN Adel, MD, Westchester County Health Dept, New Rochelle; A Novello, MD, D White, PhD, D Morse, MD, K Spitalny, MD, R Gallo, S Wong, MD, L Grady, MD, M Eidson, DVM, B Wallace, MD, P Smith, MD, State Epidemiologist, New York State Dept of Health. M Cartter, MD, R Nelson, DVM, J Hadler, MD, State Epidemiologist, Connecticut Dept of Health; T Andreadis, PhD, Connecticut Agricultural Experiment Station. J Blumenstock, J Degraaf, F Sorhage, DVM, C Campbell, DVM, J Brook, MD, M Gerwell, MD, D Adams, K Bruder, R Kent, R Eisner, DVM, N Halperin, DVM, D Roscoe, DVM, E Bresnitz, MD, State Epidemiologist, New Jersey Dept of Health and Senior Svcs. W Crans, PhD, Rutgers Univ, New Brunswick, New Jersey. US Geologic Survey. Animal Plant and Health Inspection Svc, US Dept of Agriculture. Div of Applied Public Health Training, Epidemiology Program Office; Infectious Disease Pathology Activity, Div of Viral and Rickettsial Diseases, and Arbovirus Diseases Br, Div of Vector-Borne Infectious Diseases, National Center for Infectious Diseases; and EIS officers, CDC.

Editorial Note:

Human cases of encephalitis attributable to WNLV should continue to decline in areas where WNLV activity has been documented because of the application of adulticidal and larvacidal mosquito-control compounds; however, persons in these areas should continue to use personal protective measures, including reducing outdoor exposures at dusk and at night; wearing long-sleeved shirts and pants; and applying to skin and clothing DEET-containing mosquito repellants according to label directions (1). Shorter days and the onset of colder weather eventually will lead to major declines in vector mosquito populations and will reduce human risk for exposure.

Confirmation that these WNLVs are virulent in a wide range of domestic and exotic birds has led to the formation of a cooperative federal working group. This working group, in cooperation with state and local health departments, will attempt to define the extent to which WNLVs are distributed in mosquito and bird populations outside the Northeast.

The appearance of WNLV in the Western Hemisphere will necessitate enhanced vigilance for this virus during the transmission seasons for the next several years. Enhanced human surveillance for West Nile-like encephalitis will be a fundamental part of determining geographic distribution. To assist states in augmenting surveillance, CDC has distributed surveillance guidelines to state epidemiologists and state health laboratory directors. The guidelines include early warning tools for surveillance of arbovirus activity in nature, such as mosquito trapping for virus isolation and avian serologic and viral surveillance (3).


  1. CDC. Outbreak of West Nile-like viral encephalitis--New York, 1999. MMWR 1999;48:871-4.
  2. CDC. Case definitions for infectious conditions under public health surveillance. MMWR 1997; 46(no. RR-10):12-3.
  3. CDC. Guidelines for arbovirus surveillance in the United States, 1993. Fort Collins, Colorado: US Department of Health and Human Services, CDC, 1994.4

Figure 1

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