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

Surveillance data reported to CDC indicate intensified transmission and geographic expansion of the West Nile Virus (WNV) outbreak in the northeastern United States. Increasing reports of WNV infections in American crows, other avian species, and mosquitoes are being accompanied by reports of neurologic disease caused by WNV in humans, horses, and other mammals. This report updates human data through September 12 and animal data through September 8, 2000.

Since July 20, 12 persons have been hospitalized with serious central nervous system infections caused by WNV; eight had encephalitis, and four had meningitis. Patients ranged in age from 40 to 87 years; seven were men. Eight resided in Richmond County (Staten Island), New York, two in Kings County (Brooklyn), New York, and one in Hudson County, New Jersey. One person spent substantial time both in Brooklyn, New York, and Bergen County, New Jersey. Diagnoses were confirmed by ELISA for WNV-specific IgM in cerebrospinal fluid. Subsequently, a four-fold rise in plaque-reduction neutralization antibody titer was demonstrated in four of these patients. Nine patients improved and were discharged from the hospital; three remain hospitalized.

Surveillance detected epizootic activity (15 WNV-infected dead birds and five infected mosquito pools in Staten Island; 10 infected dead birds in Hudson County; and two infected dead birds and one infected live hatch-year bird in Brooklyn) before onset of human illness on July 20 (first Staten Island case), August 6 (Hudson County), and August 15 (first Brooklyn case). The most recent onset of human illness was September 2.

Veterinary surveillance has detected WNV infection in five horses with severe neurologic disease (one horse each in Middlesex County, Massachusetts; Atlantic and Cape May counties, New Jersey; Staten Island, New York; and Washington County, Rhode Island). Onset of illness in these horses ranged from August 17 to 29. WNV infection has been confirmed in six bats (four live big brown bats [Eptesicus fuscus] from Albany County, New York, and two dead little brown bats [Myotis lucifugus] from Ontario County, New York) that originally were submitted for rabies testing. WNV infection was confirmed in a dead raccoon from New York County (Manhattan) that was found on August 19.

Mosquito surveillance has detected WNV in 237 mosquito pools in 15 counties in four states (223 pools in New York, eight in New Jersey, and three each in Connecticut and Massachusetts); 84 (36%) were from Staten Island. Of the 237 reported WNV-infected pools, 137 pools were Culex pipiens/restuans, 44 were Culex pipiens, 25 were Culex salinarius (23 from Staten Island, one from Bronx, and one from Queens, New York City), three were Culex restuans, three were Aedes japonicus (Orange, Rockland, and Westchester counties, New York), three were Aedes vexans (Brooklyn and Staten Island), two were Aedes triseriatus (Staten Island), and one was Anopheles punctipennis (Staten Island).

Avian surveillance has identified 1471 WNV-infected dead birds from 79 counties in six states (586 birds in New Jersey, 536 in New York, 241 in Connecticut, 103 in Massachusetts, four in Rhode Island, and one in New Hampshire). Since 1999, WNV has been identified in 56 avian species in the United States, 48 of which are native. In New York state, all types of submitted avian species are tested for WNV; of the 536 birds infected with WNV in 2000, 347 (65%) were American crows, 82 (15%) were blue jays, and 107 (20%) were other species. WNV antibody was documented in a serologic specimen collected August 4 from a previously seronegative sentinel chicken in Westchester County, New York.

Reported by: A Novello, MD, D White, PhD, L Kramer, PhD, C Trimarchi, MS, M Eidson, DVM, D Morse, MD, B Wallace, MD, P Smith, MD, State Epidemiologist, New York State Dept of Health; W Stone, MS, Dept of Environmental Conservation, Albany; B Cherry, VMD, PhD, B Edwin, J Kellachan, MPH, V Kulasekera, PhD, J Miller, MD, New York City Dept of Health. W Crans, PhD, Rutgers Univ, New Brunswick; F Sorhage, DVM, E Bresnitz, MD, State Epidemiologist, New Jersey Dept of Health and Senior Svcs. T Andreadis, PhD, Connecticut Agricultural Experiment Station, New Haven; M Cartter, MD, J Hadler, MD, State Epidemiologist, Connecticut Dept of Public Health. B Werner, PhD, A DeMaria, Jr, MD, State Epidemiologist, Massachusetts Dept of Public Health. U Bandy, MD, State Epidemiologist, Rhode Island Dept of Health. J Greenblatt, MD, State Epidemiologist, New Hampshire Dept of Health. National Wildlife Health Center, US Geologic Survey, Madison, Wisconsin. US Air Force. Arbovirus Diseases Br, Div of Vector Borne Infectious Diseases, National Center for Infectious Diseases; and EIS officers, CDC.

Editorial Note:

WNV primarily circulates between birds and mosquitoes and probably only incidentally infects humans, horses, and other mammals. As a result, WNV activity in birds and mosquitoes in a specific area generally precedes WNV infection in humans and horses (1). In 2000, the WNV surveillance system documented epizootic WNV infections in birds and mosquitoes as sentinel events before reports of severe neurologic WNV infection in humans and prompted immediate implementation of mosquito control. This confirms the pattern suspected in 1999 when an epizootic among American crows preceded the outbreak of 62 humans identified with WNV encephalitis and meningitis in the New York City metropolitan area (2).

Many counties with intense WNV activity in mosquito and avian populations during the summer of 2000 have not reported WNV infections in humans or other mammals. This is probably a result of a combination of intensive mosquito control activities and variable mosquito feeding behaviors, reservoir host behaviors, human outdoor activities, and use of protective measures. However, the 12 patients with severe central nervous system disease caused by WNV probably represent a small proportion of humans infected with WNV this season. Not all persons with neurologic WNV infection may have had the condition diagnosed or reported. Most persons with WNV infection are asymptomatic or have only nonspecific symptoms for which WNV testing is not performed routinely. A serosurvey in Queens, New York City, after the 1999 outbreak indicated that <1% of WNV-infected persons developed severe neurologic disease.

Health-care providers in areas with documented epizootic activity should consider WNV infection in persons with suspected viral meningitis (especially among adults) or encephalitis (regardless of age). Although severe WNV central nervous system disease may be more common in the elderly, eight of the 12 persons in this report were aged <65 years. In the 1999 WNV outbreak in the New York City area, the youngest patient was aged 5 years. WNV and other arboviruses (Eastern equine encephalitis, St. Louis encephalitis, and California serogroup viruses) can cause disease in the northeastern United States through the end of October and later in more southern locations.

The recent diagnosis of a WNV-infected horse in southern New Jersey (Cape May County), a major stopover for birds migrating south, underscores the need for enhanced avian morbidity and mortality surveillance in areas south of New York City and New Jersey. If ongoing local WNV epizootic activity is detected, public health measures should be enhanced to reduce the risk for human infection (3).


  1. CDC. Guidelines for arbovirus surveillance in the United States. Washington, DC: US Department of Health and Human Services, April 1993.
  2. CDC. Guidelines for surveillance, prevention, and control of West Nile virus infection---United States. MMWR 2000;49:25--8.
  3. CDC. Update: West Nile virus activity---Northeastern United States, January--August 7, 2000. MMWR 2000;49:714--7.

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