ArticlesEpidemic West Nile encephalitis, New York, 1999: results of a household-based seroepidemiological survey
Introduction
West Nile virus is a mosquito-borne flavivirus that belongs to the Japanese encephalitis virus serocomplex, and which was first isolated from a woman with febrile illness in Uganda.1 Early reports described endemic and epidemic “West Nile fever” in Africa and the Middle East, which was characterised clinically by low-grade fever, headache, myalgia, and profound fatigue, sometimes with gastrointestinal symptoms, lymphadenopathy, or a diffuse roseolar rash.2, 3, 4 Neurological involvement with aseptic meningitis or encephalitis was thought to be rare, and mortality was negligible. More recent epidemics in northern Africa, Eastern Europe, and Russia, however, have been notable for a preponderance of West Nile meningoencephalitis, case-fatality rates of 4–13%, and no evidence of widespread mild illness.5, 6, 7
In the summer of 1999, West Nile virus was recognised in the western hemisphere for the first time when it caused an epidemic of encephalitis and aseptic meningitis in residents of greater New York City, NY, USA.8, 9, 10 Intensive hospital-based surveillance identified 59 patients who were admitted to hospital (seven of whom died) and three people with mild illness. The true extent of symptomless and mild infections was unknown. We did a householdbased seroepidemiological survey in the outbreak epicentre to assess more accurately the public-health impact of the New York epidemic, describe the clinical range of illness, and explore risk factors for infection.
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Methods
The survey was carried out from Oct 2 to Oct 6, 1999— ie, about 6 weeks after the epidemic peak for West Nile encephalitis cases. The highest rates of illness occurred within an area of about 4·8 km2 (population 49 475 in 1990)11 of northern Queens in New York city. A multistage cluster design similar to that of the WHO Expanded Programme on Immunizations (EPI)12 was used to obtain a representative sample of households. Census blocks were sampled with probability proportional to the number of
Results
During evening hours and weekend days, 1861 households were visited. Residents were home and an adult was present in 1069 households; 470 (44%) agreed to participate. Of 1220 eligible residents, 700 (57%) were present and consented to interviews and phlebotomy. 23 individuals from 11 households were excluded because of inadequate specimens (n=14) or incorrect sampling (n=9). Of the 677 remaining survey participants, a smaller proportion were children or young adults compared with the 47 368
Discussion
Our study sheds light on the clinical manifestation and public-health importance of West Nile viral infection in immunologically naïve populations. By using a community-based design, we found evidence of a substantial, if undiagnosed, outbreak of West Nile fever that accompanied the outbreak of West Nile meningoencephalitis. The predominance of neurological illness described in recent outbreaks is probably due to hospital-based case finding, rather than the emergence of more pathogenic West
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