Epidemic West Nile encephalitis, New York, 1999: results of a household-based seroepidemiological survey

Background In the summer of 1999, West Nile virus was recognised in the wes tern hemisphere for the first time when it caused an epidemic of encephalit is and meningitis in the metropolitan area of New York City, NY, USA. Inten sive hospital-based surveillance identified 59 cases, including seven death s in the region. We did a household-based seroepidemiological survey to ass ess more clearly the public-health impact of the epidemic, its range of ill ness, and risk factors associated with infection. Methods We used cluster sampling to select a representative sample of house holds in an area of about 7.3 km(2) at the outbreak epicentre. All individu als aged 5 years or older were eligible for interviews and phlebotomy. Seru m samples were tested for IgM and IgG antibodies specific for West Nile vir us. Findings 677 individuals from 459 households participated. 19 were seroposi tive (weighted seroprevalence 2.6% [95% CI 1.2-4.1). Six (32%) of the serop ositive individuals reported a recent febrile illness compared with 70 of 6 48 (11%) seronegative participants (difference 21% [0-47]). A febrile syndr ome with fatigue, headache, myalgia, and arthralgia was highly associated w ith seropositivity (prevalence ratio 7.4 [1.5-36.6]). By extrapolation from the 59 diagnosed meningoencephalitis cases, we conservatively estimated th at the New York outbreak consisted of 8200 (range 3500-13 000) West Nile vi ral infections, including about 1700 febrile infections. Interpretation During the 1999 West Nile virus outbreak, thousands of sympt omless and symptomatic West Nile viral infections probably occurred, with f ewer than 1% resulting in severe neurological disease.