(July 27, 2001) -- In an article that appears in this week's British medical journal, The Lancet, New York City Health Department investigators provide additional detail on the previously announced results of the 1999 West Nile virus survey done in Northern Queens. The study led by Health Department epidemiologist, Farzad Mostashari, M.D., concludes that during the 1999 outbreak, for every diagnosed case of meningitis or encephalitis resulting from West Nile virus, there were possibly 140 other infections, over 20 percent of which experienced mild viral illnesses. The preliminary results of the survey were announced in March 2000.
Dr. Mostashari, said, "As West Nile virus becomes more established in the northeastern United States and threatens to extend its geographic range in the future, public-health authorities should be aware of the entire range of illness caused by West Nile virus. Physicians in communities at risk for West Nile virus disease outbreaks should consider infection with this virus in unexplained summertime fevers, especially if accompanied by headache, muscle ache, and joint pain."
In the summer of 1999, West Nile virus was recognized in the western hemisphere for the first time, when it caused an epidemic of encephalitis and meningitis in New York City. Intensive hospital-based surveillance identified 59 cases of meningitis or encephalitis (3 other cases of less sever illnesses were identified), including seven deaths in the region. The New York City Department of Health and the Centers for Disease Control and Prevention, USA, did a household-based survey in October 1999, six weeks after the peak of the outbreak. The study appearing in The Lancet assesses more clearly the public-health impact of the epidemic, its range of illness, and risk factors associated with infection.
The investigators used a representative sample of households in an area of about 7.3 km2 at the outbreak epicenter. Blood samples were tested for antibodies specific for West Nile virus. 677 individuals from 459 households took part in the survey. 19 (2.6%) were seropositive (i.e., they had the virus confirmed by blood test); nearly one third of these individuals reported a recent febrile illness, compared with 70 of 648 (11%) who were seronegative.
Mild illnesses including fever, fatigue, headache, muscle pain, and painful joints were highly associated with seropositivity. By extrapolation from the 59 diagnosed meningitis and encephalitis cases, the investigators estimated that New York City as a whole may have had as many as 8200 West Nile viral infections, including about 1700 mild illnesses related to infections. The risk of severe illness was higher among older people, with one case of meningoencephalitis for every 50 infections for those aged 65 years and over, compared with one case for every 300 infections for people aged less than 65 years.
Use of DEET-containing mosquito-repellent was protective of infection in those who spent 2 or more hours outdoors between dusk and dawn, the peak biting period for Culex mosquitoes. However, 70% of residents reported never using mosquito repellent, even after the outbreak was recognized, despite mass media and other public education messages encouraging its use.
No human infections have been detected thus far this year in New York City. The Health Department has a comprehensive West Nile virus surveillance, control and educational plan. More information can be obtained from the West Nile virus information line at 1-877-WNV-4NYC, or the Health Department's Web site at http://nyc.gov/health.
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