The outbreak of West Nile virus infection in the New York City area in 1999.

Background: In late August 1999, an unusual cluster of cases of meningoence phalitis associated with muscle weakness was reported to the New York City Department of Health. The initial epidemiologic and environmental investiga tions suggested an arboviral cause. Methods: Active surveillance was implemented to identify patients hospitali zed with viral encephalitis and meningitis. Cerebrospinal fluid, serum, and tissue specimens from patients with suspected cases underwent serologic an d viral testing for evidence of arboviral infection. Results: Outbreak surveillance identified 59 patients who were hospitalized with West Nile virus infection in the New York City area during August and September of 1999. The median age of these patients was 71 years (range, 5 to 90). The overall attack rate of clinical West Nile virus infection was at least 6.5 cases per million population, and it increased sharply with ag e. Most of the patients (63 percent) had clinical signs of encephalitis; se ven patients died (12 percent). Muscle weakness was documented in 27 percen t of the patients and flaccid paralysis in 10 percent; in all of the latter , nerve conduction studies indicated an axonal polyneuropathy. An age of 75 years or older was an independent risk factor for death (relative risk adj usted for the presence or absence of diabetes mellitus, 8.5; 95 percent con fidence interval, 1.2 to 59.1), as was the presence of diabetes mellitus (a ge-adjusted relative risk, 5.1; 95 percent confidence interval, 1.5 to 17.3 ). Conclusions: This outbreak of West Nile meningoencephalitis in the New York City metropolitan area represents the first time this virus has been detec ted in the Western Hemisphere. Given the subsequent rapid spread of the vir us, physicians along the eastern seaboard of the United States should consi der West Nile virus infection in the differential diagnosis of encephalitis and viral meningitis during the summer months, especially in older patient s and in those with muscle weakness. (N Engl J Med 2001;344:1807-14.) Copyr ight (C) 2001 Massachusetts Medical Society.