The Epidemic of West Nile Virus in the United States, 2002

Since 1999, health officials have documented the spread of West Nile virus across the eastern and southern states and into the central United States. In 2002, a large, multi-state, epidemic of neuroinvasive West Nile illness occurred. Using standardized guidelines, health departments conducted surveillance for West Nile virus illness in humans, and West Nile virus infection and illness in non-human species. Illnesses were reported to the Centers for Disease Control and Prevention (CDC) through the ArboNET system. In 2002, 39 states and the District of Columbia reported 4,156 human West Nile virus illness cases. Of these, 2,942 (71%) were neuroinvasive illnesses (i.e., meningitis, encephalitis, or meningoencephalitis) with onset dates from May 19 through December 14; 1,157 (28%) were uncomplicated West Nile fever cases, and 47 (1%) were clinically unspecified. Over 80% of neuroinvasive illnesses occurred in the central United States. Among meningitis cases, median age was 46 years (range, 3 months to 91 years), and the fatality-to-case ratio was 2%; for encephalitis cases (with or without meningitis), median age was 64 years (range, 1 month to 99 years) and the fatality-to-case ratio was 12%. Neuroinvasive illness incidence and mortality, respectively, were significantly associated with advanced age (p = 0.02; p = 0.01) and being male (p < 0.001; p = 0.002). In 89% of counties reporting neuroinvasive human illnesses, West Nile virus infections were first noted in non-human species, but no human illnesses were reported from 77% of counties in which non-human infections were detected. In 2002, West Nile virus caused the largest recognized epidemic of neuroinvasive arboviral illness in the Western Hemisphere and the largest epidemic of neuroinvasive West Nile virus ever recorded. It is unknown why males appeared to have higher risk of severe illness and death, but possibilities include higher prevalence of co-morbid conditions or behavioral factors leading to increased infection rates. Several observations, including major, multi-state West Nile virus epidemics in 2002 and 2003, suggest that major epidemics may annually reoccur in the United States. Non-human surveillance can warn of early West Nile virus activity and needs continued emphasis, along with control of Culex mosquitoes.