West Nile Virus Activity — United States, 2009

NP Lindsey, MS; JA Lehman; AL Greiner; JE Staples, MD; N Komar, ScD; E Zielinski-Gutierrez, DrPh; RS Nasci, PhD; M Fischer, MD


Morbidity and Mortality Weekly Report. 2010;59(25):769-772. 

In This Article

Abstract and Introduction


West Nile virus (WNV) was first detected in the Western Hemisphere in 1999 in New York City and has since caused seasonal epidemics of febrile illness and neurologic disease across the United States, where it is now the leading cause of arboviral encephalitis.[1] This report updates a previous report[2] and summarizes WNV activity in the United States reported to CDC in 2009. A total of 38 states and the District of Columbia (DC) reported 720 cases of WNV disease. Of these, 33 states and DC reported 386 cases of WNV neuroinvasive disease, for an incidence of 0.13 per 100,000 population. The five states with the highest incidence of WNV neuroinvasive disease were Mississippi (1.05 per 100,000), South Dakota (0.74), Wyoming (0.73), Colorado (0.72), and Nebraska (0.61). Neuroinvasive disease incidence increased with increasing age, with the highest incidence among persons aged ≥70 years. A total of 33 WNV deaths were reported, 32 from neuroinvasive disease. Calculating from the number of neuroinvasive disease cases and projections from 1999 serosurvey data, CDC estimated that 54,000 persons were infected with WNV in 2009, of whom 10,000 developed nonneuroinvasive WNV disease. The continuing disease burden caused by WNV affirms the need for ongoing surveillance, mosquito control, promotion of personal protection from mosquito bites, and research into additional prevention strategies.

WNV is a nationally notifiable disease. Data are reported to CDC through ArboNET, an Internet-based arbovirus surveillance system managed by state health departments and CDC.[2] Using standard case definitions,* human WNV disease cases are classified as WNV neuroinvasive disease (e.g., meningitis, encephalitis, or acute flaccid paralysis) or WNV nonneuroinvasive disease (e.g., acute systemic febrile illness that often includes headache, myalgia, or arthralgia). Nonneuroinvasive disease reporting varies greatly by jurisdiction, depending on disease awareness, health-care–seeking behaviors, and testing practices. Therefore, this report focuses on WNV neuroinvasive disease cases, which are thought to be identified and reported more consistently because of the severity of the illness. In addition to human disease cases, ArboNET captures data on presumptively viremic blood donors (PVDs), veterinary cases, and WNV infections in sentinel animals (most commonly chickens), dead birds, and mosquitoes. Not all jurisdictions conduct nonhuman surveillance.