INTRODUCTION

West Nile encephalitis virus (WNV) is a small, enveloped, mosquito-transmitted, positive-polarity RNA virus of the Flaviviridae family. This virus is closely related to other arthropod-borne viruses that cause human disease including dengue, yellow fever, and Japanese encephalitis viruses. WNV normally cycles in nature between mosquitoes and birds, but during epidemics will infect and cause disease in human, horses, and other vertebrate animals. Severe neurological disease in humans usually occurs within 1 to 2 weeks after mosquito inoculation and is more frequent in elderly and immunocompromised individuals.

EPIDEMIOLOGY

West Nile virus historically caused sporadic outbreaks of a mild febrile illness in regions of Africa, the Middle East, Asia, and Australia. However, in the 1990s, the epidemiology of infection appeared to change, with new outbreaks in parts of eastern Europe associated with higher rates of severe neurological disease. In 1999, WNV entered North America and caused seven human fatalities in the New York area, as well the deaths of a large number of birds and horses. Since then, WNV has spread to all 48 of the lower United States as well as to parts of Canada, Mexico, and the Caribbean. Because of the increased range, the number of human cases has continued to rise: In the United States between 1999 and 2007, there were more than 26,000 clinical cases of WNV, including nearly 1,000 deaths.