West Nile Virus: 12 Years in North America

Abstract

West Nile virus (WNV) is a mosquito-borne virus that causes disease in wild birds, horses, and humans. It is one of the best-known members of the flavivirus genus in the virus family Flaviviridae. There are approximately 70 members of the genus Flavivirus, and about 40 are known to cause human disease. Most of these are transmitted by the bite of an arthropod such as a mosquito or a tick (Fig. 3.1). Eighty percent of human WNV infections are subclinical or asymptomatic. West Nile fever is the most common disease manifestation and consists of a generalized febrile illness. Malaise, fatigue, headache, nausea, vomiting, diarrhea, and confusion are the most common symptoms. Approximately 30% of symptomatic infections involve neuroinvasive disease that can be fatal or result in long-term sequelae such as persistent movement disorders and personality changes (Sejvar et al. 2003; Murray et al. 2007). A recent report of long-term persistence of WNV RNA in human urine coupled with previous data on the presence of IgM several months after acute WNV disease raises the possibility that several of the long-term sequelae may be due to persistent WNV infection in a subset of patients (Murray et al. 2010). The immunocompromised and elderly are disproportionately affected by WNV infection and are much more likely to develop severe disease than healthy adults under the age of 70. WNV therefore produces a broad spectrum of clinical disease in human beings. The mechanisms that lead to these various manifestations are only partially understood.