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Epidemiology and pathogenesis of West Nile virus infection

Lyle R Petersen, MD, MPH
Section Editor
Martin S Hirsch, MD
Deputy Editor
Jennifer Mitty, MD, MPH


West Nile (WN) virus, a member of the Japanese encephalitis virus antigenic complex, was first isolated in a blood sample in a patient from the West Nile province of Uganda in 1937 [1]. This RNA virus was initially considered of minor public health importance [2]. It emerged from obscurity in 1999 when the first incursion of the virus into North America caused 62 cases of encephalitis and seven deaths in New York [3]. Since that time, the virus has dramatically spread, and WN virus activity has now been detected in all 48 continental states, the District of Columbia, and Puerto Rico [4]. WN virus causes both sporadic infection and outbreaks that may be associated with severe neurologic disease.

The epidemiology and pathogenesis of West Nile virus will be addressed here. The clinical manifestations, diagnosis and treatment of infection are discussed elsewhere. (See "Clinical manifestations and diagnosis of West Nile virus infection" and "Treatment and prevention of West Nile virus infection".)


West Nile (WN) virus is one of the most widely distributed of all arboviruses with an extensive distribution in the Old World, throughout Africa, the Middle East, parts of Europe and the former Soviet Union, South Asia, and Australia [2]. The virus had not been detected in the Americas before the 1999 New York City outbreak. It is unknown how WN virus got to the United States. While initial phylogenetic studies suggested a Middle Eastern origin for the North American WN virus [5,6], subsequent analysis indicated that WN strains now circulating in Europe and Israel originated from the same independent location, probably North Africa [7]. WN virus is now considered endemic in North America [8].

Patterns of illness — Since the first discovery of WN virus, infrequent human outbreaks were mostly reported in groups of soldiers, children, and healthy adults in Israel and Africa [9-12]. These outbreaks were associated with only minor illness in the majority of patients; some case fatalities were associated with increasing age. In one of the largest outbreaks reported, thousands of self-limited and relatively mild clinical cases, consisting of fever, rash, and polyarthralgias occurred in South Africa, resulting in an epidemic attack rate of 55 percent [13].

However, since the mid-1990s, outbreaks of WN virus infection associated with severe neurologic disease began to occur in North Africa (Algeria [1994, 1997], Tunisia [1997], Sudan [2002]); Europe (Romania [1996]); Russia (1999); Israel (2000); North America (United States [1999], Canada [2002]); and India (2011) [4,14-19]. In each of these outbreaks, the mortality among patients with meningitis and encephalitis was approximately 10 percent, and occurred more often in elderly patients. Since 2002, outbreaks have also occurred in multiple southern European countries, with annual epidemic cycles in some countries related to the spread of WN virus lineage 2 strains [20,21].

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Literature review current through: Oct 2017. | This topic last updated: Sep 07, 2017.
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