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Japanese encephalitis: Epidemiology, diagnosis, treatment, and prevention

Susan L Hills, MBBS, MTH
Marc Fischer, MD, MPH
Tom Solomon, FRCP, PhD
Section Editor
Martin S Hirsch, MD
Deputy Editor
Elinor L Baron, MD, DTMH


Japanese encephalitis virus (JEV), a mosquito-borne flavivirus, is the most important cause of viral encephalitis in Asia based on its frequency and severity. With the near eradication of poliomyelitis, JEV is now the leading cause of childhood viral neurological infection and disability in Asia [1]. JEV is closely related to West Nile, St. Louis encephalitis, and Murray Valley encephalitis viruses [2].

About 68,000 cases of Japanese encephalitis (JE) are estimated to occur each year. JEV is endemic throughout most of Asia and parts of the western Pacific, but local transmission has not been observed in Africa, Europe, or the Americas. For travelers to Asia, the risk of JE is very low but varies based on season, destination, duration, and activities [3]. The estimated incidence of JE among travelers to Asia from nonendemic countries is <1 case per 1 million travelers. Risk is likely to be higher for expatriates or travelers with longer duration of travel or whose plans include extensive outdoor activities in rural areas [3]. Since 1973, only 56 cases of JE among travelers from nonendemic countries have been reported [4]; since a JE vaccine was licensed in the United States in 1992, only four cases of JE were reported among United States travelers between 1992 and 2008 [5].

Most human JEV infections are asymptomatic or cause a nonspecific febrile illness. Less than 1 percent of JEV infections results in symptomatic neuroinvasive disease [6]. However, when neurological disease does occur, it is usually very severe with a high case fatality rate; among survivors, neurological sequelae are common. All travelers to JEV-endemic countries should be given advice on measures to prevent JE, and JE should be considered among the differential diagnoses for patients with suspected neurological infection who have returned from recent travel in a JEV-endemic country in Asia or the Western Pacific region.


Japanese encephalitis (JE) is endemic throughout most of Asia and parts of the Western Pacific region (figure 1). Within the JE-endemic region, there are two typical patterns of transmission:

In areas with temperate climates (including China, Japan, South Korea, Nepal, northern Vietnam, and northern India), most cases occur over a period of several months when the weather is warmest, usually after the monsoons begin or associated with heavy rainfall [2,7,8]. The peak months of transmission and the length of the season vary from place to place. There are sometimes large, explosive outbreaks.


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Literature review current through: Sep 2016. | This topic last updated: May 21, 2015.
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