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Japanese encephalitis

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]. Between 1973 and 2013, 68 cases of JE among travelers or expatriates from nonendemic countries were reported [4]. Since a JE vaccine became available in the United States in 1993, only eight cases of JE were reported among United States travelers through 2013 [5,6].

Most human JEV infections are asymptomatic or cause a nonspecific febrile illness. Less than 1 percent of JEV infections results in symptomatic neuroinvasive disease [7]. 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 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 [2,8,9]. 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: Dec 2017. | This topic last updated: Mar 22, 2017.
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