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John D. Wilson, M.D.
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Eugene, Oregon 97401 USA
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Japanese encephalitis (JE) is a common mosquito-borne viral encephalitis found in Asia. Most infections are asymptomatic, but among people who develop a clinical illness, the case-fatality rate can be as high as 30%. Permanent brain disability is reported in 50% of survivors. In endemic areas, children are at greatest risk of infection; however, multiple factors such as occupation, recreational exposure, gender (possibly reflecting exposure), previous vaccination, and naturally acquired immunity alter the potential for infection and illness. A higher case-fatality rate is reported in the elderly, but serious sequelae are more frequent in the very young, possibly because they are more likely to survive a severe infection.

JE virus is transmitted chiefly by the bites of mosquitoes in the Culex vishnui complex; the individual vector species in specific geographic areas differ. In China and many endemic areas in Asia, Culex tritaeniorhyncus is the principal vector. This species feeds outdoors beginning at dusk and during evening hours until dawn; it has a wide host range, including domestic animals, birds, and humans. Larvae are found in flooded rice fields, marshes, and small stable collections of water around cultivated fields. In temperate zones, the vectors are present in greatest numbers from June through September and are inactive during winter months. Swine and certain species of wild birds are hosts in the transmission cycle.


Habitats supporting the transmission cycle of JE virus are principally in rural, agricultural locations. In many areas of Asia, however, the appropriate ecologic conditions for virus transmission occur near or occasionally within urban centers. Transmission is seasonal and occurs in the summer and autumn in the temperate regions of China, Japan, Korea, and eastern areas of Russia. Elsewhere, seasonal patterns of disease are more extended or vary with the rainy season and irrigation practices. Risk of JE varies by season and geographic area (Table 3-1).

Risk for Travelers

The risk to short-term travelers and those who confine their travel to urban centers is very low. Expatriates and travelers living for prolonged periods in rural areas where JE is endemic or epidemic are at greatest risk. Travelers with extensive unprotected outdoor, evening, and nighttime exposure in rural areas, such as might be experienced while bicycling, camping, or engaging in certain occupational activities, might be at high risk even if their trip is brief.

Preventive Measures


JE vaccine licensed in the United States is manufactured by Biken, Osaka, Japan, and distributed by Aventis Pasteur. Other JE vaccines are made by several companies in Asia, but are not licensed in the United States. Vaccination should be considered only by people who plan to live in areas where JE is endemic or epidemic and by travelers whose activities include trips into rural, farming areas. Short-term travelers (less than 30 days), especially those whose visits are restricted to major urban areas, are at lower risk for acquiring JE and generally should not be advised to receive the vaccine. Evaluation of an individual traveler's risk should take into account his or her itinerary and activities and the current level of JE activity in the country (see Table 3-1).

The recommended primary immunization series is three doses of 1.0 milliliter (mL) each, administered subcutaneously on days 0, 7, and 30. An abbreviated schedule of days 0, 7, and 14 can be used when the longer schedule is impractical because of time constraints. Two doses given a week apart may be used in unusual circumstances, but will confer short-term immunity in only 80% of vaccinees. The last dose should be administered at least 10 days before commencement of travel to ensure an adequate immune response and access to medical care in the event of delayed adverse reactions (Table 3-2).

Immunization routes and schedules for infants and children 1 through 3 years of age are identical except that doses of 0.5 mL should be administered. No data are available on vaccine efficacy and safety in infants younger than 1 year of age. The full duration of protection is unknown; however, preliminary data indicate that neutralizing antibodies persist for at least 2 years after primary immunization. In infants and children whose primary immunization series included doses of 0.5 mL, a booster dose of 1.0 mL (0.5 mL for children younger than 3 years of age) may be administered 2 years after the primary series.

Table 3-1.--Risk of Japanese Encephalitis, by Country, Region, and Season.
Country Affected Areas Transmission Season Comments
Australia Islands of Torres Strait. Probably year-round transmission risk. Localized outbreak in Torres Strait in 1995 and sporadic cases in 1998 in Torres Strait and on mainland Australia at Cape York Peninsula.
Bangladesh Few data, but probably widespread. Possibly July to December, as in northern India. Outbreak reported from Tangail District, Dacca Division; sporadic cases in Rajshahi Division.
Bhutan No data. No data. No comments.
Brunei Presumed to be sporadic-endemic as in Malaysia. Presumed year-round transmission. No comments.
Presumed to be endemic-hyperendemic countrywide. Presumed to be May to October. Repeated outbreaks in Shan State in Chiang Mai valley.
Cambodia Presumed to be endemic-hyperendemic countrywide. Presumed to be May to October. Cases reported from refugee camps on Thai border.
India Reported cases from all states except Arunachal, Dadra, Daman, Diu, Gujarat, Himachal, Jammu, Kashmir, Lakshadweep, Meghalaya, Nagar Haveli, Orissa, Punjab, Rajasthan, and Sikkim. South India: May to October in Goa; October to January in Tamil Nadu; and August to December in Karnataka. Second peak, April to June in Mandya District.
Andrha Pradesh: September to December.
North India: July to December.
Outbreaks in West Bengal, Bihar, Karnataka, Tamil Nadu, Andrha Pradesh, Assam, Uttar Pradesh, Manipur, and Goa.
Urban cases reported (for example, Luchnow).
Indonesia Kalimantan, Bali, Nusa, Tenggara, Sulawesi, Mollucas, and Irian Jaya (Papua), and Lombok. Probably year-round risk; varies by island; peak risks associated with rainfall, rice cultivation, and presence of pigs.
Peak periods of risk: November to March; June and July in some years.
Human cases recognized on Bali, Java, and possibly in Lombok.
Japan* Rare-sporadic cases on all islands except Hokkaido. June to September, except April to December on Ryuku Islands (Okinawa). Vaccine not routinely recommended for travel to Tokyo and other major cities.
Enzootic transmission without human cases observed on Hokkaido.
Korea North Korea: No data.
South Korea: Sporadic-endemic with occasional outbreaks.
July to October. Last major outbreaks in 1982 and 1983. Sporadic cases reported in 1994 and 1998.
Laos Presumed to be endemic-hyperendemic countrywide. Presumed to be May to October. No comments.
Malaysia Sporadic-endemic in all states of Peninsula, Sarawak, and probably Sabah. Year-round transmission. Most cases from Penang, Perak, Salangor, Johore, and Sarawak.
Nepal Hyperendemic in southern lowlands (Terai). July to December. Vaccine not recommended for travelers visiting only high-altitude areas.
Pakistan May be transmitted in central deltas. Presumed to be June to January. Cases reported near Karachi; endemic areas overlap those for West Nile virus.
Lower Indus Valley might be an endemic area.
Papua New Guinea Normanby Islands and Western Province. Probably year-round risk. Localized sporadic cases.
People’s Republic of China Cases in all provinces except Xizang (Tibet), Xinjiang, Qinghai.
Hyperendemic in southern China.
Endemic–periodically epidemic in temperate areas.
Hong Kong: Rare cases in new territories.
Taiwan: Endemic, sporadic cases; islandwide.*
Northern China: May to September.
Southern China: April to October (Guangxi, Yunnan, Guangdong, and Southern Fujian, Sichuan, Guizhou, Hunan, and Jiangxi provinces).
Hong Kong: April to October.
Taiwan: April to October, with a June peak.*
Vaccine not routinely recommended for travelers to urban areas only.
Taiwan: Cases reported in and around Taipei and the Kao-hsiung–Pingtung river basins.*
Philippines Presumed to be endemic on all islands. Uncertain; speculations based on locations and agroecosystems. West Luzon, Mindoro, Negros, Palawan: April to November.
Elsewhere: year-round, with greatest risk April to January.
Outbreaks described in Nueva Ecija, Luzon, and Manila.
Russia Far Eastern maritime areas south of Khabarousk. Peak period July to September. First human cases in 30 years recently reported.
Singapore Rare cases. Year-round transmission, with April peak. Vaccine not routinely recommended.
Sri Lanka Endemic in all but mountainous areas.
Periodically epidemic in northern and central provinces.
October to January; secondary peak of enzootic transmission May to June. Recent outbreaks in central (Anuradhapura) and northwestern provinces.
Thailand Hyperendemic in north; sporadic-endemic in south. May to October. Annual outbreaks in Chiang Mai Valley; sporadic cases in Bangkok suburbs.
Vietnam Endemic-hyperendemic in all provinces. May to October. Highest rates in and near Hanoi.
Western Pacific Two epidemics reported in Guam & Saipan since 1947. Uncertain; possibly September to January. Enzootic cycle might not be sustainable; epidemics might follow introductions of virus.
  *  Local JE incidence rates might not accurately reflect risks to nonimmune visitors because of high immunization rates in local populations. Humans are incidental to the transmission cycle. High levels of viral transmission can occur in the absence of human disease.
NOTE: Assessments are based on publications, surveillance reports, and personal correspondence. Extrapolations have been made from available data. Transmission patterns can change.


Table 3-2.--Japanese Encephalitis Vaccine.
Doses Subcutaneous Route Comments
1 through 2 years of age 3 years of age or older
Primary series
1, 2, and 3
0.5 milliliter 1.0 milliliter Days 0, 7, and 30
Booster* 0.5 milliliter 1.0 milliliter 1 dose at 24 months or later

In vaccinees who have completed a three-dose primary series, the full duration of protection is unknown; therefore, definitive recommendations cannot be given.

Adverse Reactions

JE vaccine is associated with local reactions and mild systemic side effects (fever, headache, myalgias, and malaise) in about 20% of vaccinees. More serious allergic reactions, including generalized urticaria, angioedema, respiratory distress, and anaphylaxis, have occurred within minutes to as long as one week after immunization. Such hypersensitivity reactions occur in approximately 0.6% of vaccinees. Reactions have been responsive to therapy with epinephrine, antihistamines, or steroids, or a combination of these. Vaccinees should be observed for 30 minutes after immunization and warned about the possibility of delayed allergic reactions. The full course of immunization should be completed at least 10 days before departure, and vaccinees should be advised to remain in areas with access to medical care. People with a past history of urticaria appear to have a greater risk for developing more serious allergic reactions, and this must be considered when weighing the risks and benefits of the vaccine. A history of allergy to JE or other mouse-derived vaccines is a contraindication to further immunization.

Precautions and Contraindications

People with known hypersensitivity to the vaccine should not be vaccinated. People with multiple allergies, especially a history of allergic urticaria or angioedema, are at higher risk for allergic complications for JE vaccine.

Pregnancy.-- Vaccination during pregnancy should be avoided unless the risk of acquiring JE outweighs the theoretical risk of vaccination.


Travelers should be advised to stay in screened or air-conditioned rooms, to use bed nets when such quarters are unavailable, to use aerosol insecticides and mosquito coils as necessary, and to use insect repellents and protective clothing to avoid mosquito bites.


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©  John D. Wilson, M.D. 1999-2004; Last Update 11/19/2003