Transmission of Japanese Encephalitis Virus

Pig enclosed in a farm

Japanese encephalitis (JE) virus, a flavivirus, is closely related to West Nile and St. Louis encephalitis viruses. JE virus is transmitted to humans through the bite of infected Culex species mosquitoes, particularly Culex tritaeniorhynchus.

The virus is maintained in a cycle between mosquitoes and vertebrate hosts, primarily pigs and wading birds. Humans are incidental or dead-end hosts, because they usually do not develop high enough concentrations of JE virus in their bloodstreams to infect feeding mosquitoes.

JE virus transmission occurs primarily in rural agricultural areas, often associated with rice production and flooding irrigation. In some areas of Asia, these conditions can occur near urban centers.

In temperate areas of Asia, JE virus transmission is seasonal. Human disease usually peaks in the summer and fall. In the subtropics and tropics, transmission can occur year-round, often with a peak during the rainy season.

Risk areas and transmission season for Japanese encephalitis (JE), by country1,2,3

Country

Country

Country

Risk areas

Risk areas

Risk areas

Transmission season

Transmission season

Transmission season

Comments

Comments

Comments

Australia

Country

Australia

Outer Torres Strait Islands, parts of four states on mainland (New South Wales, Queensland, South Australia, Victoria)

Risk areas

Outer Torres Strait Islands, parts of four states on mainland (New South Wales, Queensland, South Australia, Victoria)

December–May

Transmission season

December–May

Four cases previously reported from Outer Torres Strait Islands and 1 case from Tiwi Islands. On Australian mainland, 1 case  reported in 1998 from Far North Queensland and cases reported in 2022 in eastern and southern states. Vaccine only recommended for travel to Outer Torres Strait Islands, unless travelers to mainland plan to work or reside near pig operations, or work directly with mosquitoes.

Comments

Four cases previously reported from Outer Torres Strait Islands and 1 case from Tiwi Islands. On Australian mainland, 1 case  reported in 1998 from Far North Queensland and cases reported in 2022 in eastern and southern states. Vaccine only recommended for travel to Outer Torres Strait Islands, unless travelers to mainland plan to work or reside near pig operations, or work directly with mosquitoes.

Bangladesh

Country

Bangladesh

Widespread

Risk areas

Widespread

Year-round with most cases reported July–November

Transmission season

Year-round with most cases reported July–November

Highest disease incidence in northwest Bangladesh

Comments

Highest disease incidence in northwest Bangladesh

Bhutan

Country

Bhutan

Presumed widespread in nonmountainous areas

Risk areas

Presumed widespread in nonmountainous areas

Unknown

Transmission season

Unknown

Risk likely highest in southern districts that share similar ecologic conditions with bordering JE-endemic states of India

Comments

Risk likely highest in southern districts that share similar ecologic conditions with bordering JE-endemic states of India

Brunei Darussalam

Country

Brunei Darussalam

Presumed widespread

Risk areas

Presumed widespread

Unknown

Transmission season

Unknown

Limited data but outbreak reported in 2013

Proximity to Sarawak, Malaysia suggests ongoing transmission likely

Comments

Limited data but outbreak reported in 2013

Proximity to Sarawak, Malaysia suggests ongoing transmission likely

Burma (Myanmar)

Country

Burma (Myanmar)

Widespread

Risk areas

Widespread

Year-round with most cases reported May–September

Transmission season

Year-round with most cases reported May–September

Greatest risk in delta and lowland areas

Comments

Greatest risk in delta and lowland areas

Cambodia

Country

Cambodia

Widespread

Risk areas

Widespread

Year-round with peak season May–October

Transmission season

Year-round with peak season May–October

Cases reported from majority of provinces, so transmission likely countrywide

Comments

Cases reported from majority of provinces, so transmission likely countrywide

China

Country

China

Andhra Pradesh, Arunachal Pradesh, Assam, Bihar, Goa, Haryana, Jharkhand, Karnataka, Kerala, Maharashtra, Manipur, Meghalaya, Nagaland, Odisha, Punjab, Tamil Nadu, Telangana, Tripura, Uttar Pradesh, Uttarakhand, West Bengal

Risk areas

Andhra Pradesh, Arunachal Pradesh, Assam, Bihar, Goa, Haryana, Jharkhand, Karnataka, Kerala, Maharashtra, Manipur, Meghalaya, Nagaland, Odisha, Punjab, Tamil Nadu, Telangana, Tripura, Uttar Pradesh, Uttarakhand, West Bengal

Peak season May–November, especially in northern India; the season may be extended or year-round in some areas, especially in southern India

Transmission season

Peak season May–November, especially in northern India; the season may be extended or year-round in some areas, especially in southern India

Comments

Indonesia

Country

Indonesia

Widespread

Risk areas

Widespread

Year-round, with peak season varying by island

Transmission season

Year-round, with peak season varying by island

Cases reported from many islands, including Sumatra, Java, Kalimantan, Bali, Nusa Tenggara, and Papua, so transmission likely on all islands

Several traveler cases reported in recent years from Bali

Comments

Cases reported from many islands, including Sumatra, Java, Kalimantan, Bali, Nusa Tenggara, and Papua, so transmission likely on all islands

Several traveler cases reported in recent years from Bali

Japan

Country

Japan

All islands

Risk areas

All islands

June–October

Transmission season

June–October

Rare sporadic cases reported from all islands except Hokkaido

Enzootic transmission without reported human cases on Hokkaido

Comments

Rare sporadic cases reported from all islands except Hokkaido

Enzootic transmission without reported human cases on Hokkaido

Lao People’s Democratic Republic

Country

Lao People’s Democratic Republic

Widespread

Risk areas

Widespread

Year-round with peak season June–September

Transmission season

Year-round with peak season June–September

Comments

Malaysia

Country

Malaysia

Widespread

Risk areas

Widespread

Year-round, with peak season in Sarawak from October–December

Transmission season

Year-round, with peak season in Sarawak from October–December

Much higher rates of disease reported from Sarawak than peninsular Malaysia

Comments

Much higher rates of disease reported from Sarawak than peninsular Malaysia

Nepal

Country

Nepal

Southern lowlands (Terai), some hill and mountain districts

Risk areas

Southern lowlands (Terai), some hill and mountain districts

Peak season June–October

Transmission season

Peak season June–October

Highest rates of disease reported from southern lowlands (Terai)

Vaccine not routinely recommended for those trekking in high-altitude areas

Comments

Highest rates of disease reported from southern lowlands (Terai)

Vaccine not routinely recommended for those trekking in high-altitude areas

North Korea

Country

North Korea

Presumed widespread

Risk areas

Presumed widespread

Unknown

Proximity to South Korea suggests peak transmission May–November

Transmission season

Unknown

Proximity to South Korea suggests peak transmission May–November

Comments

Pakistan

Country

Pakistan

Unknown

Risk areas

Unknown

Unknown

Transmission season

Unknown

Very limited data

Previous case report and serosurvey data suggest transmission possible at least in Sindh Province

Comments

Very limited data

Previous case report and serosurvey data suggest transmission possible at least in Sindh Province

Papua New Guinea

Country

Papua New Guinea

Widespread

Risk areas

Widespread

Presumed year-round

Transmission season

Presumed year-round

Sporadic cases reported from Western Province, serologic evidence of disease from Gulf and Southern Highland Provinces, and 1 case reported from near Port Moresby, so transmission likely countrywide

Comments

Sporadic cases reported from Western Province, serologic evidence of disease from Gulf and Southern Highland Provinces, and 1 case reported from near Port Moresby, so transmission likely countrywide

Philippines

Country

Philippines

Widespread

Risk areas

Widespread

Year-round with peak season April–August

Transmission season

Year-round with peak season April–August

Human, animal, and mosquito studies have indicated transmission in 32 provinces, and transmission likely on all islands

Comments

Human, animal, and mosquito studies have indicated transmission in 32 provinces, and transmission likely on all islands

Russia

Country

Russia

Primorsky Krai

Risk areas

Primorsky Krai

June–September

Transmission season

June–September

Cases previously reported from Primorsky Krai

Vaccine not routinely recommended

Comments

Cases previously reported from Primorsky Krai

Vaccine not routinely recommended

Singapore

Country

Singapore

Presumed in focal areas

Risk areas

Presumed in focal areas

Year-round

Transmission season

Year-round

Very rare sporadic cases reported

Vaccine not routinely recommended

Comments

Very rare sporadic cases reported

Vaccine not routinely recommended

Sri Lanka

Country

Sri Lanka

Widespread except in mountainous areas

Risk areas

Widespread except in mountainous areas

Year-round with peak season November–February

Transmission season

Year-round with peak season November–February

Comments

Taiwan

Country

Taiwan

Widespread

Risk areas

Widespread

Peak season May–October

Transmission season

Peak season May–October

Comments

Thailand

Country

Thailand

Widespread

Risk areas

Widespread

Year-round with peak season May–October, especially in northern Thailand

Transmission season

Year-round with peak season May–October, especially in northern Thailand

Highest rates of disease reported from Chiang Mai Valley

Several traveler cases reported in recent years from resort and coastal areas of southern Thailand

Comments

Highest rates of disease reported from Chiang Mai Valley

Several traveler cases reported in recent years from resort and coastal areas of southern Thailand

Timor-Leste

Country

Timor-Leste

Presumed widespread

Risk areas

Presumed widespread

No data

Proximity to West Timor suggests year-round

Transmission season

No data

Proximity to West Timor suggests year-round

Comments

Viet Nam

Country

Viet Nam

Widespread

Risk areas

Widespread

Year-round with peak season May–October, especially in northern Viet Nam

Transmission season

Year-round with peak season May–October, especially in northern Viet Nam

Comments

1 Destination and transmission season information should be considered in association with travel duration and activities when making decisions on vaccination.

2 Data are based on published and unpublished reports. Risk assessments should be performed cautiously, because risk can vary within areas and from year to year, and surveillance data regarding human cases and JE virus transmission are often incomplete. In some endemic areas, human cases among residents are limited because of vaccination or natural immunity among older people. However, because JE virus is maintained in an enzootic cycle between animals and mosquitoes, susceptible visitors to these areas still may be at risk for infection.

3 Outbreaks previously occurred in the Western Pacific Islands of Guam (1947–1948) and Saipan (1990), but as they are no longer considered risk areas, they are not included in the table.