Japanese encephalitis

Japanese encephalitis is a potentially deadly infectious disease found mainly in Asia, where 68.000 clinical cases are estimated to occur each year.^(source)

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Japanese encephalitis Flyer (PDF)

Japanese encephalitis Poster (PDF)

Key facts

Japanese encephalitis (JE) is the most common vaccine-preventable cause of encephalitis in Asia.⁽¹⁾
The disease is endemic in 25 countries in Asia and the Western Pacific including Australia^(5,6), not just in Japan.⁽³⁾
JE is caused by bites from Culex mosquitoes, which carry the JE virus.^(1,3)
JE can progress to inflammation of the brain (encephalitis), which can cause seizures, coma and paralysis.⁽⁴⁾
The case-fatality rate can be as high as 30% among those with disease symptoms.⁽³⁾
Among survivors, 30%–50% have serious neurologic, cognitive, or psychiatric sequelae.⁽¹⁾
There is currently no antiviral treatment for JE.⁽³⁾
Vaccination and preventing mosquito bites are reducing the risk for JE.⁽³⁾

What is JE?

Japanese encephalitis (JE) is a potentially serious disease, which is caused by the Japanese-Encephalitis-virus, a mosquito-borne Flavivirus, and is closely related to dengue, yellow fever and West Nile viruses.⁽³⁾ JE is the main cause of viral encephalitis in Asia.⁽³⁾ JE virus is transmitted to humans through the bite of an infected mosquito, primarily Culex species. The virus is maintained in an enzootic cycle between mosquitoes and amplifying vertebrate hosts, primarily pigs and wading birds.^(1,3)

Video JE

Where does JE occur?

JE map

JE is endemic in 25 countries across Asia and the Western Pacific and Australia.^(3,5,6) Transmission of the virus through mosquitoes occurs mainly in rural agricultural areas, often associated with rice cultivation and flood irrigation, but also in urban centers.⁽¹⁾ In temperate areas of Asia, transmission is seasonal, and human disease usually peaks in summer and fall.⁽¹⁾ In the subtropics and tropics, seasonal transmission varies with monsoon rains and irrigation practices and may be prolonged or even occur year-round.⁽¹⁾

Who is at risk of contracting JE?

For most travelers to Asia, the risk for JE is extremely low but varies based on their destination, duration of travel, season, and activities.⁽¹⁾ However, expatriates and travelers who stay for prolonged periods in rural areas with active JE virus transmission might be at similar risk as resident population.⁽¹⁾ Travelers on even brief trips might be at increased risk if they have extensive outdoor or nighttime exposure in rural areas during periods of active transmission.⁽¹⁾

What are the symptoms and lasting effects of JE?

Most people infected with JE are asymptomatic or only have mild symptoms.⁽³⁾ However, in a small number of cases, acute encephalitis is the most common clinical sign of the disease, observed between 5 and 15 days after infection.⁽¹⁾ Illness usually begins with sudden onset of fever, headache, and vomiting.⁽¹⁾ Mental status changes, focal neurologic deficits, generalized weakness, and movement disorders may develop over the next few days.⁽¹⁾ The classical description of JE includes a parkinsonian syndrome.⁽¹⁾ Acute flaccid paralysis has also been associated with JE virus infection. Seizures are common, especially among children.⁽¹⁾ The case-fatality ratio is approximately 20%–30%. Among survivors, 30%–50% have serious neurologic, cognitive, or psychiatric sequelae.⁽¹⁾

How can JE be treated?

There is currently no specific antiviral treatment available for JE.^(1,3,4) Treatment is supportive to relieve symptoms and stabilize the patient.^(1,3,4) To reduce the risk for JE, all travellers to Japanese encephalitis-endemic areas should take precautions to avoid mosquito bites.^(1,3)JE vaccines are available to prevent the disease.⁽³⁾

References

Centers for Disease Control and Prevention. Yellow Book 2024. Travel-Associated Infections & Diseases. Japanese Encephalitis. Available online: https://wwwnc.cdc.gov/travel/yellowbook/2020/travel-related-infectious-diseases/japanese-encephalitis (Last accessed December 2023)
WHO. Disease Outbreak. JE Australia. Online available: https://www.who.int/emergencies/disease (Last accessed December 2023)
WHO. Fact sheet. Japanese Encephalitis. Online available: https://www.who.int/news-raom/fact sheets/detail/japanese-encephalitis (Last accessed December 2023)
Centers for Disease Contra! and Prevention. Japanese Encephalitis. Frequently Asked Questions. Online available: https://www.cdc.gov/japaneseencephalitis/qa/ (Last accessed December 2023)
BlueDot’s Initial Assessment. Geographic Expansion of Japanese Encephalitis Virus (JEV) into Australia. (Last accessed December 2023)
CDC. Geographic distribution of Japanese encephalitis virus. Available at https://www.cdc.gov/japaneseencephalitis/maps/index.html (Last accessed December 2023)