Japanese encephalitis virus (JEV) is the most important cause of viral encephalitis in Asia. It is a mosquito-borne flavivirus, and belongs to the same genus as dengue, yellow fever and West Nile viruses. The first case of Japanese encephalitis viral disease (JE) was documented in 1871 in Japan. Most countries in South-East Asia and Western Pacific regions have JEV transmission risk. JEV is transmitted to humans through bites from infected mosquitoes of the Culex species, mainly Culex tritaeniorhynchus. Once infected, humans do not develop sufficient viraemia to infect feeding mosquitoes. The virus exists in a transmission cycle between mosquitoes, pigs or water birds (enzootic cycle). The disease is predominantly found in rural and periurban settings, where humans live in closer proximity to these vertebrate hosts.
In most temperate areas of Asia, JEV is transmitted mainly during the warm season, when large epidemics can occur. In the tropics and subtropics, transmission can occur year-round but often intensifies during the rainy season, during which vector populations increase. The transmission also peaks during pre-harvest period in rice-cultivating regions. The spread of JEV in new areas has been correlated with agricultural development and intensive rice cultivation supported by irrigation programmes.
The disease primarily affects children, but individuals of any age may be affected. Most JEV infections are mild with only fever and headache, or without apparent symptoms. Only less than 1% of infections results in severe clinical illness. The incubation period is between 4-14 days. Severe disease is characterized by rapid onset of high fever, headache, neck stiffness, disorientation, coma, seizures, spastic paralysis and ultimately death. The case-fatality rate can be as high as 30% among those with disease symptoms. Of those who survive, 20%–30% suffer permanent intellectual, behavioural or neurological sequelae such as paralysis, recurrent seizures or speech impairment.
Individuals who live in or have travelled to a JE-endemic area and experience encephalitis are considered a suspected JE case. A laboratory test is required in order to confirm JEV infection and to rule out other causes of encephalitis. Diagnostic testing for JEV-specific IgM antibody in a single sample of cerebrospinal fluid (CSF) or serum using enzyme-linked immunosorbent assays (ELISA) is recommended. Surveillance of the disease is mostly syndromic for acute encephalitis syndrome. Currently, there is no antiviral treatment for patients with JE. Treatment is directed primarily at relieving the symptoms, including using anti-pyretics, optimal analgesics and fluids.
Safe and effective JE vaccines are available to prevent the disease. Even if the number of JE-confirmed cases is low, vaccination should be considered where there is a suitable environment for JE virus transmission. Additionally, to reduce the risk for JE, all population at risk in endemic areas should take precautions to avoid mosquito bites. Personal preventive measures include the use of mosquito repellents, long-sleeved clothes, coils and vaporizers. Travellers spending extensive time in JE endemic areas are recommended to get vaccinated before travel.
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Garjito TA, Widiarti, Anggraeni YM, Alfiah S, Tunggul Satoto TB, Farchanny A, et al. Japanese encephalitis in Indonesia: An update on epidemiology and transmission ecology. Acta Trop. 2018;187:240-7.
World Health Organization. Japanese encephalitis [updated 9 May 2019]. Available from: https://www.who.int/news-room/fact-sheets/detail/japanese-encephalitis.