Japanese encephalitis virus (JEV) is a flavivirus, spread by mosquitoes and related to dengue, yellow fever and West Nile viruses. The virus is transmitted to humans through the bites of infected Culex mosquitoes and is predominantly found in rural parts of Asia and the western Pacific where humans live near pigs, the main animal host. It is thought that around three billion people around the world live in areas where there is a risk of the disease. However, vaccines protect against it.
Most JEV infections are mild (fever and headache) or without symptoms, but approximately one in 250 infections results in severe clinical illness. The incubation period is four to 14 days. In children, gastrointestinal pain and vomiting may be the dominant initial symptoms.
Severe disease is characterised by rapid onset of the following:
The case fatality rate can be as high as 30 per cent among those with disease symptoms. Among patients who develop encephalitis, about one in four die. It is estimated that there are around 100,000 cases of JEV-related illness a year and around 25,000 deaths. Of those who survive, 20 to 30 per cent suffer permanent cognitive, behavioural or neurological damage such as seizures, hearing or vision loss, speech, language, memory, and communication problems, or weakness of the limbs. Long-term complications can include paralysis, seizures and loss of speech.
JEV is transmitted to humans through bites from infected mosquitoes of the Culex species, mainly Culex tritaeniorhynchus. Once infected, you cannot pass the disease onto another person.
The disease is predominantly found in rural areas, where humans live near animals, in particular domestic pigs.
In temperate parts of Asia – Japanese islands, Korea and Mongolia – it is transmitted mainly during the warm season. But in the tropics and subtropics – countries including Thailand, India, Vietnam and Indonesia – it is spread year-round but is more prevalent. during the rainy season. The risk is very low for most travellers to Asia, but varies based on destination, length of travel, season, and activities.
There is no specific treatment and care is focused on symptom relief. Treatments include painkillers and steroids. Encephalitis is a medical emergency and requires urgent medical attention.
A vaccine for Japanese encephalitis is recommended if travelling to an affected country, especially if you are staying for more than a month, will be in a rural area, or visiting rice fields, wetlands, or places where pigs are kept.
You should also protect yourself from mosquito bites by using insect repellent, sleeping under a mosquito net treated with insecticide, wearing long-sleeved tops, trousers or long skirts, socks, and shoes.
The first case of Japanese encephalitis viral disease was documented in 1871 in Japan. In 1924, the infectious agent was isolated from human brain tissue, again in Japan.
The first vaccines against the disease were developed and licenced in the 1960s, with more sophisticated jabs available since 2011. The widespread rollout of immunisation has seen a drop in incidence rate of JEV in south east Asia from a high of 3.1 cases per million people in 2007 to 0.3 cases per million people in 2024.
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