Japanese encephalitis is a rare but serious mosquito-borne infection. Learn who should consider vaccination before travel and how the virus spreads.
What Japanese encephalitis is and why travellers ask about it
Japanese encephalitis is a rare but serious infection caused by a virus spread through mosquito bites. The NHS states that it is not found in the United Kingdom and is mostly found in rural areas of Asia, although it is rare for travellers to get it. The World Health Organization reports that 24 countries in its South-East Asia and Western Pacific Regions carry transmission risk, covering more than 3 billion people. For most people who are infected, there are no symptoms at all. A small proportion develop severe illness. The virus belongs to the same antigenic complex as West Nile virus, according to the European Centre for Disease Prevention and Control. Knowing the geography and the mosquito habits helps travellers judge their own risk calmly.
How the virus spreads between mosquitoes, birds and pigs
The virus is maintained in an enzootic cycle between mosquitoes, water birds and pigs. The European Centre for Disease Prevention and Control describes the cycle as involving Culicidae mosquitoes, Ardeidae water birds such as egrets and herons, and pigs. In Asia, Culex tritaeniorhynchus and Culex vishnui appear to be the most important maintenance vectors. Culex pipiens pipiens, Culex pipiens molestus and Culex quinquefasciatus show moderate transmission efficiency, and transovarial transmission has been demonstrated in some Aedes species. The World Health Organization states that the virus circulates between mosquitoes, pigs and water birds. Humans and horses are dead-end hosts, meaning they can be infected but do not contribute to the transmission cycle (ECDC). The World Health Organization adds that infected people do not develop sufficient viraemia to infect feeding mosquitoes (WHO), so patients remain dead-end hosts in this cycle.
When and where transmission risk peaks
Transmission follows mosquito seasons and farming patterns. The World Health Organization says that in temperate Asia transmission occurs mainly during the warm season, when large epidemics can occur. In tropical and subtropical areas it can occur year-round, but often intensifies during the rainy season and the pre-harvest period in rice-growing regions. The European Centre for Disease Prevention and Control links epidemics to monsoons in the tropics and to summer in temperate climates. Many mosquitoes are most active at dusk and dawn, according to the same agency (ECDC). Rice fields, wetlands and places where pigs are kept bring the mosquito cycle close to people, and irrigation flooding around settlements can add to the risk for travellers who sleep outdoors without a net.
Symptoms, severity and who is most at risk
Most infections are asymptomatic or cause only fever and headache. The European Centre for Disease Prevention and Control reports that about 1 in 250 infected people develops severe neuroinvasive illness. Case fatality in severe disease is up to 30%, and up to 50% of survivors have neurologic or psychiatric sequelae. The World Health Organization estimates about 100,000 clinical cases a year and approximately 25,000 deaths, with a case fatality rate as high as 30% among people with symptoms and 20 to 30% of survivors left with permanent cognitive, behavioural or neurological sequelae. Higher-risk groups include rural residents, long-term travellers with rural exposure, and travellers to irrigation-flooded areas who sleep outdoors without a net. The World Health Organization reports that 20 to 30 percent of survivors suffer permanent cognitive, behavioural or neurological after-effects from the disease.
Who should discuss vaccination before travel
The World Health Organization recommends that travellers who spend extensive time in Japanese encephalitis endemic areas be vaccinated before travel. It also recommends that vaccination be integrated into national immunisation schedules where the disease is a recognised public health issue. The NHS advises vaccination for people travelling to a part of the world where the virus is found, especially if they are staying more than a month, staying in a rural area, or staying near or visiting rice fields, wetlands or places where pigs are kept. The vaccine must be paid for, and it is available from travel health clinics and pharmacies with travel health services. A travel health professional can weigh itinerary, season and activities against the low but real risk.
How the vaccine is given and what the evidence shows
The European Medicines Agency states that Ixiaro is an inactivated Japanese encephalitis vaccine produced in Vero cells. It is indicated for active immunisation of adults, adolescents, children and infants aged two months and older, particularly people at risk through travel or occupation. The standard course is two doses four weeks apart: a full 0.5 ml dose for adults and children aged three years and older, and 0.25 ml for children aged two months to three years. Adults aged 18 to 65 can instead receive a rapid course with the second dose seven days after the first. The second dose should be given at least one week before potential exposure. In a study of 867 adults, 96% of those who received both doses developed protective antibody levels.
Practical steps alongside vaccination
Vaccination is one layer of protection, not the only one. The NHS advises travellers to use insect repellent on exposed skin, ideally containing at least 50% DEET, to sleep under an insecticide-treated mosquito net, and to wear long-sleeved tops, trousers or long skirts, socks and shoes. The European Centre for Disease Prevention and Control likewise points to insecticide-treated nets, screened rooms and covering clothing, because the mosquitoes are most active at dusk and dawn. There is no specific treatment for the disease, so prevention carries the weight. Seriously unwell patients need hospital treatment, as the illness can be life-threatening and can sometimes cause long-term complications such as paralysis, seizures and loss of speech. The World Health Organization notes that three main Japanese encephalitis vaccine types are in use, and one inactivated vaccine plus two live vaccines hold prequalification status.
Sources
- Japanese encephalitis: World Health Organization
- Disease information about Japanese encephalitis: European Centre for Disease Prevention and Control
- Japanese encephalitis: NHS
- Ixiaro: European Medicines Agency