What is it?
Japanese encephalitis (JE) is a rare but potentially serious disease caused by infection with Japanese encephalitis virus (JEV). JEV is a flavivirus (other flaviviruses cause diseases including yellow fever, dengue and Murray Valley encephalitis). Spread to humans through mosquito bites, JEV is a leading cause of vaccine-preventable encephalitis (brain inflammation) in Asia and the Western Pacific.
What to look for?
Most people infected with JEV do not develop symptoms or develop mild symptoms only. However, approximately 1 in 250 people infected will experience severe clinical illness. If symptoms develop, they will occur 5 to 15 days after exposure and can include fever, diarrhoea and rigors (episodes of shivering and feeling cold). Features of severe illness with encephalitis include headache, confusion, vomiting, paralysis, seizures, neck stiffness, disorientation and coma. JE has a mortality rate of 20 to 30%, with permanent neurological disability occurring in 30 to 50% of survivors.
How is it transmitted?
JE is a zoonotic disease and can spread to humans when a mosquito bites an infected animal (mostly water birds or pigs) and then bites a human. It cannot be transmitted from person to person, or through eating pork or pig products. Horses can be infected with JEV; however, they cannot transmit the disease further.
When JE occurs during pregnancy, infection can be transmitted to the unborn baby.
In most temperate areas of Asia and Australia, JEV is transmitted mainly during the warmer months. In the tropics and subtropics, transmission can occur year-round but often intensifies during the rainy season when mosquito numbers are greater.
Epidemiology
There are an estimated 100,000 clinical cases of JE globally each year, resulting in approximately 25,000 deaths. JE is most common across Asia and the Western Pacific. In the 1990s, cases were reported in the Torres Strait Islands and in Cape York (mainland Australia). In 2022 and 2023, cases occurred in the Murray and Riverina regions of south-eastern Australian states.
Where JEV is present, people with increased exposure to mosquitoes are at higher risk of infection, particularly those spending time outdoors or working with pigs. Children under 5 years of age and older people who are infected with JEV are at higher risk of more severe illness, including encephalitis.
Prevention
Physical barriers
Preventing mosquito bites with physical barriers is the most effective way to avoid infection with JEV. Exposure to mosquito bites can be prevented by:
- wearing long, loose fitting clothes when outdoors
- using mosquito repellents containing picaridin or DEET
- limiting outdoor activity if there are lots of mosquitoes
- using fly sprays, mosquito coils and plug-in repellent devices
- sleeping under mosquito nets treated with insecticides if you don’t have flywire screens on your windows or are sleeping in an untreated tent/in the open outdoors.
Vaccines
Vaccines are also available to prevent the development of disease. There are currently two JE vaccines available in Australia: Imojev and JEspect. Vaccination is recommended for:
- all research laboratory workers at risk of exposure to JEV
- travellers spending 1 month or more in areas outside of Australia with endemic JEV transmission
- people who live or work in the outer islands in the Torres Strait (for a cumulative total of 30 days or more) during the wet season (December to May).
Other individuals with specific risk factors are prioritised and funded to receive vaccines through state-led programs:
Table 1: Vaccine brands and dose recommendations
| Vaccine brand | Age | Dose and route | Primary course | Booster |
| Imojev | ≥ 9 months – < 18 years | 0.5 mL subcutaneous (SC) | 1 dose | 1–2 years after primary course if ongoing risk of exposureΩ |
| ≥ 18 years | 0.5 mL SC* | 1 dose | NA | |
| JEspect^ | ≥ 2 months – < 3 years | 0.25 mL intramuscular (IM) | 2 doses, at least 28 days apart | 1–2 years after primary course if ongoing risk of exposureΩ |
| ≥ 3 years – < 18 years | 0.5 mL IM | |||
| ≥ 18 years | 0.5 mL IM | 2 doses, at least 28 days apart# |
shaded boxes indicate live-attenuated vaccines, which are contraindicated for pregnant women and people who are immunocompromised.
* Intradermal (ID) administration of Imojev (single dose of 0.1 mL) may be used as a dose-sparing strategy for heatlhy adults (non-pregnant, non-breastfeeding, no immunocompromise) aged ≥ 18 years in circumstances where public health benefits outweigh potential risk as determined by State/Territory health authorities.
^JEspect is an inactivated vaccine and is therefore suitable for administration in pregnant women or immunocompromised individuals.
# The interval between doses of JEspect can be shortened to 7 days for those aged ≥ 18 years, if the individual is at imminent risk of JEV exposure.
Ω Where possible the same brand of vaccine should be used for the booster dose as the primary course. Based on first prinicples, use of the alternate brand should provide a satisfactory immune response however there are no studies confirming this.
Side effects
Expected side effects following either vaccine are usually mild and short lived. They can include injection site pain, redness and swelling, as well fever, headaches and muscle aches.
Resources
- Victorian Department of Health: Japanese encephalitis in Victoria
- Australian Immunisation Handbook: Japanese encephalitis
- ATAGI statement on the intradermal use of Imojev Japanese encephalitis vaccine
- Australian Government Department of Health and Aged Care: Japanese encephalitis virus situation declared a Communicable Disease Incident of National Significance
- CDNA advice regarding vaccination against Japanese encephalitis virus
Authors: Rachael Purcell (Immunisation Fellow, Royal Children’s Hospital), Nigel Crawford (Director, SAEFVIC, Murdoch Children’s Research Institute), Rachael McGuire (MVEC Education Nurse Coordinator) and Raffaela Armiento (Consultant Paediatrician, Victorian Immunisation Specialist Services)
Reviewed by: Rachael McGuire (MVEC Education Nurse Coordinator)
Date: October 2024
Materials in this section are updated as new information and vaccines become available. The Melbourne Vaccine Education Centre (MVEC) staff regularly reviews materials for accuracy.
You should not consider the information in this site to be specific, professional medical advice for your personal health or for your family’s personal health. For medical concerns, including decisions about vaccinations, medications and other treatments, you should always consult a healthcare professional.