Please note this factsheet is currently under review, however the content is safe to be used.

By Dr Benedict Michael, NIHR Doctoral Research Fellow, University of Liverpool and Professor Tom Solomon, Chair of Neurological Science, Head of Institute for   Infection and Global Health, University of Liverpool and reviewed by Katherine Dodd, Specialist Registrar in Neurology


Epidemics of Encephalitis were described in Japan from the 1870s onwards and Japanese Encephalitis virus was first isolated from a patient who died with the condition in the 1930s.  It is a small ‘Flavivirus’, named after the original Yellow fever virus (in Latin yellow = flavus). The flaviviruses are relatively new viruses, arising from a common ancestor 10-20,000 years ago and they are rapidly evolving and involving new areas.

Japanese Encephalitis virus is transmitted between small birds by mosquitoes, called ‘Culex’ mosquitoes. Additionally, when pigs are infected, they can make a lot of the virus and this can then infect a lot of mosquitos. Humans become infected by mosquitoes coincidentally and are not part of the natural cycle.

Japanese Encephalitis virus has always been recognised as a killer. Over the last 50 years it has spread relentlessly across South-East Asia, India, southern China, and the Pacific reaching Australia in 1998.  Culex mosquitoes are unavoidable in rural Asia, and almost everyone is exposed to the virus. Only about 1 in 300 people who are infected with the virus, get symptoms. This can range from a simple raised temperature illness to a severe brain infection (Encephalitis), as well as causing paralysis of an arm or leg similar to that seen with polio. 


There are estimated to be 67,900 people with Japanese Encephalitis per year with a 20%-30% case-fatality rate and neurologic or psychiatric sequelae in 30%-50% of survivors. The actual numbers may become clearer with the application of new simple rapid diagnostic tests.  

Treatment and Prevention

There is no specific treatment for Japanese virus Encephalitis. Current management consists of treating the complications of the disease such as high fever and aches, low blood pressure, blood loss, convulsions (fits) or raised intracranial pressure.

The simplest preventative measure is to avoid bites from the mosquitoes that carry the virus. This means wearing long sleeves and trousers, especially during the evening when the mosquito bites and avoiding areas where stagnant water can be found as mosquito larvae need still water to develop. For further protection use an insect spray containing at least 30% DEET (N,N-diethyl-3methlybenzamide) and sleep under bed-nets.

Several vaccines are now available for Japanese Encephalitis.  In the last few years routine vaccination has started to be given to local people in some countries such as China and India. UK travellers are recommended to get the vaccination if staying for a long time in rural affected areas. 

Future Research

Recent findings in Asia raise important issues about the spread and control of Japanese Encephalitis. It is thought to be spread by birds, but mosquitoes blown between Pacific islands may contribute too. The majority of the 2.8 billion people living in affected regions of Japanese Encephalitis still do not have access to the vaccine. For them, the factors determining who, of all those infected with Japanese Encephalitis virus, develops brain or spine disease may be critically important. The contributions of the human immune response and different types of virus are currently being investigated, as well as blood ‘biomarkers’ to help reach a faster diagnosis.

FS006V2 Japanese Encephalitis

Page Created: March 2002/Last Updated June 2014/ Review date: June 2017

Disclaimer: We try to ensure that the information is easy to understand, accurate and up-to-date as possible. If you would like more information on the source material the author used to write this document please contact the Encephalitis Society. None of the authors of the above document has declared any conflict of interest which may arise from being named as an author of this document