Japanese encephalitis is a viral disease spread by mosquitoes, which transfer the virus from infected animals - usually pigs and wading birds - to humans. Areas such as rice fields, where mosquitoes thrive and there is a lot of pig farming, are especially risky.
It was first recognised in Japan in the late 1800s (hence the name) and has since been found throughout most countries of east and South East Asia where it is the leading cause of viral encephalitis.
Approximately 30,000 to 50,000 cases are reported every year, and there are about 10,000 deaths, mostly in children. In fact it’s now thought that many more people have the infection (research shows that by the age of 15 most people in South East Asia have had it) but symptoms are usually minimal so it doesn’t get reported.
Causes And Risk Factors
Japanese encephalitis is caused by a Flaviviridae virus (or flavivirus), which is transmitted by the bite of an infected mosquito.
Transmission of the disease is most likely during the summer months in temperate areas and during the rainy season and early dry season in tropical areas, when the mosquito populations are the highest.
Japanese encephalitis is rare in travellers and the risk to short-term visitors to the region is very low, especially if they are just visiting urban areas. However, it has a high fatality rate, and can cause chronic complications so it should be taken seriously.
Symptoms of Japanese Encephalitis
Most people who are infected show only mild symptoms or no symptoms at all. However, in severe cases the disease may be fatal.
Japanese encephalitis begins like flu with headache, fever, and weakness. As it progresses to inflammation of the brain there may be confusion and delirium. Gastrointestinal problems, including vomiting, may also be present. About one third of these patients will die, and 25-30 per cent have neurological damage including paralysis, speech difficulties, Parkinson’s-like syndrome or psychological problems. Children are most vulnerable.
Treatment And Prevention Of Japanese Encephalitis
At present, there is no medical ‘cure’ for Japanese encephalitis once infection has occurred although supportive care in hospital can help. A vaccine has been developed and is used in Asia to immunize children. It is licensed for use in the UK and the USA for people who plan to travel to South East Asia. Allergic reactions can occur in up to one in 100 people vaccinated but are mostly minor.
Reducing The Risk
The best way to reduce the risk of contracting the disease is to reduce exposure to mosquitoes. This can be done by avoiding being outside in the cooler hours between dusk and dawn when mosquitoes are most actively feeding. The use of mosquito repellent, portable bednets, aerosol room insecticides and permethrin, which can be applied to clothing, is also recommended.
Scientists researching the Malaysian outbreak in 1998 have isolated a mysterious second virus, a member of the Paramyxovirus family, which is believed to have caused some of the deaths. The new virus is similar to the Hendra virus that killed two people and 15 horses during outbreaks in Australia in 1994 and 1995. The method of transmission is as yet unclear. Health experts believe it may be killed through the use of soap, detergent or disinfectant.