Japanese encephalitis: Definition, symptoms, transmission, and diagnosis

Japanese encephalitis is a mosquito-borne viral infection. It is the leading cause of viral encephalitis in Asia. Humans can get the disease a mosquito that carries the virus bites them.

Japanese encephalitis virus (JEV) cannot transmit from one person to another.

JEV is related to the viruses that cause St. Louis encephalitis and Murray Valley encephalitis, West Nile virus, dengue fever, and yellow fever.

Encephalitis is an inflammation of the brain that can cause fever, headache, confusion, seizures, and, in some cases, death.

Fewer than 1 percent of people infected with the virus develop symptoms.

However, according to the World Health Organization (WHO), it is fatal for 30 percent of those who do develop symptoms.

Researchers estimate the number of fatalities from Japanese encephalitis is to be between 13,600 and 20,400 a year.

What is Japanese encephalitis?

culex mosquito
Culex mosquitos transmit Japanese encephalitis to humans.

Japanese encephalitis is a virus in the flavivirus family. The Culex mosquito passes it on.

The virus can infect horses and pigs, as well as humans. This can lead to encephalitis in horses and miscarriage in pigs.

A host is the source of a virus, and the vector passes it on. Wild birds are likely to be the natural hosts of JEV, and mosquitoes are the vectors. A vector does not cause disease but passes it on.

When mosquitoes infect an animal, the animal might become a carrier of the virus. When other mosquitos feed on these animals that have newly acquired the virus, they take it on board and infect other animals.

People are at the highest risk in rural areas where the virus is common. Japanese encephalitis is common around towns and cities.

It is more likely to affect children because adults in areas where the virus is endemic generally become immune as they get older.

Where is it most common?

Japanese encephalitis is most common throughout Southeast Asia.

China, Korea, Japan, Taiwan, and Thailand have had outbreaks in the past, but they have mainly controlled the disease through vaccination. Vietnam, Cambodia, Myanmar, India, Nepal, and Malaysia still experience occasional epidemics.

There have been cases in northern Australia, but health authorities in mainland Australia consider the disease to be low-risk.

The United States has seen a few reports of Japanese encephalitis in people who have traveled to places in which the disease is active.

Overall, the chance of catching Japanese encephalitis while traveling in Asia is extremely low. However, it depends on the season, the travel destination, the duration of stay, and which activities a traveler in Asia would be doing.

The risk is highest during the transmission season, but this varies from place to place in the following ways:

In temperate regions, transmission is highest during the summer and early fall, between roughly May and September.

In subtropical and tropical areas, the season depends on the rainfall and patterns of bird migrations.

In some tropical areas, transmission might occur at any time of the year, depending partly on agricultural practices.

It is more common in areas where people grow rice.

Japanese encephalitis fever
Japanese encephalitis can lead to a high fever.

A person with Japanese encephalitis will probably have no symptoms at all, but if there are symptoms, they will appear 5 to 15 days after being infected.

A person with mild Japanese encephalitis might only develop a fever and a headache, but in more severe cases, more serious symptoms can develop quickly.

Possible symptoms include:

a headache

high fever




stiff neck

spastic paralysis

A person might also undergo changes to brain function, including:




convulsions in children

The testicles can also swell.

The brain symptoms of Japanese encephalitis can cause lifelong complications, such as deafness, uncontrollable emotions, and weakness on one side of the body.

The chance of surviving the disease varies, but children face the highest risk of fatal consequences.

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