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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.

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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.
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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
  • tremors
  • nausea
  • vomiting
  • stiff neck
  • spastic paralysis

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

  • stupor
  • disorientation
  • coma
  • 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.

The best ways to prevent Japanese encephalitis is through vaccination and insect repellent.


A safe and effective vaccine is available to prevent infection. A doctor will give this as an injection over two doses.

The second dose will occur 28 days after the first. An accelerated schedule is also possible, in which only 7 days pass between the two doses. The accelerated schedule is only safe and suitable for people between 18 and 65 years old.

Be sure to complete either course at least 7 days before traveling to the prevalent region.

Doctors recommend the vaccine for the following people:

  • those traveling to areas where the disease is endemic
  • those on short-term trips lasting less than a month
  • people visiting areas where there is an outbreak or after a recent outbreak
  • those participating in outdoor activities while in an endemic area

A person who fits any of these categories should speak to a doctor 6 to 8 weeks before traveling to the region.

The Japanese encephalitis vaccine can cause some short-term side effects, including:

  • red, swollen, and sore skin at the site of injection
  • a headache
  • muscle pain
  • hives and breathing difficulties, in rare instances

Some people are allergic to certain ingredients in the vaccine. Speak to a primary care physician to confirm that the vaccine will not trigger any allergies.

The doctor may postpone the injection for people who are pregnant or breastfeeding, as well as people who have a fever.

Infants who are younger than 2 months old or people with severe allergies to any of the components of the vaccine should not receive this injection.

Precautions and DEET repellent

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DEET insect repellents can keep disease-spreading mosquitos at bay.

People who spend time outdoors in rural areas should use protective clothing and bed nets, and they should sleep in air-conditioned or well-screened rooms.

A person who is new to an area of prevalence does not usually have a natural immunity to the Japanese encephalitis virus.

This means that travelers of all ages are more vulnerable to infection than those who have always lived in an area to which the disease is common.

In the case of an outbreak, people who live in communities experiencing Japanese encephalitis should remove pools of standing water, where mosquitoes can breed, and use an insect repellent.

Loose-fitting clothing can also help keep mosquitos from the skin.

The most effective insect repellents contain a chemical called DEET. An excellent range with thousands of customer reviews is available here.

Mosquitos acquire the virus ahead of transmission to humans by feeding on infected wild birds and domestic pigs.

The mosquitoes then pass the virus to humans and animals. Birds and other animals cannot transmit the infection to humans. Only infected mosquitoes can pass it on.

The mosquitoes mainly live in agricultural and rural areas. Their larvae breed in pools of water, such as those in flooded rice fields.

To diagnose Japanese encephalitis, the doctor will examine any symptoms, verify where the individual lives, and ask about the destinations of any recent visits from which the infection could have emerged.

If a doctor suspects encephalitis, the patient will undergo tests, such as a CT or MRI scan of the brain.

The physician might use a lumbar puncture or spinal tap to draw fluid from the spine. The results can show which virus is causing the encephalitis.

Immunofluorescence tests can detect human antibodies. The antibodies show up after the doctor tags them with a fluorescent chemical.

There is no treatment or cure for Japanese encephalitis.

Once a person has the disease, treatment can only relieve the symptoms. Antibiotics are not effective against viruses, and effective anti-viral drugs are available.

Prevention is the best form of treatment for Japanese encephalitis.


How do I stop a case of Japanese encephalitis being fatal?


The main therapy is supportive care. There are medications to help the swelling in the brain, and sometimes a person might require heavy sedation and a breathing tube for a period of time until the swelling in the brain begins to improve

Answers represent the opinions of our medical experts. All content is strictly informational and should not be considered medical advice.
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