Japanese Encephalitis

What is Japanese encephalitis, how is it spread, and how is it treated?

Japanese encephalitis (JE) is inflammation of the brain caused by a virus that is spread by a bite from the culex mosquito.

Within 5 to 15 days of the mosquito bite, symptoms will develop that range from headache and fever to lethargy and may include vomiting, diarrhea, seizure, or coma. Only 1 out of 250 infected persons have any symptoms, but 5 to 30 percent of persons who develop symptoms die. Also, 30 to 50 percent of survivors have some permanent damage to their central nervous system such as paralysis, convulsions, memory loss, or behavioral disturbances. The only treatment for JE is supportive care.

From 1978 to 1992, only 11 cases of JE were reported in U.S. citizens living or traveling in Asia, and at least 6 of these cases were soldiers living in field conditions. Fewer than 1 in 1 million unvaccinated American tourists in Asia have contracted JE. Those persons who lived in conditions of intense exposure for a month during peak transmission season increased their risk to 1 in 5,000.

Many factors determine the risk of contracting JE, including the region of the world, the habitat, the season, and even the time of day. JE is reported in Southeast Asia, India, China, Japan, and Korea. The mosquitoes that spread JE often live where they have plenty of water to breed in and domestic animals, especially pigs, to feed upon. The mosquitoes feed in the evening and at night. So, to some extent, Japanese encephalitis can be prevented by avoiding rural-agricultural, rice-growing, pig-farming regions of Asia once the sun has gone down. The right conditions for spread of the virus also exist near or within many Asian cities, but risk to travelers in these areas is very low.

Although JE is a threat year-round in tropical regions of Asia and Oceania, in most temperate areas transmission is highest from April to September. In northern India and Nepal, transmission peaks from June to November.

How effective is the Japanese encephalitis vaccine, and who should get it?

Although it is difficult to determine how effective the vaccine is at protecting Western travelers, field testing with children in Taiwan showed the vaccine to be 80 percent effective. The ACIP recommends three doses: the second follows the first by 7 days and the third follows the first by 30 days. If departure is imminent, the third dose may follow the first by only 14 days. Just two doses given a week apart confer short-term immunity in 80 percent of vaccinees, but stopping there is not recommended. A booster is recommended two years after the initial series if the risk of disease persists. Protection starts about 10 days after the vaccine series.

The JE vaccine should be considered for travelers who will be in the risk areas of Asia, the Indian subcontinent, and the western Pacific for a month or longer. Persons who will be in these regions for a shorter period but intensively exposed to the mosquitoes may also benefit from vaccination.

Who should not get the vaccine?

• Persons with a history of a severe allergic or systemic reaction following a prior dose of JE vaccine or any of its components should not receive this vaccine.

• Children less than 12 months old should not receive the vaccine.

• Persons with a moderate or severe acute illness should postpone vaccination.

• Pregnant women should avoid this vaccine. A pregnant woman should only receive the JE vaccine if the benefits outweigh the risks.

What are the vaccine risks and side effects?

About 20 percent of vaccinated persons experience pain, swelling, and redness at the vaccine injection site. These symptoms may begin soon after the shot and usually resolve within a few days. About 10 percent of vaccinated persons experience malaise, fever, headache, rash, dizziness, muscle aches, vomiting, or abdominal pain.

Severe allergic reactions and other severe reactions occur rarely after JE vaccine. Such reactions usually begin within a day or two, but they may be delayed until a week to two after a second dose of vaccine. For this reason-and to ensure maximum protection-you should schedule the last dose of this vaccination series at least 10 days before departure.

Additional notes

To minimize your risk when traveling to a JE-infected region during transmission season, wear mosquito repellent, wear a long-sleeved shirt and long pants, avoid outdoor activities in the evening, and sleep under permethrin-impregnated mosquito netting or in a screened or air-conditioned room.