JAPANESE ENCEPHALITIS

• Japanese encephalitis is a potentially severe viral disease that is spread by infected mosquitoes in the agricultural regions of Asia.
• It is one of several mosquito-borne virus diseases that can affect the central nervous system and cause severe complications and death.
• Japanese encephalitis can be a risk to travelers to rural areas where the disease is common.
• There is no specific treatment for Japanese encephalitis.

What is Japanese encephalitis?

Japanese encephalitis is a disease that is spread to humans by infected mosquitoes in Asia. It is one of a group of mosquito-borne virus diseases that can affect the central nervous system and cause severe complications and even death.

What is the infectious agent that causes Japanese encephalitis?

The Japanese encephalitis virus, an arbovirus, causes Japanese encephalitis. Arbovirus is short for arthropod-borne virus. Arboviruses are a large group of viruses that are spread by certain invertebrate animals (arthropods), most commonly blood-sucking insects. Like most arboviruses, infected mosquitoes spread Japanese encephalitis.

Where is Japanese encephalitis found?

Japanese encephalitis is found throughout rural areas in Asia. Transmission can also occur near urban areas in some developing Asian countries.

Japanese encephalitis is a seasonal disease that usually occurs in the summer and fall in the temperate regions of China, Japan, and Korea. In other places, disease patterns vary with rainy seasons and irrigation practices.

How do people get Japanese encephalitis?

The Japanese encephalitis virus has a complex life cycle involving domestic pigs and a specific type of mosquito, Culex tritaeniorhynchus, that lives in rural rice-growing and pig-farming regions. The mosquito breeds in flooded rice fields, marshes, and standing water around planted fields. The virus can infect humans, most domestic animals, birds, bats, snakes, and frogs. After infection, the virus invades the central nervous system, including the brain and spinal cord.

What are the signs and symptoms of Japanese encephalitis?

Most infected persons develop mild symptoms or no symptoms at all. In people who develop a more severe disease, Japanese encephalitis usually starts as a flu-like illness, with fever, chills, tiredness, headache, nausea, and vomiting. Confusion and agitation can also occur in the early stage. The illness can progress to a serious infection of the brain (encephalitis) and can be fatal in 30% of cases. Among the survivors, another 30% will have serous brain damage, including paralysis.

How soon after exposure do symptoms appear?

Symptoms usually appear 6-8 days after the bite of an infected mosquito.

How is Japanese encephalitis diagnosed?

Diagnosis is based on tests of blood or spinal fluid.

How common is Japanese encephalitis?

Japanese encephalitis is the leading cause of viral encephalitis in Asia, where 30,000 to 50,000 cases are reported each year. The disease is very rare, however, in U.S. travelers to Asia.

The chance that a traveler to Asia will get Japanese encephalitis is very small: 1) only certain mosquito species can spread Japanese encephalitis; 2) in areas infested with mosquitoes, only a small portion of the mosquitoes are usually infected with Japanese encephalitis virus; 3) among persons who are infected by a mosquito bite, only 1 in 50 to 1 in 1,000 will develop an illness. As a result, fewer than 1 case per year is reported in U.S. civilians and military personnel traveling to and living in Asia. Only 5 cases among Americans traveling or working in Asia have been reported since 1981.

Who is at risk for Japanese encephalitis?

Anyone can get Japanese encephalitis, but some people are at an increased risk:

• People living in rural areas where the disease is common
• Active-duty military deployed to areas where the disease is common
• Travelers to rural areas where the disease is common (very small increased risk)

What is the treatment for Japanese encephalitis?

There is no specific treatment for Japanese encephalitis. Antibiotics are not effective against viruses, and no effective anti-viral drugs have been discovered. Care of patients centers on treatment of symptoms and complications.

How can Japanese encephalitis be prevented?

A vaccine is licensed for use in U.S. travelers to rural areas where the disease is common. The vaccine is recommended only for persons who plan to travel in these areas for 4 weeks or more, except in special circumstances such as an ongoing outbreak of disease.

Because of the potential for other mosquito-borne diseases in Asia, all travelers should take steps to avoid mosquito bites. The mosquitoes that transmit Japanese encephalitis feed mainly outside during the cooler hours at dusk and dawn. Travelers should minimize outdoor activities at these times, use mosquito repellent on exposed skin, and stay in air-conditioned or well-screened rooms. Travelers to rural areas should use a bed net and aerosol room insecticides.

 This fact sheet is for information only and is not to be used for self-diagnosis or as a substitute for consultation with a health-care provider. If you have any questions about the disease described above, consult a health-care provider.

Description
Japanese encephalitis (JE) is a common mosquito-borne viral encephalitis found in Asia. Most infections are asymptomatic, but among people who develop a clinical illness, the case-fatality rate can be as high as 30%. Neuropsychiatric sequelae are reported in 50% of survivors. In endemic areas, children are at greatest risk of infection; however, multiple factors such as occupation, recreational exposure, sex (possibly reflecting exposure), previous vaccination, and naturally acquired immunity alter the potential for infection and illness. A higher case-fatality rate is reported in the elderly, but serious sequelae are more frequent in the very young, possibly because they are more likely to survive a severe infection.

JE virus is transmitted chiefly by the bites of mosquitoes in the Culex vishnui complex; the individual vector species in specific geographic areas differ. In China and many endemic areas in Asia, Culex tritaeniorhyncus is the principal vector. This species feeds outdoors beginning at dusk and during evening hours until dawn; it has a wide host range, including domestic animals, birds, and humans. Larvae are found in flooded rice fields, marshes, and small stable collections of water around cultivated fields. In temperate zones, the vectors are present in greatest numbers from June through September and are inactive during winter months. Swine and certain species of wild birds function as viremic amplifying hosts in the transmission cycle.

Occurrence
Habitats supporting the transmission cycle of JE virus are principally in rural, agricultural locations. In many areas of Asia, however, the appropriate ecologic conditions for virus transmission occur near or occasionally within urban centers. Transmission is seasonal and occurs in the summer and autumn in the temperate regions of China, Japan, Korea, and eastern areas of Russia. Elsewhere, seasonal patterns of disease are more extended or vary with the rainy season and irrigation practices. Risk of JE varies by season and geographic area.

Risk for Travelers
The risk to short-term travelers and persons who confine their travel to urban centers is very low. Expatriates and travelers living for prolonged periods in rural areas where JE is endemic or epidemic are at greatest risk. Travelers with extensive unprotected outdoor, evening and night-time exposure in rural areas, such as bicycling, camping, or engaging in certain occupational activities, may be at high risk even if their trip is brief.

 

Preventive Measures

Vaccine
JE vaccine licensed in the United States is manufactured by Biken, Osaka, Japan, and distributed by Aventis Pasteur. Other JE vaccines are made by several companies in Asia, but are not licensed in the United States. Vaccination should be considered only by people who plan to live in areas where JE is endemic or epidemic and by travelers whose activities include trips into rural, farming areas. Short-term travelers (less than 30 days), especially those whose visits are restricted to major urban areas, are at lower risk for acquiring JE and generally should not be advised to receive the vaccine. Evaluation of an individual traveler’s risk should take into account his or her itinerary and activities and the current level of JE activity in the country

The recommended primary immunization series is three doses of 1.0 milliliter (ml) each, administered subcutaneously on days 0, 7, and 30. An abbreviated schedule of days 0, 7, and 14 can be used when the longer schedule is impractical because of time constraints. Two doses given a week apart may be used in unusual circumstances, but will confer short-term immunity in only 80% of vaccinees. The last dose should be administered at least 10 days before commencement of travel to ensure an adequate immune response and access to medical care in the event of delayed adverse reactions (see Table below).

Table: Japanese encephalitis vaccine.
DOSES	SUBCUTANEOUS ROUTE		COMMENTS
	1 through 2 years of age	3 years of age or older
Primary series 1, 2, and 3	0.5 milliliter	1.0 milliliter	Days 0, 7, and 30
Booster*	0.5 milliliter	1.0 milliliter	1 dose at 24 months or later

 

 

 

 

 

* In vaccines who have completed a three-dose primary series, the full duration of protection is unknown; therefore, definitive recommendations cannot be given.

Immunization routes and schedules for infants and children 1 through 3 years of age are identical except that doses of 0.5 ml should be administered. No data are available on vaccine efficacy and safety in infants younger than 1 year of age. The full duration of protection is unknown; however, preliminary data indicate that neutralizing antibodies persist for at least 2 years after primary immunization. In infants and children whose primary immunization series included doses of 0.5 ml, a booster dose of 1.0 ml (0.5 ml for children younger than 3 years of age) may be administered 2 years after the primary series.

Adverse Reactions
JE vaccine is associated with local reactions and mild systemic side effects (fever, headache, myalgias, and malaise) in about 20% of vaccines. More serious allergic reactions, including generalized urticaria, angioedema, respiratory distress, and anaphylaxis, have occurred within minutes to as long as one week after immunization. Such hypersensitivity reactions occur in approximately 0.6% of vaccines. Reactions have been responsive to therapy with epinephrine, antihistamines, or steroids, or a combination of these. Vaccinees should be observed for 30 minutes after immunization and warned about the possibility of delayed allergic reactions. The full course of immunization should be completed at least 10 days before departure, and vaccinees should be advised to remain in areas with access to medical care. People with a past history of urticaria appear to have a greater risk for developing more serious allergic reactions, and this must be considered when weighing the risks and benefits of the vaccine. A history of allergy to JE or other mouse-derived vaccines is a contraindication to further immunization.

Precautions and Contraindications
People with known hypersensitivity to the vaccine should not be vaccinated. People with multiple allergies, especially a history of allergic urticaria or angioedema, are at higher risk for allergic complications for JE vaccine.

Pregnancy - Vaccination during pregnancy should be avoided unless the risk of acquiring JE outweighs the theoretical risk of vaccination.

Other
Travelers should be advised to stay in screened or air-conditioned rooms, to use bed nets when such quarters are unavailable, to use aerosol insecticides and mosquito coils as necessary, and to use insect repellents and protective clothing to avoid mosquito bites.