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Japanese Encephalitis

Japanese Encephalitis


Disease History, Characteristics, and Transmission

Scope and Distribution

Treatment and Prevention

Impacts and Issues



Encephalitis is an inflammation of the brain that is most often caused by a virus. The Japanese encephalitis virus (JEV) is the leading cause of viral encephalitis in Asia, but the infection is relatively rare in the West. Although only a minority of cases of Japanese encephalitis causes symptoms, such as headache, seizures and paralysis, the disease is potentially fatal, and there can be long-lasting disability among survivors. There is no cure for Japanese encephalitis, but there are vaccines available. Countries that vaccinate their populations against JEV, including Japan, tend to have fewer cases of encephalitis than those where vaccination is less routine, such as in India and Vietnam. Vaccination is often recommended for travelers, especially if they expect lengthy stays in rural endemic areas (where the disease occurs consistently within a specific region/locality). People who intend to reside in an area where JEV is endemic also need vaccination to protect themselves.

Disease History, Characteristics, and Transmission

Japanese encephalitis virus (JEV) is a flavivirus, a type of single-stranded RNA virus that is related to the St. Louis encephalitis virus and West Nile virus. The incubation period of JEV is 5–15 days, and persons with symptoms will usually have a history of exposure to mosquitoes in an endemic area in Asia. Most JEV infections are subclinical, that is, the infected person has no symptoms or only mild symptoms, such as headache and fever. One person with JEV in 250 will develop acute (rapid-onset) symptoms, including headache, neck stiffness, stupor, disorientation, tremor, seizures, paralysis, and even coma. Japanese encephalitis can be difficult to distinguish from the other types of viral encephalitis; tests of blood or cerebrospinal fluid can give a definitive diagnosis if this is needed. The mortality rate among the symptomatic cases is between 10% and 30%, and is higher where there is only limited access to intensive care facilities which may be required if paralysis leads to breathing or feeding problems. Up to 30% of survivors of Japanese encephalitis are left with morbidity (complications) including long-term disabilities such as movement problems, changes in behavior, blindness, and seizures. Because intensive care is often needed in Japanese encephalitis to help the patient feed and breathe, there may also be various complications arising from the bacterial infections, such as pneumonia and urinary tract infection, that are common to any patient requiring incubation for breathing, elimination, or nutrition.


ARTHROPOD-BORNE DISEASE: A disease caused by one of a phylum of organisms characterized by exoskeletons and segmented bodies.

ENDEMIC: Present in a particular area or among a particular group of people.

INCUBATION PERIOD: Incubation period refers to the time between exposure to disease causing virus or bacteria and the appearance of symptoms of the infection. Depending on the microorganism, the incubation time can range from a few hours (an example is food poisoning due to Salmonella)to adecadeormore(an example is acquired immunodeficiency syndrome, or AIDS).

MORBIDITY: The term “morbidity” comes from the Latin word “morbus,” which means sick. In medicine it refers not just to the state of being ill, but also to the severity of the illness. A serious disease is said to have a high morbidity.

RESERVOIR: The animal or organism in which the virus or parasite normally resides.

Japanese encephalitis is an arthropod-borne virus, and is transmitted through the bite of the rice paddybreeding Culex mosquito, which is why the disease tends to occur mainly in rural areas. Mosquitoes become infected with JEV through feeding on the natural animal reservoirs of JEV, which are wild birds and domestic pigs. Once JEV has been transmitted to a human host, through a mosquito bite, it may spread through the body and reach the brain. The transmitting mosquitoes prefer to bite humans outdoors and are at their most active during the evening and night. JEV cannot be transmitted via direct person-to-person contact.

Scope and Distribution

Children and the elderly are the most likely to develop the symptomatic form of Japanese encephalitis. The disease is endemic in the countries of the Indian sub-continent, South East Asia, and North East Asia, including Japan. It is transmitted by Culex mosquitoes living in rural rice-growing and pig-farming regions, breeding in flooded rice fields, marshes, and standing water around rice fields. Research has shown that most people in endemic areas have been exposed to JEV, even though they may not have had any symptoms of encephalitis. The rate of symptomatic disease in an endemic area is estimated at about one per 150,000 of the population.

Japanese encephalitis is seasonal, as might be expected from a disease transmitted by mosquitoes whose activity depends upon temperature. In temperate regions, it occurs from June to September; in the sub-tropics, the season is extended from April to October, and in tropical regions, Japanese encephalitis occurs all year round. In the United States, just 12 cases were recorded between 1978 and 1993, and these were among expatriates, travelers, or military personnel returning from parts of the world where Japanese encephalitis is endemic. Currently, the rate of infection among U.S. citizens remains at less than one case per year. In endemic areas, it is those living in rural areas that are most at risk; the disease tends to occur less frequently in towns and cities. In general, the risk of travelers contacting JEV infection is low, but much depends on where they are residing and the length of potential exposure.

Treatment and Prevention

As of early 2007, there are no specific anti-viral drugs effective against JEV. Treatment of Japanese encephalitis involves supportive treatment dealing with the symptoms of the disease. For instance, anticonvulsant drugs can be used to treat seizures. Intensive care is often needed, if neurological problems like paralysis set in, to provide feeding and airway support. There are a number of vaccines against JEV, some of which are only available in Asia. One of these is a vaccine composed of killed JEV that sometimes causes adverse reactions, but can be used to protect those who intend an extended stay of more than a month to an area where Japanese encephalitis is endemic. If a traveler is sleeping in a rural area where JEV is endemic, then avoiding mosquito exposure is crucial by using bednets treated with the proven mosquito repellent and insecticide DEET (diethyltoluamide). It is best to avoid the outdoors during the evenings and at night, and to stay in well-screened rooms. However, only certain Culex species transmit JEV and only a small number of these mosquitoes are infected. Among those travelers who are infected with a JEV-bearing mosquito bite, only one in 50 to one in 1,000 will become ill with JEV.


In 2005, after an unusually heavy monsoon season, an epidemic of Japanese encephalitis occurred in India's most populous state, Uttar Pradesh. The outbreak soon spread into the neighboring state of Bihar and eventually crossed the border into Nepal. Officials in overcrowded hospitals filled every available space with Japanese encephalitis patients, and some families with children suffering from the disease camped outside hospitals hoping to gain access to treatment. After running out of oxygen masks, one Indian hospital fashioned makeshift oxygen masks out of cardboard rolled into the shape of a cones. The outbreak resulted in over 5,000 Japanese encephalitis cases and approximately 1,300 deaths, mostly among children.

Impacts and Issues

Travelers are still considered to be at low risk of contracting Japanese encephalitis. Interest in vacations to Asia has been on the increase in recent years, therefore, there are potentially more people at risk of exposure to JEV. Advice on precautions and prevention changes frequently, so those traveling to countries such as Vietnam, Japan, India, or almost anywhere in Asia are recommended to seek travel health advice from their physician prior to departure. Vaccination may or may not be recommended, depending on the traveler's specific plans, but advice on reducing exposure to mosquitoes should always be heeded.

There is a clear need for improved and cheaper vaccines against JEV. This may enable whole populations at risk to be protected. Where vaccination is practiced as routine, such as China, Korea and Japan, have tended not to have the epidemics that still occur in India, Nepal and Myanmar, where vaccination is not yet the norm. In May 2006, the World Health Organization (WHO) adopted a 10-year strategy to increase immunization coverage worldwide for several preventable diseases, including Japanese encephalitis. Advanced clinical trials of a new vaccine for children are also underway in India. In the meantime, it also appears that the range of JEV may be extending and may continue to do so with global warming and increased frequency of international travel.

There have been two outbreaks of Japanese encephalitis in Australia—one in 1995, on islands in the Torres Strait and another in 1998 on the Cape York Peninsula. In 2004, JEV was found in mosquitoes in the Cape York Peninsula, indicating an ongoing risk from Japanese encephalitis.

See AlsoAfrican Sleeping Sickness (Trypanosomiasis); Arthropod-borne Disease; Climate Change and Infectious Disease; Mosquito-borne Diseases; St. Louis Encephalitis; West Nile.



Mackenzie, J.S., et al. Japanese Encephalitis and West Nile Viruses New York: Springer, 2002.

Web Sites

Centers for Disease Control and Prevention (CDC) Division of Vector-Borne Infectious Diseases. “Japanese Encephalitis Fact Sheet.” June 21, 2001 <> (accessed July 20, 2007).

World Health Organization. “Japanese Encephalitis.” <> (accessed March 25, 2007).

Susan Aldridge

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