Japanese encephalitis

Japanese encephalitis is an acute inflammatory viral disease of short duration that involves parts of the brain, spinal cord, and meninges (linings of the brain). It is closely related to West Nile virus, St. Louis Encephalitis virus, and other encephalitic viruses (viruses that affect the brain). Japanese encephalitis disease is caused by the mosquito-borne Japanese encephalitis virus.

The risk to short-term travellers and those who confine their travel to urban centres is very low. Expatriates and travellers living for prolonged periods in rural areas where Japanese encephalitis is endemic or epidemic are at greater risk. Travellers with extensive outdoor, evening and night-time exposure in rural areas (for example, while bicycling, camping, working outdoors, or sleeping in unscreened structures without bed nets) may be at high risk even if their trip is brief. Transmission is seasonal and occurs in the summer and autumn in the temperate regions. Sometimes the transmission season extends or varies with the rainy season and irrigation practices. In tropical parts of Asia, mosquitoes that breed in rice paddies may also be important vectors, and Japanese encephalitis virus transmission may occur over a longer season or even throughout the year.

Susceptibility to Japanese encephalitis disease is usually highest in infancy and old age. Once infected, a person obtains lifelong immunity. In highly endemic areas, adults are largely immune to local strains because they have already had a mild or unapparent infection.

The majority of human Japanese encephalitis viral infections do not show any symptoms, and only about 1 in 250 of those infected will develop symptoms of the disease. Japanese encephalitis usually begins with the sudden onset of fever with gastrointestinal symptoms and headache. Encephalitis (inflammation in the brain) is the most common manifestation of the infection. Mental or behavioural changes, neurologic deficits, generalized weakness, and movement disorders may develop after the onset of encephalitis. Seizures are fairly common among children. Milder forms of the disease, such as aseptic meningitis (inflammation of the lining of the brain without an identifiable bacteria or virus) or fever with headache, can occur more commonly among adults. 

Among patients with encephalitis symptoms, approximately 20%-30% of patients die, and 30%-50% of survivors have neurologic, cognitive, or behavioural disabilities. There is no specific antiviral treatment for Japanese encephalitis. Therapy consists of supportive care, treatment of symptoms, and management of complications.

Japanese encephalitis occurs in India and throughout most of Asia (especially China, Japan, Korea, eastern Russia, Vietnam, Thailand, Laos, Malaysia, Indonesia, Cambodia, and some islands in the western Pacific). It has not been identified in Africa, Europe, or the Americas.