Questions & Answers
Japanese encephalitis - The Disease
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  1. What is Japanese encephalitis (JE)?

    It is a severe infection of the brain and spinal cord caused by the bite of a Culex mosquito carrying the JE virus.  The disease is most common in rural, agricultural areas of Asia and parts of the western Pacific where mosquito larvae inhabit flooded rice fields and marshes.  These mosquitoes feed outdoors from dusk until dawn.  Disease transmission occurs when mosquitos are active, usually during summer and fall.

    JE is only transmitted by mosquitos – never from person-to-person.  It is the most common vaccine-preventable cause of encephalitis in Asia with 30,000 to 50,000 cases reported annually.

  2. Who is at risk for getting JE?

    JE occurs in Asia and parts of the Western Pacific. The risk of infection is highest in rural agricultural areas where rice production and flooding irrigation take place, and large numbers of mosquitoes breed in proximity to pigs and wading birds.  Individuals are at an increased risk of JE if they spend lots of time outdoors, especially during the evening;  when mosquitos are biting; participate in extensive outdoor activities (e.g., camping, hiking, trekking, biking, fishing, hunting or farming); or stay in accommodations without air conditioning, screens, or bed nets.
    In most temperate areas of Asia, JE transmission is seasonal, and substantial epidemics can occur, usually peaking in summer and fall.  In the subtropics and tropics, transmission can occur year-round, often intensifying during the rainy season.  JE can be prevented with a vaccine, but risk can also be reduced by the use of countermeasures, such as insect repellent, permethrin-treated clothing, and vector avoidance.

    Visit the CDC JE webpage to see a geographic distribution of the JE virus.