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Diseases / Japanese Encephalitis

Japanese Encephalitis

What is it?: 

Japanese encephalitis (JE) is a vector-borne zoonotic viral disease. JE virus (JEV) is the leading cause of viral encephalitis in Asia. JE occurs in nearly all Asian countries, whether temperate, subtropical, or tropical, and has intruded into new areas through importation of infected vectors. Currently, an estimated 3 billion people live in the 24 countries, mainly in the WHO South-East Asia and Western Pacific Regions, considered at risk of JE.1

In temperate locations, the period of transmission of JEV typically starts in April or May, and lasts until September or October. In tropical and subtropical areas, transmission exhibits less seasonal variation, or intensifies with the rainy season.1

As JE surveillance is not well established in many countries, and laboratory confirmation is challenging, the true extent and prevalence of the virus and burden of disease are not well understood. It is estimated that 67 900 clinical cases of JE occur annually despite widespread availability of vaccine, with approximately 13 600 to 20 400 deaths, and an overall incidence rate of 1.8/100 000 in the 24 countries with JE risk. 1

Clinical features: 

Mild infections may occur without apparent symptoms other than fever with headache. More severe infection is marked by quick onset of headache, high fever, neck stiffness, impaired mental state, coma, tremors, occasional convulsions (especially in children) and paralysis.4

Mode of transmission: 

JEV is transmitted through a zoonotic cycle between mosquitoes, pigs and water birds. Humans get accidentally infected when bitten by an infected mosquito and are a dead end host. Humans do not participate in the spread of JE because of low level and short-lived viremia. 2

Distribution of JE virus vector mosquitoes (Culex) in Hong Kong:3

*Adapted from Food and Environmental Hygiene Department, The Government of HKSAR

Incubation period: 

Symptoms usually start at around 4-14 days after being infected.4


The case-fatality rate can be as high as 20-30% among those with symptoms. Of those who survive, 30% suffer1

  • permanent intellectual
  • behavioral or neurological problems such as paralysis, recurrent seizures or inability to speak.

WHO recommendations: JE vaccination should be integrated into national immunization schedules in all areas where JE is recognized as a public health priority. Even if the number of JE-confirmed cases is low, vaccination should be considered where there is a suitable environment for JEV transmission, i.e. presence of animal reservoirs, ecological conditions supportive of virus transmission, and proximity to other countries or regions with known JEV transmission.1

Japanese encephalitis vaccine is not available in Hong Kong for individuals aged 9-month above.5

Besides, to prevent contracting the disease, one should take general measures to prevent mosquito bites, apply effective insect repellents (containing DEET) to exposed parts of the body and avoid going to rural areas from dusk till dawn when the mosquitoes spreading this virus are most active.4


General Measures on Preventing Mosquito-borne Diseases:4

  • Wear loose, light-coloured, long-sleeved tops and trousers, and apply effective insect repellents containing DEET to exposed parts of the body & clothing.
  • Use mosquito screens or bed nets when the room is not air-conditioned.
  • Apply household pesticide to kill adult mosquito with a dosage according to the label instructions. Do not spray the pesticides directly against functioning electrical appliances or flame to avoid explosion.
  • Place mosquito coil or electric mosquito mat / liquid near possible entrance, such as window, to prevent mosquito bites.
  • Prevent the accumulation of stagnant water
  • Put all used cans and bottles into covered dustbins.
  • Change water for plants at least once a week, leaving no water in the saucers underneath flower pots.
  • Cover tightly all water containers, wells and water storage tanks.
  • Ensure air-conditioner drip trays are free of stagnant water.
  • Keep all drains free from blockage.
  • Top up all defective ground surfaces to prevent the accumulation of stagnant water.

There is no specific anti-viral treatment currently for Japanese encephalitis. Physician may provide supportive treatment to patient.1

High risk groups: 
  • Higher risk in children: In JE endemic countries, the incidence exceeds 9 times in children under 14 years old compared to 15 years old or above.1
  • Greater risk of severe disease in young children: Children under 10 years old have a greater risk of severe disease and a higher case-fatality rate.1
  • According to the latest recommendations from the Scientific Committee on Vector-borne Diseases in 2017:  areas within 2km of pig farms in Hong Kong are considered as high-risk for local JE cases and it covers 633,000 population.6

+Locations of pig farms are provided by Agriculture, Fisheries and Conservation Department, The Government of HKSAR


  1. World Health Organization. Japanese encephalitis: WHO position paper. 2015.
  2. Misra UK, Kalita J. Overview: Japanese encephalitis. Prog Neurobiol 2010; 91: 108–120.
  3. Food and Environmental Hygiene Department, 2010. Distribution of Culex tritaeniorhynchus (JE vector survey 10/04-10/05). Available at:  [Accessed: February 2, 2021].
  4. Centre for Health Protection (2018), Centre for Health Protection - Japanese Encephalitis. Available from: Department of Health, HKSAR, Web site: [Accessed: February 2, 2021].
  5. Imojev package insert. May 2015 version.
  6. Scientific Committee On Vector-borne Disease (2017), Consensus Statement on Prevention and Control of Japanese Encephalitis. Available from: Centre for Health Protection, Web site [Accessed: February 2, 2021]