Japanese encephalitis is a mosquito-borne disease caused by the Japanese encephalitis virus. The disease occurs mainly in the rural and agricultural areas of Asia and the Western Pacific Region, especially in hot seasons. In the subtropics and tropics, transmission can occur year-round, often with a peak during the rainy season.
Most infections occur without apparent symptoms or with mild symptoms such as fever and headache. More severe infection is characterised by rapid onset of headache, high fever, neck stiffness, impaired mental state, coma, convulsions (especially in children), spastic paralysis and even death.
Mode of transmission
Japanese encephalitis virus is principally transmitted by the bites of infected mosquitoes. The principal type of mosquito that transmits the disease is the Culex species, especially Culex tritaeniorhynchus. The mosquito breeds where there is abundant water such as rice paddies and becomes infected by feeding on pigs and wild birds infected with the Japanese encephalitis virus. The infected mosquito transmits the virus to humans and animals while biting. While Japanese encephalitis is principally mosquito-borne, a human case transmitted by blood transfusion was recorded in Hong Kong. In addition, overseas scientific literature showed that, based on the nature of similar flaviviruses, organ transplant is also considered to be a potential mode of transmission.
Symptoms usually start around 4 – 14 days after being infected.
There is no specific treatment for this disease. Supportive therapy is the mainstay of treatment. The case-fatality rate can be as high as 30% among those with symptoms. Of those who survive, 20% – 30% suffer permanent intellectual, behavioural or neurological problems such as paralysis, recurrent seizures or inability to speak.
To prevent contracting the disease, one should take general measures to prevent mosquito bites and avoid going to rural areas from dusk till dawn when the mosquitoes spreading this virus are most active. People planning to travel to areas in which Japanese encephalitis is endemic should take special note.
Vaccination for Japanese Encephalitis
Japanese encephalitis vaccination is safe and effective in preventing the disease. Vaccination for Japanese encephalitis is available in Hong Kong but it is generally not recommended to members of the general public. Vaccination is recommended for travellers who plan to stay one month or longer in endemic areas, particularly in rural areas; and for short-term (less than one month) travellers if they plan to have significant/ extensive outdoor or night-time exposure in rural areas during the transmission season of the disease. For further information on Japanese encephalitis vaccination and outbreak news in other countries, please visit the Hong Kong Travel Health Service website at https://www.travelhealth.gov.hk/eindex.html.
General measures on preventing mosquito-borne diseases
(I) Prevention of mosquito bites
(II) Prevention of mosquito proliferation
For more information about control and prevention of mosquito breeding, please visit the website of the Food and Environmental Hygiene Department (FEHD) at http://www.fehd.gov.hk/english/pestcontrol/handbook_prev_mos_breeding.html
(III) Advice for travelers