Japanese Encephalitis

Epidemiology of Infectious Diseases: Japanese Encephalitis

Causal agent
A mosquito-borne viral encephalitis caused by a flavivirus1.
(Culex tritaeniorhynchus group)

Common clinical features

  • The majority of cases of JE infections are mild (fever and headache) or are asymptomatic.
  • Children and the elderly suffer most clinical disease.
  • However, approximately 1 in 200 infections result in severe disease characterized by rapid onset of high fever, headache, neck stiffness, disorientation, coma, seizures, spastic paralysis and death4.
  • Cases-fatality is up to 60% of cases with symptoms of severe disease.
  • 30% of persons who survive suffer from lasting damage to the central nervous system.

Epidemiology

  • JE is endemic in tropical parts of South East Asia and the Far East including; Australia (islands of Torres Strait), Bangladesh, Bhutan, Brunei, Myanmar, Cambodia, China, India, Indonesia, Japan, Korea, Laos, Malaysia, Nepal, Pakistan, Papua New Guinea, Philippines, Russia, Singapore, Sri Lanka, Thailand, Vietnam and the Pacific Islands3.
  • JE occurs primarily in rural agricultural areas where flooding irrigation is practiced, especially where rice growing and pig farming coexist. However, in many areas of Asia these ecological conditions may occur near or occasionally within urban area3,4.
  • Transmission is seasonal and occurs in the summer and autumn in temperate regions of China, Japan, Korea and eastern Russia. Elsewhere, seasonal patterns of transmission vary within the rainy season and by irrigation practices3.
  • JE is the leading cause of childhood viral encephalitis in Asia with 30,000-50,000 clinical cases reported each year4.
  • The risk of infection with JE in travellers is extremely low.  

Reservoir
Ardeid birds (herons and egrets), pigs.

Mode of transmission
Transmitted by the bite of infected mosquitoes from the Culex tritaeniorhynchus and Culex vishnui groups.

No person to person transmission.

Incubation period
5-15 days.

Period of Communicability
Virus not usually demonstrable in human blood after the onset of disease.

Mosquitoes remain infective for life.

Prevention and control
Travellers should undergo careful risk assessment that takes into consideration their itinerary, season of travel, duration of stay and planned activities. This risk of JE should be balanced against the risk of potential adverse affects ofvaccination4. 

Immunisation recommended for those going to reside in areas where JE is endemic or epidemic. 

Travellers to South East Asia and the Far East should be immunised if staying for a month or longer in endemic areas during the transmission season, especially if travelling in rural areas4.

References

  1. Hawker J, Begg N, Blair I, Reintjes R, Weinberg J. Communicable Disease Control Handbook, Blackwell, 2005.
     
  2. Heymann D L, editor, Control of Communicable Disease Manual. 18th ed. American Public Health Association; 2004.
     
  3. CDC, Health Information for Travellers, Japanese Encephalitis Fact Sheet.
     
  4. Salisbury DM, Begg NT.  Immunisation against infectious disease (The green Book). London: HMSO, 1996. 2005 Update available at: http://www.dh.gov.uk/assetRoot/04/12/32/38/04123238.pdf

© CM Kirwan 2006