Japanese B. encephalitis (JBE)

This disease primarily affects piglets. The key clinical signs include defective birth of piglets; degenerated tentacles in the boar; nervous signs in piglets. The disease appears only in South East Asia. This disease is notifiable – contact your vet and local authorities if you believe this disease is affecting your herd.
calendar icon 14 November 2018
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Background and history

Japanese B. encephalitis (JBE) is caused by a virus (a member of the flavivirus group) which is spread by mosquitoes. The pig is the natural amplifier and reservoir host. In other words it serves to multiply up the virus and keep it going. Many other species can be infected including most domestic animals and many wild animals such as rabbits, mice, birds, bats, snakes and lizards.

JBE is confined to a large area of South Asia, centred on the China Sea and stretching from Ceylon to Japan and from New Guinea to Mongolia and Eastern Russia. This includes such major pig producing countries as China, Taiwan, Korea, Thailand, the Philippines and Japan.

If you live outside this area of South Asia, there is no risk to your herd and no risk to you unless you decide to visit pig farms in the area, in which case get yourself vaccinated twice, preferably three times, before you go.

Its main importance is its threat to public health – it causes an encephalitis in the human.

Clinical signs


  • Mummified piglets
  • Stillborn piglets
  • Abnormal piglets.
  • Weak piglets.
  • Nervous signs in piglets – Fits and convulsions.
  • Subcutaneous oedema (excess clear bodily fluid under the skin)
  • Hydrocephalus (water on the brain)


  • Infertility in boars.
  • Degeneration and swelling of testicles.


This requires laboratory examination of dead stillborn piglets and affected boars' testicles.

Definitive diagnosis depends on the isolation of the virus in tissue culture and demonstration of antibodies in stillborn piglets serum, usually by an enzyme-linked immunosorbent assay (ELISA) or haemagglutination inhibition (HI) test.

JE should be distinguished from similar reproductive problems caused by parvovirus, aujeszky's disease, porcine reproductive and respiratory syndrome (PRRS), certain strains of influenza, hog cholera (CSF) and some enteroviruses.


  • The presence of mosquitoes.
  • Susceptible pigs (naive).
  • Viral reservoirs in other species.


  • Vaccination
    • In countries in which this disease is endemic young breeding gilts and boars are vaccinated twice before the mosquito season starts commonly with an attenuated vaccine but inactivated vaccines are also available.

Trying to control mosquitoes in pig herds is a waste of time.


There is no effective treatment.

Emily Houghton

Editor, The Pig Site

Emily Houghton is a Zoology graduate from Cardiff University and was the editor of The Pig Site from October 2017 to May 2020. Emily has worked in livestock husbandry, and has written, conducted and assisted with research projects regarding the synthesis of welfare and productivity of free-range food species.

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