Japanese encephalitis

Alert

Japanese encephalitis has been confirmed in a small number of properties with pigs in Queensland. There have also been detections in piggeries in New South Wales, Victoria and South Australia.

Japanese encephalitis is category 1 restricted matter.

Under Queensland legislation, if you suspect the presence of this disease in any species of animal, you must report it to Biosecurity Queensland on 13 25 23 or contact the Emergency Animal Disease Watch Hotline on 1800 675 888.

Japanese encephalitis is an acute mosquito-borne viral disease associated with reproductive losses in pigs and inflammation of the brain in horses and people.

Japanese encephalitis is spread by mosquitoes. Waterbirds act as natural reservoirs for the virus, and mosquitoes can spread the virus to people, horses, pigs and other animals.

In animals, signs of disease are most common in pigs and horses. Other animals can be infected but typically do not show signs of illness. These include cattle, sheep, goats, dogs, cats, bats, rodents, reptiles, amphibians, and birds.

Japanese encephalitis is not a food safety concern. Commercially produced pork meat or pork products are safe to consume.

Scientific name

Japanese encephalitis

Cause

Japanese encephalitis virus (JEV), is a member of the Flavivirus genus of the family Flaviviridae.

Distribution

In 2022, JEV has been detected in properties with pigs in Queensland, New South Wales, Victoria and South Australia. At this time, it is not known how the virus has come into mainland Australia. The movement of infected mosquitoes or migratory water birds may have played a part in the virus' spread, combined with recent rain events. A case of Japanese encephalitis in a person was reported in the Tiwi Islands in early 2021.

JEV is widely distributed in southeast and southern Asia extending from China and India east to Japan and south into Indonesia and Papua New Guinea.

The far north of Australia's Cape York Peninsula is considered an area at risk and seasonal incursions of Japanese encephalitis are occasionally detected in people in the Torres Strait.

Hosts

  • waterbirds
  • pigs
  • mosquitoes, mainly Culex annulirostris.

Life cycle

Japanese encephalitis virus is primarily transmitted by bites from infected mosquito vectors and is maintained in mosquito–waterbird or mosquito–waterbird–pig cycles. Australia has a number of mosquito species that are capable of transmitting the virus.

Pigs are known as 'amplifiers' of the virus as they develop levels of virus in their blood sufficient to infect mosquitoes for around 4 days.

Horses and a wide range of other species may be infected but do not develop levels of virus in their blood sufficient to infect mosquitoes, so are not involved in maintaining the disease life cycle.

There are reports of transmission of infection via artificial insemination or embryo transfer, but this does not appear to be an important route of transmission.

Japanese encephalitis does not survive for long in the environment and windborne spread of the virus is not reported.

Affected animals

  • horses and donkeys
  • pigs
  • humans

Clinical signs

Pigs

Most infections with JEV are subclinical, meaning the animal is infected but shows no signs of disease. Clinical disease in animals is most commonly associated with pigs and horses; reports of disease in other species are rare.

In pigs the most common clinical signs are mummified and stillborn or weak piglets, some with neurological signs.

Piglets infected after birth can develop encephalitis (paddling, other neurological signs) in the first 6 months of life. In other cases, wasting, depression or hindlimb paralysis may be seen in suckling piglets and weaner pigs.

Adult sows do not typically show overt signs of disease. Boars may experience infertility and oedematous, congested testicles.

Learn more about information for pig owners.

Horses

Many cases of Japanese encephalitis infection in horses are subclinical, meaning they are infected but show no signs of the disease.

For those cases that show signs of disease, most are mild, however more severe encephalitis can occur which may be fatal.

Signs can include:

  • fever
  • jaundice
  • lethargy
  • anorexia
  • neurological signs (incoordination, difficulty swallowing, impaired vision, hyperexcitability).

Overseas, disease has also been reported in donkeys.

Learn more about information for horse owners.

How it is spread

Japanese encephalitis virus is primarily transmitted by bites from infected mosquito vectors and is maintained in mosquito–waterbird or mosquito–waterbird–pig cycles.

Waterbirds, particularly wading birds such as herons and egrets, are the main source from which transmission of JE virus can occur.

Risk period

The main mosquito vector of Japanese encephalitis, Culex annulirostris, feeds during the night, particularly in the period shortly after sunset and in the early morning.

In temperate endemic areas, infection builds up in waterbirds and then in pigs in late spring and early summer. People and horses are more commonly infected in summer and autumn.

In tropical areas, Japanese encephalitis circulates more or less continuously between mosquitoes, birds, and pigs.

Control

Vaccination

A vaccine for horses and pigs is available in countries where the disease is endemic. No JE vaccines for animals are registered for general use in Australia.

Seek medical advice about JE vaccination in people.

Treatment

There is no effective treatment for JE in animals.

Quarantine

Australia has strict import conditions for animals to prevent the entry of JE and other diseases.

In addition, the Northern Australia Quarantine Strategy (NAQS) provides an early warning system for exotic pest, weed and disease detections across northern Australia (including JE) that helps address the unique biosecurity risks facing the region.

Further information