Japanese encephalitis: Information for local government and industry

JEV is transmitted by mosquitoes that occur across much of Asia. Whilst rare, infection with JEV can result in a disease known as Japanese encephalitis (JE), both in humans and animals, including pigs and horses.

The transmission cycle of JEV involves mosquito vectors and vertebrate hosts. Water birds (particularly water wading birds e.g., herons, egrets and bitterns) and pigs are the most important amplifying hosts. Humans and other animals, such as horses, are generally considered incidental hosts, as they do not contribute to ongoing transmission of the virus.

JEV had not previously been detected in Australia outside of the Torres Strait, far north Queensland (QLD) and Tiwi Islands in the Northern Territory (NT).

2024/25 Outbreak

In October 2024, JEV seroconversions were confirmed in the Derby sentinel chicken flock for the first time. This indicated that the virus had been active in the Kimberley for two consecutive seasons, highlighting the ongoing risk to public health that the virus now presented.

In January/February 2025, JEV infections were reported in mosquitoes, commercial and feral pigs in QLD, NSW and Victoria. Human cases of JE were also  reported in the eastern states.

To date, no human cases of JE have been reported in WA.

For updated information on the national JEV response, please visit the outbreak website (external site).

2022/2023 Outbreak

In March 2022, JEV was detected in the southeast of Australia for the first time. Affected jurisdictions, including QLD, New South Wales (NSW), Victoria, South Australia and the NT reported 45 human cases of JE, 7 of which were fatal. Significant economic impact was experienced, with more than 80 commercial piggeries declared infected premises.

During this outbreak, the WA Department of Health confirmed detection of JEV antibodies in sentinel chicken flocks in the Kimberley (Kununurra, Halls Creek, Wyndham) and Pilbara (Newman) regions, as well as feral pigs in the northern Kimberley.

Culex annulirostris is suspected of being the most likely mosquito vector responsible for transmitting JEV in eastern Australia. This mosquito species occurs throughout WA.

Culex tritaeniorhynhus (external site), the primary JEV vector species in south-east Asia, has been detected in the Kimberley. It is now established in the region and is expanding its geographic distribution. The Northern Territory is the only other jurisdiction in Australia to have reported the mosquito. Its presence, along with the ubiquitous native species Culex annulirostris, are expected to facilitate efficient transmission of JEV in the region. 

Another efficient vector species, Cx. gelidus is also present in the Kimberley region, but its distribution is far more restricted than Cx. annulirostris.

The role of other ‘native’ Culex species is yet to be determined but cannot be ruled out. This includes Cx quinquefasciatus which breeds in standing polluted/stagnant water and is therefore relevant in the context of piggeries.

WA has large commercial pig farming operations, mostly in the southern half of the state and a widespread population of feral pigs. Water birds also occur in abundance across much of WA, often in close association with seasonal and permanent freshwater wetlands where Cx. annulirostris and Cx. tritaeniorhynchus mosquitoes are abundant. This close proximity of mosquito vectors to amplifying hosts creates an ideal environment for JEV transmission.

Current evidence indicates that transmission of JEV via direct contact between pigs (pig to pig) is possible, although the risk associated with this mode of transmission is considered very low. Importantly, there is no evidence of pig to human transmission.

Less than 1% of people infected with JEV will experience symptoms. Some infected people experience an illness with fever and headache. People with a severe infection may experience neck stiffness, disorientation, tremors, coma and seizures. If people have any of these symptoms, they should seek medical treatment. Among those who develop severe infection, some will go on to experience permanent neurological complications or possibly death.

There are two JE vaccines available for use in Australia – Imojev® and JEspect® (also known as Ixario). 

Due to the current outbreak of JEV in Australia, the Communicable Disease Network of Australia (external site) has identified at-risk groups who have been identified nationally as a priority for vaccination:

• Persons with an occupational/residential exposure to, or a planned, non-deferable visit to a piggery or pork abattoir/rendering plant.
• Persons working directly with mosquitoes (surveillance, control, or management) and indirectly through management of vertebrate mosquito-borne disease surveillance systems (e.g. sentinel chicken flocks).
• All diagnostic and research laboratory workers who may be exposed to the virus.

Additionally, the WA Department of Health is currently offering free JE vaccine to eligible persons in specific areas of the Kimberley. These areas are considered to be at highest risk of JEV transmission in WA. For more information, visit the Japanese encephalitis webpage on Healthy WA.

People who request JE vaccine for travel purposes are not eligible for a free vaccine under the JEV vaccination program, and instead should visit their GP, travel doctor or other immunisation provider.

Further information:

• Japanese encephalitis | The Australian Immunisation Handbook (external site)
• Japanese encephalitis virus (JEV) vaccines | Australian Government Department of Health and Aged Care (external site)
• NCIRS learning resource: Japanese encephalitis vaccination | NCIRS (external site)

Animals, including pigs and horses, can exhibit clinical signs associated with infection.

The most common clinical signs of JE in pigs are reproductive losses, including abortions, mummified foetuses and stillborn or weak piglets. Tremors and convulsions are occasionally seen in pigs up to six months of age.

In horses, clinical disease is generally mild, however fever, decreased or no appetite, lethargy, and incoordination may occur. In rare cases, JE infection may be fatal.

JE does not pose a food safety concern. Commercially produced pork meat or pork products are safe for human consumption.

WA Health is currently working with state and national authorities to increase surveillance and undertake response preparedness. As part of this response, local government may be called upon to assist with the following activities:

• undertake targeted mosquito surveillance
• refine mosquito management programs to target Culex mosquito species
• provide mosquito management advice to piggery operators/managers
• assist in refining piggery and hobby pig farm mapping to inform mosquito surveillance efforts.

In those local government areas, particularly in regional WA, where environmental health officers (EHOs) and rangers may interact with pig and horse owners/managers, it will be important to encourage individuals to report of any signs of illness to their local veterinarian, DPIRD or the national Emergency Animal Disease Watch Hotline on 1800 675 888. For DPIRD webpage information please visit the JE information page (external website).

WA Health will keep local governments informed about these next steps and issue formal requests for assistance or relevant public health measures as soon as required.

DPIRD has previously issued a media statement (external website), advising all commercial piggeries of the need to undertake appropriate mosquito management. Piggery operators may seek the advice of local government EHOs. EHOs may wish to consider the following advice:

• ensure effluent ponds or water bodies have sufficient capacity to contain all excess wastewater from pig production and are well maintained to prevent mosquito breeding
• ensure misting/irrigation systems do not create standing water in or around pig sheds
• level or drain depressions in the ground that may hold water
• empty, cover or discard any water holding containers, including rubbish/debris, to reduce mosquito breeding on the property
• screen water tanks with insect‑proof mesh, including inlet, overflow and inspection ports.
• ensure guttering is not blocked and does not hold water
• apply residual barrier sprays to buildings, structures and vegetation (ensure any application of pesticides is in accordance with the registered product label and will not compromise livestock production standards or withholding periods)
• cut grass and vegetation regularly around housing and communities to stop mosquitoes seeking shelter.
• staff to apply an effective mosquito repellent containing diethyltoluamide (DEET), picaridin or oil of lemon eucalyptus (OLE) according to the label instructions to prevent mosquito bites.

The following advice can be provided to horse owners seeking information about ways to reduce the risk of exposure to biting mosquitoes for their animals:

• horse owners can also put measures in place to help their horses avoid mosquito bites. During hotter months put a light cotton rug on them, a fly mask, and if the horse allows, apply a safe insect repellent. Do not spray the repellent around or above their eyes.
• the Australian mosquito that transmits JEV feeds at night and is reluctant to enter dwellings, so stabling horses between dusk and dawn is a valuable action
• rugging and hooding with lightweight permethrin fabric may help protect horses not stabled overnight.

Mosquito surveillance in proximity to piggeries

A Japanese encephalitis virus – Information for industry fact sheet (PDF 258KB) has been prepared to provide information to the piggery industry on Japanese encephalitis virus and the importance of mosquito surveillance around piggeries and wetlands, to maintain industry confidence.

Piggery operators are advised of the need for field officers from WA Health and local government to set mosquito traps in close proximity to piggeries for the purposes of enhanced mosquito surveillance. It is WA Health’s preference to advise piggery operators (by phone) when we will be trapping mosquitoes near their premises. This will also provide an opportunity for piggery operators to ask specific questions about surveillance activities or seek advice about mosquito management.

To ensure you are advised prior to mosquito trapping being undertaken in proximity to your piggery, or to express interest in on-property mosquito trapping, contact Medical.Entomology@health.wa.gov.au

Mosquito management on piggeries

The most effective way to prepare for the possibility that JEV may be introduced into WA, and to reduce the likelihood that pigs and industry workers will be infected, will be to minimise mosquitoes on a property. It is important that an assessment of on-site mosquito breeding and mosquito management is undertaken now, rather than waiting for JEV to be detected within the State.

If JEV is already present at undetected levels, the need to undertake mosquito management is even more critical, as it will prevent the cycle of JEV transmission being established. The following resources provide useful information on mosquito management, specific to piggeries:

• Controlling mosquitoes around piggeries (external link)
• Integrated mosquito management principles for piggeries (external link).

Non-chemical control methods are often very effective and can be undertaken by piggery operators immediately. There are a range of chemical larvicide products that are also safe and effective to use, from a chemical residue and environmental perspective. However, it is recommended that you seek professional advice from a licensed pest control operator or a local government environmental health officer before undertaking chemical control.