Japanese encephalitis - Frequently asked questions
Japanese encephalitis (JE) is a mosquito-borne viral disease that can cause reproductive losses and encephalitis in susceptible animal species. It is caused by the Japanese encephalitis virus (JEV), which is a member of the Flavivirus genus.
The Japanese encephalitis virus is primarily spread by mosquitoes feeding on infected animals and does not usually spread directly between animals (i.e. it is not considered contagious). Certain mosquito species, such as Culex sp., can become infected with and transmit JEV. These mosquitoes are generally most active at dawn and dusk.
Infected pigs and wading birds do not transmit the infection to other animals but may infect mosquitoes if bitten while they still have the virus in their blood.
Generally, JEV is spread through the movement of infected mosquitoes, often over long distances due to wind dispersal, and the movement of some migratory water birds. While water birds can carry the virus, they are not affected by it.
Japanese encephalitis disease occurs most commonly in pigs. Horses can also be infected and rarely other animals.
Pigs and waterbirds (such as herons and egrets) play an important part in the transmission cycle by amplifying the virus and providing an ongoing source of Japanese encephalitis virus to infect local mosquito populations.
In 2022, South Australia had a confirmed case of Japanese encephalitis in an alpaca in the Adelaide Plains local government area.
Alpacas, like horses, are considered a ‘dead-end’ host, as they do not develop a blood infection that will reinfect mosquitoes.
Pigs
Adult non-pregnant sows typically show no obvious signs of infection. However, if sows are infected before 60–70 days of pregnancy, reproductive failure may be evident, which can present as:
- an increase in stillborn, mummified or weak piglets
- piglets born with obvious deformities or abnormalities
- neurologically affected piglets trembling, shaking or making paddling motions
- increased abortions
- over-term sows with delayed farrowing, extending beyond 118 days.
If present on farm, infected boars may experience infertility and swollen or inflamed testicles.
Horses
In horses many cases are asymptomatic and most clinical disease is mild, however more severe encephalitis (inflammation of the brain) can occur which may be fatal. Signs include:
- Fever
- lethargy
- anorexia
- neurological signs which vary with severity of the clinical disease.
Neurological signs can include incoordination, difficulty swallowing, impaired vision, and rarely a hyperexcitable form occurs. Disease has also been reported in donkeys.
Other species
Reports of disease in other species are rare and affected animals may show non-specific clinical signs such as fever and decreased appetite or neurological signs.
Japanese encephalitis is a notifiable disease in Victoria. If you suspect it in any animal, immediately contact your local Agriculture Victoria staff or phone the all-hours Emergency Animal Disease Watch Hotline on 1800 675 888.
Most JE virus infections are asymptomatic. A small number of people infected with JE virus may become seriously ill with encephalitis (inflammation of the brain). Other people may develop an illness with fevers, headaches and aches and pains.
Symptoms of encephalitis may include rapid onset of fever, headache, sensitivity to light, neck stiffness, vomiting, confusion or disorientation and sometimes seizures, progressing to coma.
Anyone experiencing symptoms, particularly if they’ve visited regions near the border of Victoria and New South Wales or been in contact with pigs or with mosquitoes should seek urgent medical attention.
Most human cases of Japanese encephalitis will have no symptoms or very mild symptoms, but anyone who develops a sudden onset of fever, headache and vomiting should see their doctor immediately.
Any person who is experiencing any of the symptoms should seek urgent medical attention. Please visit the Department of Health website for public health advice.
The Department of Health recommends people protect themselves and their families against mosquito-borne diseases:
- Wear long, loose-fitting clothes outdoors.
- Use effective mosquito repellents containing picaridin or DEET on all exposed skin.
- Try to limit outdoor activity if lots of mosquitoes are about.
- Use ‘knockdown’ fly sprays and plug-in repellent devices indoors.
- Mosquito coils can be effective in small outdoor areas where you gather to sit or eat.
- Make sure your accommodation is mosquito-proof.
- Sleep under mosquito nets treated with insecticides if you don’t have flywire screens on windows on your home or are sleeping in an untreated tent or out in the open.
- Remove mosquito breeding sites like stagnant water around your property.
- Remember dusk and dawn are when mosquitoes are more active, but some will also bite during the day.
People with increased exposure to mosquitoes may be at a higher risk of infection, particularly those in regional areas and who work with or are in contact with pigs, and people camping, working or spending time outdoors in these regions.
In Victoria, state-funded JE vaccine has been made available free of charge for people who may be at risk of exposure to the JE virus. Speak to your health practitioner or local public health unit, or visit the Victorian Department of Health to find out more details.
Japanese encephalitis virus is primarily spread through mosquitoes, or animal management practices that involve penetrating the skin, e.g. vaccination or blood collection.
Insecticide can be used to reduce mosquito numbers and minimise further spread of the virus. Reducing mosquito habitats and exposure to mosquitoes is an important part of breaking the transmission cycle.
For more information visit the Farm Biosecurity Website:
To minimise the risk of spreading the virus through management practices, avoid sharing needles between animals when injecting, and thoroughly wash and decontaminate other equipment between animals.
Agriculture Victoria is actively monitoring the situation and continues to investigate and test horses, pigs and other livestock (for example, alpaca) that exhibit clinical signs consistent with JE virus infection.
A surveillance project using oral fluids collected via chew ropes from pigs has been established for the 2024–25 mosquito season. This project aims to establish an early warning system for the potential circulation of JE virus in the environment, benefiting both human and livestock health.
Agriculture Victoria continues to support the Victorian Department of Health in its ongoing public health response activities.
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