Japanese Encephalitis


Published: 27 March 2022

Prior to 2022, Japanese encephalitis virus (JEV), which causes inflammation of the brain, had only affected two people in mainland Australia (van den Hurk et al. 2019; Queensland Health 2022).

Both cases were locally acquired in far-north Queensland, two decades ago in 1998 (van den Hurk et al. 2019).

Fast-forward to March 2022, where there are now at least 20 confirmed human cases of the virus, not only in Queensland but as far south as New South Wales, South Australia and Victoria - where it has never before been detected (DoH 2022a; Lanese 2022).

This unprecedented outbreak has caused the virus to be declared a Communicable Disease Incident of National Significance by the Australian Government (DoH 2022).

The surge in cases is believed to be associated with climate change and flooding, which have encouraged waterbirds to migrate to newly formed inland waterways. The warm, wet conditions have also facilitated an increase in the mosquito population (Lanese 2022).

In light of this emerging situation, it’s crucial for healthcare professionals to remain up-to-date with the latest information.

So, what exactly is Japanese encephalitis, and should there be cause for concern in the community?

What is Japanese Encephalitis?

Japanese encephalitis culex mosquito bite
Japanese encephalitis is spread to humans via Culex mosquitoes, which act as disease vectors.

Japanese encephalitis (JE) is a mosquito-borne illness caused by the Japanese encephalitis virus (JEV) (NSW Health 2022).

Like other types of encephalitis such as Murray Valley encephalitis, JE is an inflammation of the brain. While 99% of cases are asymptomatic, severe cases can lead to lifelong disability or even death (Healthdirect 2020; Queensland Health 2022).

The disease predominantly occurs in rural and agricultural areas in Asia but is also endemic (found often) in the Torres Strait and Papua New Guinea (Queensland Health 2022).

What Causes Japanese Encephalitis?

JE is a zoonotic virus, meaning it’s spread from animals to people. However, animals can’t spread the virus to humans directly, nor can humans spread the virus to other humans. Instead, mosquitoes that feed on infected animals acquire the virus themselves and become vectors (i.e. living organisms that transmit pathogens from infected animals to other animals, or humans). If an infected mosquito feeds on a human, the virus will be transmitted to that individual (Davey 2022; Australia Wide First Aid 2022; EFSA 2020).

Only Culex genus mosquitoes are able to transmit the illness, predominantly the Culex annulirostris species (Lanese 2022; Business Queensland 2022).

The JEV virus is predominantly found in pigs and waterbirds (particularly herons and egrets), which are amplifying hosts, meaning they facilitate the rapid multiplication of infectious agents and can reinfect new mosquitoes. Therefore, the virus is maintained in nature through mosquito-waterbird or mosquito-waterbird-pig transmission cycles (Business Queensland 2022; Biology Online 2021).

Humans and other animals, on the other hand, are ‘dead end’ hosts that once infected do not play any further role in transmission (Outbreak 2022).

Other animals that can be infected with JEV include:

  • Horses
  • Cattle
  • Sheep
  • Dogs
  • Cats
  • Bats
  • Rodents
  • Reptiles
  • Amphibians.

(Outbreak 2022)

Excluding pigs and horses, animals infected with JEV are usually asymptomatic. Signs of JEV in pigs typically include mummified, stillborn or weak piglets, possibly with neurological issues, and in horses, fever, jaundice, lethargy, anorexia and neurological issues (Outbreak 2022).

It’s not possible for humans to contract JEV by touching infected animals or eating animal products such as pork (NSW Health 2022).

Risk Factors for Japanese Encephalitis

Those at most risk of locally-acquired infection in Australia are people in close proximity to infected animals and mosquitoes. These include:

  • Those who work at, visit or live close to piggeries or pork abattoirs
  • Those participating in outdoor activities like camping, fishing or hiking near waterways or areas with high mosquito populations
  • Those who work directly with mosquitoes (e.g. environmental health officers and entomologists)
  • Diagnostic and research laboratory personnel working with mosquitoes or JEV cultures.

(NSW Health 2022; DoH 2022b)

Japanese encephalitis pigs
Pigs, along with waterbirds, are amplifying hosts of the Japanese encephalitis virus.

Symptoms of Japanese Encephalitis

Less than 1% of JEV infections in humans are symptomatic (NSW Health 2022). If symptoms do occur, they typically develop 5 to 15 days after being bitten by an infected mosquito. The individual might experience:

  • Fever
  • Headache.

(Queensland Health 2022)

And in severe cases:

  • High fever
  • Chills
  • Severe headache
  • Stiff neck
  • Disorientation
  • Sensitivity to light
  • Nausea and vomiting
  • Tremors
  • Coma
  • Seizures.

(NSW Health 2022; Queensland Health 2022)

One-third of severe cases will result in death, and another third will result in permanent disability (Queensland Health 2022).

Diagnosis is typically made via a blood test or spinal fluid sample (SA Health 2022).

There is no specific treatment for JE. Mild symptoms can be managed by resting, ensuring adequate fluid intake and taking paracetamol, while more severe cases may require admission to hospital for observation and supportive care (DoH 2022b).

Preventing Japanese Encephalitis

There is a vaccine available to protect against JEV, but it is currently only indicated for those who are at increased risk of contracting the virus or those travelling to Asia or the Torres Strait region (NSW Health 2022; DoH 2022b).

JE can also be prevented by avoiding mosquito bites through strategies such as:

  • Being careful outdoors during dawn, dusk and the evening, when Culex mosquitoes are most active
  • Wearing light-coloured, loose-fitting, long-sleeved clothing, as well as covered shoes and socks, when outdoors
  • Applying mosquito repellent that contains diethyltoluamide (DEET), picaridin or oil of lemon eucalyptus to all exposed skin (after sunscreen)
  • Reapplying repellant after swimming
  • Using flyscreens in the home
  • Using flyscreens or mosquito nets when camping
  • Using insecticide sprays, vapour dispensing units and mosquito coils to remove mosquitoes
  • Removing water-holding containers around the home, as these are breeding grounds for mosquitoes.

(NSW Health 2022; DoH 2022b)

Japanese encephalitis prevention wetlands
The risk of being infected with Japanese encephalitis virus can be reduced by taking precautions to prevent mosquito bites in outdoor environments, especially in areas with high mosquito populations.


Despite the recent outbreak, the risk of contracting JEV in Australia is still low (Better Health Channel 2022).

However, staying up-to-date with the situation, especially while the environmental conditions are ideal for JEV transmission, is crucial in the case that you do encounter cases of JE.

You can check the most current information about the Australian JEV outbreak on the Department of Health website.

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