Japanese encephalitis virus (JEV) infection for health professionals

Japanese encephalitis virus (JEV) spreads to humans and other animals through infected mosquitoes. It is a nationally notifiable disease. We support a One Health approach to JEV and report on national case numbers and epidemiological trends. We publish case definitions, guidelines and advice.

For health professionals For everyone

Disease

  • Japanese encephalitis virus (JEV) is a mosquito-borne flavivirus closely related to Murray Valley encephalitis virus and West Nile virus. 

  • Most human infections are asymptomatic. 

    Estimates of the proportion of infected people who develop symptoms vary in different populations, ranging from fewer than 1% to up to 4%. 

    Of those infected with JEV, fewer than 1% develop neuroinvasive disease.

    The most recognised clinical presentation of JEV infection is acute encephalitis. Milder presentations include non-specific febrile illness with headache and aseptic meningitis.

    Symptoms are initially non-specific and typically include:

    • fever
    • diarrhoea
    • headache
    • vomiting
    • generalised weakness.

    Neurological manifestations might then develop over the following few days, including:

    • confusion and agitation
    • focal neurological deficits, such as cranial nerve palsies and paralysis
    • convulsions and seizures
    • coma.

    As many as 1 in 3 people who develop encephalitis die from the disease. 

    Of those who survive, up to half suffer permanent cognitive, behavioural or neurological complications.

    Read about diagnosing JEV infection.

Public health importance

  • Japanese encephalitis is a serious, vaccine-preventable disease with no specific treatment. 

    JEV infection has a high rate of morbidity and mortality in people who develop encephalitis.

    Because of these major impacts, we:

    • monitor and report on cases of JEV infection in Australia 
    • take a One Health approach by working with colleagues in the animal and environmental sectors to monitor and report on detections of JEV in mosquitoes and animals
    • plan for how to manage outbreaks.  

  • JEV infection is a major public health problem in many parts of Asia and the western Pacific.

    In Australia, before 2021, occasional outbreaks occurred only in the Torres Strait and Cape York Peninsula. 

    During 2021 and 2022, an outbreak in south-eastern Australia resulted in 45 cases and 7 deaths. 

    JEV detections in feral pigs, mosquitoes and sentinel chickens show there is ongoing JEV transmission in animals in south-eastern Australia. 

    Studies in 2022 found evidence of past JEV infection in humans in:

    • 8.7% of participants in 5 higher-risk New South Wales regional towns
    • 3.3% of participants in 3 higher-risk Victorian regions.

    The risk of JEV infection varies depending on location, season, weather and other local factors.

    For the latest information on JEV infection cases in Australia, see the National Notifiable Diseases Surveillance System data visualisation tool.

Spread of infection

  • JEV transmits through bites of infected mosquitoes after they have fed on infected waterbirds or pigs.

    The primary host of JEV is believed to be wild birds, especially those in the Ardeidae family (including herons and egrets).

    The main vector for JEV in Australia is Culex annulirostris, though several mosquito species are likely involved.

    Culex annulirostris lives across Australia in permanent and semi-permanent freshwater bodies, usually in heavily vegetated sites. 

    Adult Culex annulirostris mosquitoes are active:

    • from mid-spring to late autumn in south-eastern Australia
    • all year in northern Australia.

    Humans are incidental and dead-end hosts – this means we cannot pass on the disease, as we do not develop high enough concentrations of JEV in our blood to infect mosquitoes.

    JEV does not spread through:

    • contact with infected people 
    • eating meat from an infected animal.

  • Symptoms usually start between 5 and 15 days after an infected mosquito bite.

  • JEV does not transmit from person to person. 

Priority populations

  • Anyone with JEV infection is at risk of severe disease. 

    Evidence about risk factors for severe disease is limited. Those who might be at greater risk include:

    • children aged under 5 years
    • non-immune adults
    • people who are immunocompromised
    • pregnant people.

    Read more about measures people can take to protect themselves.

  • People are at risk of exposure to JEV if they travel to, work or live in areas with JEV. 

    People are more likely to be exposed if they:

    • undertake outdoor work or leisure activities – like camping, fishing, hiking or gardening – in areas with JEV
    • work with or live close to pigs and waterbirds
    • work with mosquitoes, such as environmental health officers or entomologists
    • travel to endemic areas during the JEV transmission season
    • live or work on the outer islands of the Torres Strait.

    Read more about measures people can take to protect themselves.

  • High-risk settings include piggeries, pork abattoirs and pork rendering plants.

    These settings should actively promote measures people can take to protect themselves, including to:

    • get vaccinated
    • take steps to prevent mosquito bites.

    Read more about:

Prevention

  • JEV infection is vaccine preventable. 

    Routine JEV vaccination is recommended for:

    • laboratory workers who may be exposed the virus
    • travellers spending 1 month or more in endemic areas during the JEV transmission season
    • people who live or work on the outer islands of Torres Strait.

    States and territories also provide free vaccination to eligible people who live or regularly work in areas of high JEV concern. Free vaccination may also be available for people outside those areas who:

    • work with or live close to pigs and waterbirds
    • work with mosquitoes, such as environmental health officers or entomologists.

    See who is eligible in your state or territory: 

    Read more about JEV vaccination. 

  • The best protection against JEV is to avoid mosquito bites by:

    • using insect repellents that contain diethyltoluamide (DEET), picaridin or oil of lemon eucalyptus (OLE) – always follow the instructions on the label
    • wearing long, loose-fitting, light coloured clothing and enclosed footwear when outside
    • using mosquito netting or screens on windows, doors and vents, and patching any splits or tears
    • using insecticide sprays, vapour-dispensing units (indoors) and mosquito coils (outdoors) to repel mosquitoes 
    • removing standing water where mosquitoes may breed, such as plant trays or saucers, buckets or kiddie pools
    • regularly cleaning out permanent water containers – such as ponds and pet water bowls – 
    • keeping gutters clear
    • covering openings to water storage – like rainwater and sceptic tanks – with mosquito-proof mesh
    • avoiding areas of high mosquito activity, especially at dawn and dusk.

Diagnosis and clinical management

  • JEV infection is usually diagnosed through laboratory testing of a sample of cerebrospinal fluid or blood to detect the JEV nucleic acid. A serum sample is necessary for serological diagnosis. 

    Laboratories need extensive experience in diagnostic testing for flaviviruses, as cross-reactivity with other flaviviruses is common in serological tests.

    If JEV infection is part of a differential diagnosis, it’s important to specifically request JEV testing when referring patients to pathologists.

    Read more about the diagnosis and treatment of Japanese encephalitis.

  • Read more about laboratory testing and the laboratory case definitions.

  • There is no specific antiviral treatment for JEV infection. Clinical management consists of supportive care. 

    People with suspected JEV infection are generally referred to hospital for management and further investigation.

Notification and reporting

Public health response

  • Public health authorities adopt a One Health approach when responding to locally acquired JEV infection cases. This recognises the connections between humans, animals and the environment that contribute to JEV outbreaks, such as:

    • mosquito numbers for types capable of spreading JEV – which is influenced by season, weather patterns and temperature for breeding
    • the presence of host animals that maintain and amplify the virus, including pigs and waterbirds
    • how close mosquitoes and host animal populations are to one another and to humans. 

    Public health actions focus on: 

    • finding the sources of exposure
    • supporting local communities to control mosquitoes
    • communicating advice on preventing mosquito bites
    • offering vaccination to eligible people. 

    Animal health authorities, industry workers, landowners and work health safety officers can help identify and manage any domestic or wild pigs near likely exposure sites. 

    Local councils may be involved in mosquito control measures – such as fogging and spraying of insecticides – when there are large numbers of mosquitoes. They may also provide mosquito warnings and signage in high-risk areas.

    Environmental health officers undertake mosquito surveillance and determine the need for extra mosquito control measures. 

  • Public health units urgently investigate human cases of JEV, with a focus on determining the likely source of the infection. 

    The person affected, their family and any other people who might have been exposed to the same source should be provided with information about JEV. 

    Human cases do not need to isolate. 

  • Public health authorities provide information to people who might have been exposed about: 

    • how JEV spreads
    • the signs and symptoms of JEV infection 
    • vaccination.  

    For detailed guidance, see state and territory advice, including:

  • The joint national JEV outbreak response plan provides a framework for a national One Health response to manage JEV outbreaks. 

    How public health agencies respond to JEV infection cases and outbreaks depends on: 

    • state or territory legislation
    • local reporting requirements 
    • the nature of the cases or outbreak 
    • available resources. 

    Control measures used during outbreaks focus on humans, animals and the environment, and include:

    • animal surveillance to
      • understand disease epidemiology 
      • support targeted measures to reduce the immediate transmission risk
    • mosquito vector control measures, including environmental, biological and chemical controls 
    • health education and promotion of mosquito bite prevention and behaviour change measures
    • vaccination of people at high risk of exposure to JEV.

  • See guidelines for pig producers and horse owners to help protect animals from mosquito bites and reduce the risk of infection.

Resources

We are adding new content to this website and expanding these pages, including a suite of disease-related information. In the meantime, information about diseases in Australia is available on the Department of Health and Aged Care’s website. 

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