Update on Japanese encephalitis and Murray Valley encephalitis in Victoria

What is the issue?

Mosquitoes can spread diseases such as Murray Valley encephalitis (MVE), Japanese encephalitis (JE), and West Nile (Kunjin) virus infections. While most infections are asymptomatic or mild, some people can have severe illness with encephalitis or meningitis which can lead to long term neurological complications or death.

The risk of mosquito-borne diseases remains high in the coming weeks. Human cases of MVE continue to be detected in Victoria. In addition, MVE virus and West Nile (Kunjin) virus have been detected in mosquitoes trapped in parts of northern Victoria. For up-to-date information on human cases of mosquito-borne diseases and virus detections in mosquitoes, visit the Department of Health Mosquito-borne diseases page.

Human cases of JE and MVE have been reported in Victoria and other south-eastern Australian states and territories this mosquito season. In Victoria, cases of JE were reported for the first time in 2022 while cases of MVE were last reported in 1974.

Who is at risk?

Anyone is potentially at risk of mosquito bites and mosquito-borne diseases. However, most mosquito bites only cause minor swelling and irritation and do not transmit infection.

People with increased exposure to mosquitoes have a higher risk of infection. These include people who live, work, visit or spend time outdoors in northern Victoria, particularly inland riverine regions and near the Murray River.

People who live or work at properties with pigs may also be at higher risk of JE virus infection.

Symptoms and transmission

JE, MVE and West Nile (Kunjin) viruses are spread to humans through the bite of an infected mosquito. They do not spread directly from person-to-person.

Most people infected with JE virus or MVE virus do not have symptoms. If symptoms develop, they typically start 1 to 2 weeks after exposure. Symptoms may include fever, headache, nausea, vomiting and muscle aches.

Less than one per cent of people develop severe disease. People with severe disease (meningitis or encephalitis) may have symptoms of severe headache, neck stiffness, sensitivity to bright lights, drowsiness, confusion, cranial nerve pathology, muscle weakness or paresis, movement disorders, seizures, loss of consciousness or coma. Severe disease can result in death or long-term neurological complications.

Recommendations

For health professionals

Clinicians should consider JE, MVE and West Nile (Kunjin) virus infections, and other mosquito-borne diseases in patients presenting with a compatible illness, particularly in those with extensive exposure to mosquitoes or who have spent time in northern Victoria.

JE and MVE are clinically indistinguishable. Clinicians should test for both JE and MVE viruses in the appropriate clinical situation.

JE and MVE are urgent notifiable conditions. Clinicians and pathology services must notify the Department of Health of suspected or confirmed cases immediately by phoning 1300 651 160 (24 hours).

The usual investigations for common causes of encephalitis or meningoencephalitis should be conducted concurrently, including CSF sampling if safe and appropriate.

Recommended laboratory testing for JE and MVE virus infections includes all of the following:

• Blood – serum (2 to 5mL in children, 5 to 10 mL in adults, in a serum tube) for:
  • JE, MVE and West Nile (Kunjin) virus serology
  • Repeat convalescent serology testing at 2 to 4 weeks post onset of illness
• Blood – whole blood (2 to 5mL in children, 5 to 10 mL in adults, in a dedicated EDTA tube) for:
  • JE, MVE and West Nile (Kunjin) virus PCR and culture
• CSF (1 to 3mL, in a sterile collection tube) for:
  • JE, MVE and West Nile (Kunjin) virus serology
  • JE, MVE and West Nile (Kunjin) virus PCR and culture
• Urine (2 to 5mL, in a sterile urine jar) for:

JE, MVE and West Nile (Kunjin) virus PCR and culture.

Samples should be sent urgently to the Victorian Infectious Diseases Reference Laboratory (VIDRL). Request forms should be appropriately labelled and include relevant clinical and epidemiological history including symptom onset, vaccination, travel history and country of birth, to guide laboratory interpretation. Contact the on-call lab manager at VIDRL to provide information on samples being sent. Samples should be transported at 4 °C.

For the public

Victorians should be aware of the risk of mosquito-borne diseases, particularly those spending time outdoors in northern Victoria.

People with symptoms should seek urgent medical care.

The best prevention is to protect against mosquito bites. For further information, see the Better Health Channel mosquito bite prevention webpage.

A Japanese encephalitis vaccine is available for eligible people. See JE vaccine eligibility criteria for further details. People in the specified priority groups are advised to contact their general practitioner, local public health unit, local council or community pharmacy to confirm eligibility and arrange a vaccination appointment.

For high-risk areas

Residents and people visiting northern Victoria are strongly advised to take additional measures to reduce their risk of mosquito bites:

• Wear an appropriate repellent at all times when outdoors.
• Avoid time spent outdoors at dusk and dawn as much as possible.
• Event organisers must consider their obligations to keep event patrons safe. Inform patrons of the risk of mosquito exposure and recommend the use of protective measures, including appropriate clothing and use of repellent. Organisers may wish to postpone outdoor events or relocate them indoors during this high risk period, particularly for events scheduled around dawn and dusk.