Department of Health

New Murray Valley encephalitis virus detections in northern Victoria

Health alert

Status:
Active
Alert number:
230125
Date issued:
24 Jan 2023 - Update to alert issued 21 January 2023
Issued by:
Associate Professor Deborah Friedman, Deputy Chief Health Officer (Communicable Disease)
Issued to:
Health professionals and the Victorian community

Update

There have been more detections of Murray Valley encephalitis (MVE) virus in mosquitoes trapped at multiple locations across northern Victoria. These results mean that MVE virus is circulating widely in the mosquito population and indicate a significant risk to people in these areas, particularly Mildura and northern Victoria’s inland river regions. The risk area for MVE virus is similar to that for Japanese encephalitis virus (JEV) and the risk of human cases in the coming weeks is very high. This risk is expected to continue while mosquito numbers remain elevated. 

Public health advice for Mildura and northern Victoria’s inland river regions

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

  • Residents, visitors and event patrons should wear an appropriate repellent at all times when outdoors. 
  • Cover up – wear long, loose-fitting, light-coloured clothing.
  • Avoid time spent outdoors at dusk and dawn as much as possible.
  • Event organisers must consider their obligations to keep event patrons safe. This requires informing patrons of the risk of mosquito exposure and the necessary 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.

Local councils are undertaking mosquito control activities in affected areas.

Key messages

  • Murray Valley encephalitis (MVE) virus has been detected in mosquitoes in northern Victoria this mosquito season.
  • MVE virus can cause a rare but potentially serious infection of the central nervous system and is spread to humans by infected mosquitoes. 
  • Symptoms may include fever, headache, nausea, vomiting and muscle aches, although most infected people do not have symptoms. In serious cases, people can develop meningitis or encephalitis.
  • Anyone with symptoms should seek urgent medical care.
  • Clinicians should test for MVE virus and Japanese encephalitis virus in patients with a compatible illness.
  • Clinicians should notify the Department of Health immediately of suspected cases by calling 1300 651 160.
  • The risk of MVE virus infection and other mosquito-borne diseases is high this mosquito season. The best prevention is to protect against mosquito bites.

What is the issue?

Murray Valley encephalitis (MVE) virus has been detected in mosquitoes in northern Victoria this mosquito season. This marks the first detection of the virus in Victoria in over 10 years through surveillance programs.   

MVE virus can be transmitted to humans by infected mosquitoes and can cause potentially serious illness. 

There is no effective treatment or vaccine for MVE. The best prevention is to protect against mosquito bites. 

The risk of mosquito-borne diseases, such as MVE, Japanese encephalitis, Kunjin/West Nile virus, and Ross River and Barmah Forest virus infections, is high due to recent weather conditions and elevated mosquito numbers across Victoria. 

MVE virus is endemic to northern Australia and the virus was previously detected in Victoria in 2011 from animal surveillance along the Murray River. The last human cases of MVE virus infection in Victoria were reported in 1974 following significant flooding. Human cases were reported in New South Wales and South Australia in 2011.

For up-to-date information on mosquito viral detections, visit the Department of Health's webpage on mosquito-borne diseases.

Who is at risk?

Anyone is potentially at risk of mosquito bites and mosquito-borne diseases, such as MVE virus infection. However, most mosquito bites do not transmit infection. 

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

Symptoms and transmission

MVE virus is spread to humans through the bite of an infected mosquito. It does not spread directly from person-to-person. 

Most people infected with MVE virus do not have symptoms. Less than one per cent of people develop clinical disease. If symptoms develop, they typically start 7 to 12 days after exposure but may occur anywhere from 5 to 28 days after exposure. 

Symptoms include fever, headache, nausea, vomiting and muscle aches. In rare cases, people can develop meningitis or encephalitis and have symptoms of severe headache, neck stiffness, sensitivity to bright lights, drowsiness, confusion, seizures, loss of consciousness or coma.

Serious illness can result in death or long-term neurological complications. 

People who have been exposed to MVE virus are likely to have long-lasting immunity to subsequent infections. 

Recommendations

For health professionals

Clinicians should consider MVE virus infection, 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 rural or regional Victoria. 

MVE may be clinically indistinguishable from Japanese encephalitis. Clinicians should consider testing for both viruses in the appropriate clinical situation.

MVE virus infection is an urgent notifiable condition. Clinicians and pathology services must notify the Department of Health of suspected or confirmed cases immediately by phoning 1300 651 160 (24 hours).

Recommended laboratory testing for MVE virus infection includes:

  • Blood (5 to 10 mL, in a serum tube) for MVE virus and flavivirus serology
    • Repeat convalescent serology testing at 3 to 4 weeks post onset of illness
  • Blood (5 to 10mL, in a dedicated EDTA tube) for MVE virus PCR/culture
  • CSF (1 to 3mL) for MVE virus and flavivirus serology and PCR/culture
  • Urine (2 to 5mL in a sterile urine jar) for MVE virus PCR/culture

Samples should be sent urgently to the Victorian Infectious Diseases Reference Laboratory (VIDRL). Request forms should be appropriately labelled and the on-call pathologist at VIDRL should be contacted to provide information on samples being sent. Samples should be transported at 4 degrees Celsius.

For the public

Victorians should be aware of the risk of mosquito-borne diseases including MVE virus infection. 

People with symptoms of MVE virus infection should seek urgent medical care. 

The best prevention is to protect against mosquito bites:

  • Cover up – wear long, loose-fitting, light-coloured clothing. 
  • Use mosquito repellents containing picaridin or DEET on all exposed skin.
  • Limit outdoor activity if lots of mosquitoes are about.
  • Remove stagnant water where mosquitoes can breed around your home or campsite.
  • On holidays make sure your accommodation is fitted with mosquito netting or screens.
  • Use ‘knockdown’ fly sprays and plug-in repellent devices indoors.
  • Don’t forget the kids – always check the insect repellent label. On babies, you might need to spray or rub repellent on their clothes instead of their skin. Avoid applying repellent to the hands of babies or young children.
  • Sleep under mosquito nets treated with insecticides if you don’t have insect screens on windows on your home or are sleeping in an untreated tent or out in the open.
  • Mosquito coils can be effective in small outdoor areas where you gather to sit or eat.
     

Reviewed 25 January 2023

Health.vic

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Communicable Disease Prevention and Control Department of Health

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