Murray Valley encephalitis virus and West Nile virus Kunjin strain

Murray Valley encephalitis (MVE) virus and West Nile virus Kunjin strain (WNVKUN) are two mosquito-borne viruses in Western Australia (WA).

MVE virus can cause potentially fatal encephalitis (inflammation of the brain) in humans, while WNVKUN is associated with milder febrile illness and only occasionally cause encephalitis.

The disease caused by these viruses was previously known as Australian encephalitis but is now generally referred to as Murray Valley encephalitis (MVE) and Kunjin disease (KUN).

In nature MVE virus and WNVKUN cycle between mosquitoes and water birds, while humans are only incidental hosts. The common banded mosquito (Culex annulirostris) is the main vector of both of these viruses in Australia, with water birds, particularly herons, considered to be the major vertebrate hosts.

Where do MVE and WNVKUN occur in WA?

Murray Valley encephalitis and Kunjin diseases are endemic in the northern two thirds of WA (specifically the Kimberley region). It is occasionally active in other regions, such as the Gascoyne, Goldfields, and Midwest.

MVE virus and WNVKUN can be active during and in the months following heavy wet season rains. February to April is the season of most risk, but the risk can commence as early as December and extend to June or July in very wet years.

MVE virus and WNVKUN infections are a greater risk near swamps, floodplains, river systems, irrigation areas and major dams.

What are the symptoms of MVE and KUN?

Symptoms of Murray Valley encephalitis disease

The incubation time for MVE varies from 5 to 15 days, but symptoms usually appear within 8 to 10 days after becoming infected.

MVE virus causes disease symptoms in approximately 1 in 1000 people that is bitten by an infective mosquito. Most do not develop any symptoms at all. 

In adults and older children, symptoms include:
  • fever
  • drowsiness
  • bad headache and stiff neck
  • nausea
  • muscle tremors
  • dizziness.
In young children the symptoms include:
  • fever
  • floppiness
  • irritability
  • drowsiness
  • fits.

The disease can be mild or severe, but in severe cases can lead to coma and death. Patients with the severe form of MVE get worse very quickly with confusion and worsening headaches, increasing drowsiness and possible fits.

People with suspected Murray Valley encephalitis should be taken to the nearest hospital without delay.

Symptoms of Kunjin disease

WNVKUN causes similar but generally less severe disease symptoms compared to Murray Valley encephalitis. The majority of infected individuals do not show any symptoms, while the minority of people with Kunjin disease can develop encephalitis which may require immediate medical attention.

Are treatments available for MVE and KUN?

People with suspected MVE or KUN should be taken to the nearest hospital. There are no vaccines or cures for either of these diseases.

A doctor who diagnoses one of these diseases in a patient is required under the Health Act to immediately notify the Department of Health.

More information

Medical Entomology
Postal address: PO BOX 8172, Perth Business Centre, WA 6849
Street Address: 1A, Brockway Road, Claremont, WA 6010
Phone: (08) 9285 5500
Email: medical.entomology@health.wa.gov.au