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Murray Valley encephalitis virus and Kunjin disease

Murray Valley encephalitis and Kunjin disease

 

Culex annulirostris mosquitoMurray Valley encephalitis (MVE) virus and West Nile virus Kunjin strain (WNVKUN) are two mosquito-borne viruses that occur in the north of Western Australia (WA).

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

MVE is a rare but potentially fatal disease characterised by severe neurological symptoms, including encephalitis. KUN is a milder febrile illness, but can occasionally be associated with encephalitis.

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

Distribution in WA

MVE distribution map in landscape orientationMVE and WNVKUN viruses occur in the northern two thirds of WA only. The risk of infection is highest if you are travelling to the Kimberley and Pilbara regions during or shortly after the west season. The viruses are occasionally active in other regions, including the Gascoyne, Goldfields, and Midwest.

MVE virus and WNVKUN can be active during and in the months following heavy wet season rains. The highest risk months are February to April, but virus activity can commence as early as December and extend as late as July in very wet years.

MVE virus and WNVKUN activity is more likely to occur near swamps, floodplains, river systems, irrigation areas and major dams where both mosquitoes and water birds are present.

Disease symptoms

Symptoms of Murray Valley encephalitis

MVE virus causes disease symptoms in approximately 1 in 1000 people bitten by an infective mosquito. Most people remain asymptomatic and do not develop any signs of disease. Once an individual has developed antibodies to MVE, it is unlikely that they will be reinfected in the future.

The incubation time for MVE varies from 5 to 15 days, but symptoms usually appear within 8 to 10 days after becoming infected. Whilst MVE is rare, it is important to note that severe cases can lead to coma and death. In young children, fever may be the only early sign of infection. Patients with the severe form of MVE get worse very quickly and present with confusion, worsening headaches, increasing drowsiness and in some cases, seizures. People with suspected MVE should be taken to the nearest hospital without delay.

A summary of MVE symptoms in adults and children is provided below: 

 MVE symptoms in adults and older children  MVE symtoms in young children 
 fever
 fever
 drowsiness
 floppiness
 bad headache and stiff neck
 irritability
 nausea
 drowsiness 
 muscle tremors
 seizures
 dizziness  

Symptoms of Kunjin disease

WNVKUN causes similar but generally less severe disease symptoms compared to MVE. While the majority of infected individuals are asymptomatic, those people that do develop symptoms of KUN can present with encephalitis, requiring immediate medical attention.

Prevention and treatment

There is no vaccine or specific treatment for either of these diseases. People with suspected MVE or KUN should be taken to the nearest hospital without delay. 

The only way to prevent infection is to avoid being bitten by mosquitoes (Healthy WA). This can be best achieved by: 

  • avoiding the outdoors when mosquitoes are most active
  • covering up in long, light coloured, loose-fitting clothing
  • applying an effective insect repellent evenly to all areas of exposed skin
  • ensuring insect screening on housing, tents and caravans remains in good working order.

Practitioners involved in the diagnosis of MVE or KUN in WA have a legal obligation to report the diagnosis to the WA Department of Health.

More information

Medical Entomology
Phone: (08) 9285 5500
Email: medical.entomology@health.wa.gov.au
Last reviewed: 09-09-2020
wa.gov.au