Key messages

  • Murray Valley encephalitis (MVE) is an ‘urgent’ notifiable condition that must be notified immediately to the department by medical practitioners and pathology services.
  • MVE virus can cause severe human disease if encephalitis is a clinical feature.
  • Clinical evidence may be present as non-encephalitic illness, encephalitic illness or asymptomatic disease.
  • Only one person in about every 800 of those infected with MVE virus develops clinical disease.

Arboviruses are viruses that are spread by the bite of arthropods, particularly mosquitoes. They are divided into alphaviruses and flaviviruses. Murray Valley encephalitis (MVE) virus is a flavivirus.

Notification requirement for Murray Valley encephalitis

MVE is an ‘urgent’ notifiable condition and must be notified by medical practitioners and pathology services immediately by telephone upon initial diagnosis (presumptive or confirmed). Pathology services must follow up with written notification within 5 days. 

This is a Victorian statutory requirement.

Primary school and children’s services centres exclusion for Murray Valley encephalitis

Primary School and Children’s Services Centres exclusion is not required.

Infectious agent of Murray Valley encephalitis

MVE virus has the capacity to cause severe human disease, with encephalitis as the most notable clinical feature.

MVE virus was first isolated from patients who died from encephalitis in the Murray Valley in Victoria and South Australia in 1951. It was previously included as one of the causative agents in the disease called ‘Australian encephalitis’, which also included disease caused by Kunjin virus, another flavivirus. These viruses are now accepted as causing two separate diseases: MVE and West Nile/Kunjin. Since 2010, Kunjin has been known as West Nile/Kunjin virus.

Identification of Murray Valley encephalitis

Clinical features

The MVE virus commonly infects humans without producing apparent disease (subclinical infection). It may also cause a comparatively mild disease with features such as fever, headache, nausea and vomiting. In a small percentage of all people infected, mild disease may be a prodrome to disease progression and the involvement of the central nervous system. This can result in meningitis or encephalitis of variable severity. Signs of brain dysfunction, such as drowsiness, confusion, fitting, weakness or ataxia, indicate the onset of encephalitis.

Diagnosis

Infection is confirmed by a significant rise in antibody titre to the virus in two blood specimens taken 7–10 days apart. The viral identification laboratory at the Victorian Infectious Diseases Reference Laboratory performs both polymerase chain reaction (PCR) and serological testing for MVE virus. PCR testing for MVE virus will only be performed when a flavivirus has been detected in the specimen. Cross-reaction of antibodies to other flaviviruses is possible.

A diagnosis of MVE should be considered in any patient who presents with encephalitis and who has been in the Murray Valley area within the incubation period of the disease, especially in the period between November and March. The disease may also be acquired at any time in northern parts of Australia or in Papua New Guinea.

Laboratory definitive evidence requires one of the following:

  • isolation of MVE virus
  • detection of MVE virus by nucleic acid testing
  • IgG seroconversion, or a significant increase in antibody level, or a fourfold or greater rise in titre to MVE virus
  • detection of MVE virus-specific IgM in cerebrospinal fluid (CSF) in the absence of IgM to West Nile/Kunjin, Japanese encephalitis and dengue viruses
  • detection of MVE virus-specific IgM in serum in the absence of IgM to West Nile/Kunjin, Japanese encephalitis and dengue viruses; this is accepted as laboratory evidence for encephalitic illnesses only.
  • focal neurological disease or clearly impaired level of consciousness
  • an abnormal CT, MRI scan or EEG
  • presence of pleocytosis in the CSF.

CSF and serum (clotted blood) are appropriate samples for PCR testing. Serological testing is done on serum alone.

Confirmation of laboratory results by a second arbovirus reference laboratory is required if the case occurs in areas of Australia not known to have established enzootic/endemic activity or regular epidemic activity.

Clinical evidence

Clinical evidence may be present as non-encephalitic illness, encephalitic illness or asymptomatic disease.

Non-encephalitic illness

Acute febrile illness with headache, myalgia and/or rash.

Encephalitic disease

Acute febrile meningoencephalitis characterised by one or more of the following:

Asymptomatic disease

Case detected as part of a serosurvey should not be notified.

Incubation period of Murray Valley encephalitis virus

The incubation period is usually 7–28 days.

Public health significance and occurrence of Murray Valley encephalitis

Serological studies show that only one person in about every 800 of those infected with MVE virus develops clinical disease. Of those presenting with encephalitis in Victoria in the 1974 epidemic, approximately one-third died, one-third were left with residual brain damage and one-third recovered completely.

MVE virus is endemic in northern Australia and Papua New Guinea, where sporadic cases or small outbreaks of MVE occur every few years, usually at the end of the wet season. Seven outbreaks of MVE have occurred at irregular intervals in south-eastern Australia since 1917. The most recent was in 1974. During those times, there was heavy rainfall leading to widespread flooding, which promoted large increases in waterbird and vector mosquito populations. The MVE virus numbers were amplified in the bird–mosquito–bird cycle, and humans became infected when bitten by mosquitoes carrying the virus. MVE seems to occur in people who receive large numbers of mosquito bites during a single exposure.

There are two theories as to how the MVE virus appears and causes outbreaks of MVE in south-eastern Australia; both may be correct. The first one postulates that the virus is carried from northern parts of Australia by birds migrating south in search of food after heavy rainfall in the south-eastern parts of the continent. This occurs in repeated mosquito–bird–mosquito amplification cycles. The other theory suggests that the MVE virus persists during inter-epidemic periods in cryptic foci along the Murray River and amplifies and becomes evident only when weather conditions are conducive to massive local mosquito and bird multiplication.

Reservoir of Murray Valley encephalitis virus

In Victoria, the primary hosts of MVE virus during years of high virus activity are waterbirds. Ardeiformes (herons), particularly the Nankeen (rufous) night-heron, and the Pelicaniformes (cormorants/darters) are the most commonly infected.

Mode of transmission of Murray Valley encephalitis virus

The virus is spread by the bite of an infected mosquito. Mosquitoes become infected when they feed on people or animals that are infected with MVE virus. The primary mosquito vector during epidemics is Culex annulirostris. Other mosquitoes, such as Culex australicus and some Aedes and Ochlerotatus species, may be involved in other aspects of MVE virus ecology.

Period of communicability of Murray Valley encephalitis

There is no evidence of person-to-person transmission.

Susceptibility and resistance to Murray Valley encephalitis

Infection with MVE virus confers lifelong immunity.

Control measures for Murray Valley encephalitis

Preventive measures

MVE infection can be prevented by:

  • mosquito control measures
  • personal protection measures, such as wearing long sleeves
  • using personal insect repellents containing diethyl toluamide (DEET) or picaridin
  • avoiding mosquito-prone areas and vector biting times at dusk and dawn.

There is no preventive vaccine available.

Control of case

Patients can be managed at any hospital, but facilities for providing intensive care and artificial respiration must be available.

Investigate the source of infection.

The patient with a suspected infection or one of their friends or relatives should be asked to recall whether, in the month prior to the onset of symptoms, the patient:

  • was bitten by mosquitoes
  • visited regions where arboviruses are endemic
  • participated in recreational or other activities involving exposure to bushland or other mosquito habitat, such as gardening, bushwalking, camping or picnicking.

Control of contacts

Not applicable.

Control of environment

To reduce or prevent virus transmission, environmental control measures include suppression of the vector mosquito population by applying mosquito control measures in local municipalities.

Outbreak measures for Murray Valley encephalitis

Following Victorian notification of a seroconversion to MVE virus or information of human notification:

  • an emergency meeting of the Victorian Arbovirus Task Force (VATF) will be convened by the department
  • the presence of MVE virus in the area will be notified to relevant regional offices and local government health personnel
  • suitable media releases will be made available
  • appropriate VATF members will visit the area to consult and advise local councils, health and tourism authorities
  • depending on the actual or potential severity of the epidemic, meetings of relevant personnel will be arranged in the affected area to consider control measures.

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