Key messages

  • Murray Valley encephalitis (MVE) virus 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. 
  • Only one person in about every 800 of those infected with MVE virus develops clinical disease.
  • The virus is transmitted to humans by infected mosquitoes.The main prevention measures include, protecting oneself from mosquito bites while in endemic areas.
     

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 exclusion for Murray Valley encephalitis

Exclusion is not required. 

Infectious agent of Murray Valley encephalitis virus

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 virus

Clinical features

MVE virus commonly infects humans without producing apparent disease (subclinical infection). Only one person in about 800 of those infected with MVE virus develops clinical disease. 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.

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.

Diagnosis

Cases require laboratory and clinical evidence. 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.

CSF and serum (clotted blood) are appropriate samples for polymerase chain reaction (PCR) testing. Serological testing is most often performed on serum.  It is important that both acute and convalescent serum samples are collected in order to demonstrate a rise in IgG as this confirms recent infection.  Cross reaction of antibodies to other flaviviruses is possible. The Victorian Infectious Diseases Reference Laboratory performs both PCR and serological testing for MVE virus

Clinical evidence requires one of the following:

  • Non-encephalitic disease:  acute febrile illness with headache, myalgia and/or rash
  • Encephalitic disease, acute febrile meningoencephalitis characterised by one or more of the following:
    • focal neurological disease or clearly impaired level of consciousness
    • an abnormal computerised tomograph (CT) scan, magnetic resonance image (MRI)or electronencephalogram (EEG)
    • presence of pleocytosis in the CSF.
  • 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 seven–28 days but can range from five to 28 days.

Public health significance and occurrence of Murray Valley encephalitis virus

The last human cases recorded in Victoria occurred during an outbreak of MVE in 1974. However the virus was detected in sentinel chicken flocks located along the Murray River in 2008, and again in 2011.

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. The primary mosquito vector during epidemics is Culex annulirostris, which are fresh water breeders. 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 virus

There is no evidence of person-to-person transmission. Transmission requires a mosquito vector.

Susceptibility and resistance to Murray Valley encephalitis virus

Infection with MVE virus confers lifelong immunity.

Control measures for Murray Valley encephalitis virus

Preventive measures

There is no preventative vaccine available.

MVE infection can be prevented by:

  • personal protection measures, such as wearing long, loose-fitting, light-coloured clothing
  • using personal insect repellents containing diethyl toluamide (DEET) or picaridin
  • avoidance of  mosquito-prone areas  especially at dusk and dawn, which are peak vector biting times 
  • mosquito control measures

Control of case

No specific treatment is available for MVE virus disease and care of symptomatic cases is largely supportive.  Given the potential for neurological deterioration, cases with encephalitis should ideally be managed in hospitals with intensive care facilities and expertise in the management of complicated neurological disease.

  • The source of infection requires investigation. A detailed history should be completed with the case or next of kin.  Focusing on travel history, mosquito bit history and recreational or other activities involving exposure to bushland or other mosquito habitats in the month prior to symptom onset.  

Control of contacts

If others are unwell, it is advisable that they see their own doctor for testing.

Control of environment

To reduce or prevent virus transmission, it is necessary to interrupt human–mosquito contact by: 

  • avoiding mosquito-prone areas. 
  • preventing mosquitoes from entering the home or accommodation
  • suppressing the mosquito population, through removal of stagnant water, using knockdown sprays or long acting surface sprays if mosquitoes are particularly bad

Outbreak measures for Murray Valley encephalitis virus

In addition to prevention activities, outbreak measures for MVE virus include: 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

Contact details