Page content: Overview | Victorian statutory requirement | Infectious agent | Identification | Incubation period | Public health significance & occurrence | Reservoir | Mode of transmission | Period of communicability | Susceptibility & resistance | Control measures | Outbreak measures | Additional sources of information

Overview

Arboviruses are viruses which are spread by the bite of arthropods, particularly mosquitoes. They are divided into alphaviruses and flaviviruses.

Victorian statutory requirement

Murray Valley encephalitis (Group A disease) must be notified immediately by telephone or fax followed by written notification within five days.

School exclusion is not required.

Infectious agent

Murray Valley encephalitis virus is a flavivirus. It has the capacity to cause severe human disease, with encephalitis being the most notable clinical feature.

Murray Valley encephalitis virus (MVEV) 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.

Identification

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 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.

Method of diagnosis
Infection is confirmed by a significant rise in antibody titre to the virus in two blood specimens taken seven to ten days apart. Sera for diagnosis should be sent to the Director of Virology, Victorian Infectious Diseases Reference Laboratory (VIDRL), preceded by telephone contact via the Royal Melbourne Hospital on (03) 9342 7000 advising the on-call Virologist that sera has been sent for urgent testing.

A diagnosis of MVEV encephalitis 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 Papua New Guinea.

Laboratory evidence requires one of the following:

• isolation of MVEV from clinical material
• detection of MVEV RNA in clinical material
• IgG seroconversion or a significant increase in antibody level or a fourfold rise in titre of MVEV specific IgG proven by neutralisation or another specific test
• MVEV-specific IgM detected in the CSF in the absence of IgM to Kunjin, Japanese encephalitis or dengue viruses
• MVEV-specific IgM detected in serum in the absence of IgM to Kunjin, Japanese encephalitis or dengue viruses. This is only accepted as laboratory evidence for encephalitic illnesses.

Confirmation of the laboratory result by a second arbovirus reference laboratory is required if the case occurs in areas of Australia not known to have established enzootic, endemic 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:

• focal neurological disease or clearly impaired level of consciousness
• an abnormal CT, MRI scan or EEG
• presence of pleocytosis in the CSF.

Incubation period

The incubation period is usually 7–28 days.

Public health significance & occurrence

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 virus encephalitis occur every few years. This is usually at the end of the wet season. Seven outbreaks of MVE virus encephalitis have occurred at irregular intervals in southeastern Australia since 1917. The last of these was in 1974. During these times there was heavy rainfall leading to widespread flooding which promoted large increases in water bird 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 virus encephalitis 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 virus encephalitis in southeastern 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 down the southeastern parts of the continent. This occurs in repeated mosquito-bird-mosquito amplification cycles. The other suggests that the virus persists during inter-epidemic periods in cryptic foci along the Murray River and the MVE virus only amplifies and becomes evident when weather conditions are conducive to massive local mosquito and bird multiplication.

Reservoir

The primary hosts in Victoria of MVE virus during years of high virus activity are water birds. Ardeiformes (herons), particularly the Rufous night-heron and the Pelicaniformes (cormorants/ darters) are the most commonly infected.

Mode of transmission

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

There is no evidence of person to person transmission.

Susceptibility & resistance

Infection with MVE virus confers lifelong immunity.

Control measures

Preventive measures
Patients can be managed at any hospital, but facilities for providing intensive care and artificial respiration must be available. There is no preventative vaccine available.

Control of case
Investigate the source of infection. Search for unreported or undiagnosed cases of encephalitis from the Murray-Darling drainage basin.

The patient with suspected infection or friend or relative, should be asked to recall if in the month prior to onset of symptoms he or she had:

• been 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 and picnicking.

Control of contacts
Not applicable.

Control of environment
To reduce or prevent virus transmission, interruption of human-mosquito contact is required by:

• suppression of the vector mosquito population
• avoidance of vector contact at biting times at dusk and dawn
• applying mosquito control measures in local municipalities
• using personal protection measures such as long sleeves, long trousers and mosquito repellents
• avoiding mosquito-prone areas.

Outbreak measures

Following 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 of Human Services
• the presence of MVE virus in the area will be notified to relevant regional offices and local health council 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.

Additional sources of information

Victorian Department of Human Services, Victorian Arbovirus Task Force contingency plan for outbreaks of MVE.