Murray Valley encephalitis

Notification requirement for Murray Valley encephalitis

MVE virus infection is an ‘urgent’ notifiable condition and must be notified by medical practitioners and pathology services to the Department of Health immediately by calling 1300 651 160 (24 hours). 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 is caused by the Murray Valley encephalitis virus, which is a flavivirus.

Other flaviviruses known to cause similar clinical presentations include Japanese encephalitis virus and West Nile/Kunjin virus.

MVE virus is endemic to northern Australia and Papua New Guinea. The virus has been sporadically detected in Victoria, particularly around inland riverine regions and extending up towards the Murray River, and other south-eastern Australian states. The highest risk of infection is between November and March, particularly following significant flooding.

Clinical features of Murray Valley encephalitis

Most people infected with MVE virus do not have symptoms, with less than one percent developing clinical illness. About 1 in 800 infected people develop severe illness (encephalitis).

Illness usually starts with headache, nausea, vomiting and myalgia. People with severe infection may develop severe headache, neck stiffness, sensitivity to bright lights (photophobia), drowsiness, confusion, seizures, weakness or ataxia, loss of consciousness and coma.

For some people, encephalitis may lead to long-term neurological complications or death.

Diagnosis

Diagnosis of MVE virus infection is made by isolating or detecting the virus from a clinical sample or by a rising antibody titre (laboratory evidence) in conjunction with compatible clinical evidence.

Recommended laboratory testing for MVE virus infection includes:

• Blood (5 to 10 mL, in a serum tube) for MVE virus and flavivirus serology
  • Repeat convalescent serology testing at 3 to 4 weeks post onset of illness
• Blood (5 to 10mL, in a dedicated EDTA tube) for MVE virus PCR/culture
• CSF (1 to 3mL) for MVE virus and flavivirus serology and PCR/culture
• Urine (2 to 5mL in a sterile urine jar) for MVE virus PCR/culture

Collect acute and convalescent serology samples. Cross reaction of antibodies to other flaviviruses is possible.

Samples should be sent urgently to the Victorian Infectious Diseases Reference Laboratory (VIDRL) which performs testing for MVE virus and other flaviviruses in Victoria. Request forms should be appropriately labelled and the on-call pathologist at VIDRL should be contacted to provide information on samples being sent. Samples should be transported at 4 degrees Celsius.

Incubation period of Murray Valley encephalitis virus

The incubation period is usually 7 to 12 days but can range from 5 to 28 days.

Public health significance and occurrence of Murray Valley encephalitis virus

MVE virus has been detected in Victoria and other south-eastern Australian states in early 2023. The virus was previously detected in Victoria in sentinel chicken flocks located along the Murray River in 2011. The last human cases of MVE in Victoria were reported in 1974 as part of an outbreak. Human cases of MVE virus infection have been reported in New South Wales and South Australia in 2011.

Of those presenting with encephalitis in Victoria in the 1974 epidemic, approximately one-fifth died and one-third were left with residual neurological complications.

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 following widespread flooding, which led to large increases in waterbird and vector mosquito populations. The virus was amplified in the bird–mosquito–bird cycle, and humans became infected when bitten by mosquitoes carrying the virus.

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 transmitted to humans through 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. It is not transmitted to humans from contact with birds or other animals.

Susceptibility and resistance to Murray Valley encephalitis virus

Anyone is potentially at risk of mosquito bites and mosquito-borne diseases, such as MVE virus infection.

People who work, live or spend time outdoors in northern Victoria, particularly inland riverine regions and extending up towards the Murray River, are at increased risk of infection.

People who have been exposed to MVE virus are likely to have long-lasting immunity to subsequent infections.

Prevention and control measures for Murray Valley encephalitis virus

There is no preventative vaccine available for MVE.

A mosquito surveillance and control program is in place in Victoria to monitor the number and species of mosquitoes, presence of viruses in mosquitoes (including Murray Valley encephalitis virus) and to support local councils to manage the risk of mosquitoes in in their local areas.

The best prevention is to protect against mosquito bites:

• Cover up – wear long, loose-fitting, light-coloured clothing.
• Use mosquito repellents containing picaridin or DEET on all exposed skin.
• Limit outdoor activity if lots of mosquitoes are about.
• Remove stagnant water where mosquitoes can breed around your home or campsite.
• On holidays make sure your accommodation is fitted with mosquito netting or screens.
• Use ‘knockdown’ fly sprays and plug-in repellent devices indoors. Don’t forget the kids – always check the insect repellent label. On babies, you might need to spray or rub repellent on their clothes instead of their skin. Avoid applying repellent to the hands of babies or young children.