Murray Valley encephalitis

Notification requirement for Murray Valley encephalitis

MVE is an ‘urgent’ notifiable condition and must be notified by medical practitioners and pathology services immediately to the Department of Health upon initial diagnosis (suspected or confirmed) 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

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 is rarely detected in Victoria and other south-eastern Australian states. Certain conducive environmental conditions, such as multiple years of above average rainfall, increase the likelihood of virus circulation in inland riverine regions and around the Murray River. The highest risk of infection is between November and April, particularly following significant flooding.

Identification of Murray Valley encephalitis

Clinical features

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 with encephalitis or meningoencephalitis.

Illness usually starts with headache, fever, nausea, vomiting and myalgia. People with severe infection may develop drowsiness, confusion or meningism with severe headache, neck stiffness and photophobia. There may be cranial nerve pathology, muscle weakness or paresis, movement disorders such as ataxia or Parkinsonism, seizures, loss of consciousness and coma.

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

Clinical illness from MVE closely resembles other flavivirus infections from Japanese encephalitis and West Nile/Kunjin viruses and these should be considered in the differential diagnosis.

Diagnosis

Diagnosis of MVE 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.

Bilateral thalamic involvement on CT or MRI Brain is classical. Other areas that may be involved include the basal ganglia and brainstem.

The usual investigations for common causes of encephalitis or meningoencephalitis should be conducted concurrently, including CSF sampling if safe and appropriate. Where CSF is obtained, it should be tested for Herpes Simplex Virus (HSV), varicella-zoster virus (VZV), enteroviruses and other common causes of meningoencephalitis by multiplex PCR and culture. Flavivirus testing should be considered in the appropriate clinical context. It is especially important to exclude bacterial meningitis and HSV as they are treatable conditions.

Recommended laboratory testing for MVE includes all of the following:

Collect acute and convalescent (2 to 4 weeks post symptom onset) serology samples. This is essential as antibodies may not be detectable at symptom onset.

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 include relevant clinical and epidemiological history including symptom onset, vaccination, travel history and country of birth, to guide laboratory interpretation.

The on-call lab manager 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

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

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 2023 as part of an outbreak. Human cases of MVE were 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. Eight outbreaks of MVE have occurred at irregular intervals in south-eastern Australia since 1917. The most recent was in 2023 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

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.

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

Treatment of MVE is supportive. Suspected cases should be discussed with the local Infectious Disease service.

There is no preventative vaccine available for MVE.

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 (diethyltoulamide) 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.

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 MVE virus) and to support local councils to manage the risk of mosquitoes in their local areas.

Serosurvey for Murray valley encephalitis virus in northern Victoria

The Victorian Department of Health conducted a study between October 2023 and April 2024 across 11 local government areas to determine how widespread MVEV infection has been in northern Victoria. This information will inform public health planning and prevention strategies in future mosquito seasons. More information can be found at Serosurvey for Murray valley encephalitis virus in northern Victoria.