Murray Valley encephalitis

Murray Valley encephalitis is a potentially serious infection closely related to Japanese encephalitis. It is a nationally notifiable disease.

There have been recent cases of Murray Valley encephalitis infections in mainland Australia, some of these infections have resulted in serious illness and death. 

Murray Valley encephalitis is spread through the bite of infected mosquitoes. Water birds are the primary host for the virus. Person-to-person transmission does not occur. 

What is Murray Valley encephalitis?

• Murray Valley encephalitis is a potentially serious infection closely related to Japanese encephalitis. 
• Murray Valley encephalitis is a nationally notifiable disease.
• The mosquito primarily involved in transmitting Murray Valley encephalitis Virus is C. annulirostris. This mosquito is not currently found in Tasmania.
• There have been recent cases of Murray Valley encephalitis infections in mainland Australia, some of these infections have resulted in serious illness and death. 
• Murray Valley encephalitis is spread through the bite of infected mosquitoes. Water birds are the primary host for the virus. Person-to-person transmission does not occur. 
• The incubation period is usually seven to 12 days but can be between five to 28 days. It is thought that once a person has been infected, they acquire long lasting immunity. 

Symptoms

• People infected with Murray Valley encephalitis often don’t develop any symptoms or they develop mild symptoms such as fever, headache, nausea and vomiting. 
• Rarely, meningitis or encephalitis (brain inflammation) can occur. 
• People with severe infection may develop drowsiness, severe headaches, neck stiffness, photophobia, muscle weakness, neurological symptoms, seizures, or loss of consciousness. 
• In severe cases, infection may result in permanent disability or death.

Who is at risk?

• Everyone is at risk of mosquito bites and mosquito borne diseases. Everyone should take steps to protect themselves.
• People who have more frequent exposure to infected mosquitos are at increased risk. This includes people participating in outdoor activities such as camping and water sports. 
• Outcomes may be worse in the very young and people over 50 years old. It is also thought that immunosuppressed individuals have increased susceptibility to disease. 
• Find out more about what you can do to protect yourself and reduce the risk.

How is it diagnosed?

• Clinical history, examination, and time spent in an area where Murray Valley Encephalitis virus has been detected may lead a doctor to suspect the infection.
• The diagnosis can be confirmed by laboratory testing of blood or cerebrospinal fluid (the fluid surrounding the brain and spinal cord) collected through a lumbar puncture procedure.
• Repeat testing may be required to confirm the diagnosis. 
• Other illnesses with similar symptoms may need to be ruled out.

How is it treated?

• It is important to seek medical attention if you think you have Murray Valley encephalitis virus. 
• There is no specific treatment available for Murray Valley encephalitis. 
• For people who experience symptoms, treatment aims to ease the discomfort of milder symptoms.

How is it spread?

• Murray Valley encephalitis virus is spread through the bite of an infected mosquito. 
• The main mosquito that spreads the virus is Culex annulirostris, however other mosquito species including other Culex and some Aedes species may also be involved. 
• There is no evidence of person-to-person transmission. 
• Water birds are the primary reservoir for the virus. 

Where is it found? 

• Murray Valley encephalitis virus is endemic to northern Australia and Papua New Guinea, with single cases or small outbreaks every few years.
• Less frequent outbreaks also occur in the south-easterly states (excluding Tasmania). In early 2023, the virus was detected in Victoria and south-eastern Australia for the first time since 1974.
• Environmental conditions increase the circulation of the virus in some areas, for example when there has been higher than average rainfall and particularly following floods.
• The mosquito primarily involved in transmitting Murray Valley encephalitis Virus is C. annulirostris. This mosquito is not currently found in Tasmania.

Prevention

• Avoiding mosquito bites and taking care during travel are important to avoid getting Murray Valley encephalitis. 
• There is no vaccine available for the Murray Valley encephalitis.

Where to get help

• In an emergency call ‘000’ (triple zero) for an ambulance
• You can also present to the emergency department of your nearest hospital or speak to your General Practitioner.
• For other queries, contact the Communicable Diseases Prevention Unit on 1800 671 738 (option 3 to speak to a Communicable Diseases Clinical Nurse Consultant).

Find out more about Mosquito-borne diseases