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Health authorities in the Northern Territory have issued warnings for residents and visitors to avoid mosquito bites after two people from Alice Springs died from Murray Valley encephalitis.

It’s a reminder that mosquitoes in many parts of Australia can be more than just a nuisance.

Without a vaccine for Murray Valley encephalitis, preventing disease relies on preventing mosquito bites.

What is Murray Valley encephalitis virus?

The virus takes its name from the Murray Valley in southeastern Australia where it was first identified in patients who died during an outbreak in 1951.

Infections are rare, but can be deadly. Most people infected don’t develop symptoms. For those that do, symptoms can range from fever and headache to encephalitis (inflammation of the brain) , with coma and paralysis. Around 40% of people who develop symptoms won’t fully recover and about 25% die.

The virus circulates between water birds and mosquitoes, most commonly Culex annulirostris, but a small number of other species may be involved too.

The virus is detected most years in northern Australia, most notably the Kimberley in Western Australia and the Top End of the Northern Territory.

But when environmental conditions are favourable, the virus can emerge in southern regions of Australia. These conditions are generally associated with increased rainfall and flooding generally brought on by La Niña influenced weather patterns.

The more water there is across the wetlands of the Murray Darling Basin and surrounds, the more likely we are to see high numbers of mosquitoes and waterbirds.

It is suspected that infected birds migrating from northern regions bring the virus south.

The most notable outbreak, in 1974, resulted in 58 cases and 12 deaths across Australia.

During outbreaks in 2011 and 2023, there were 17 and 26 cases respectively with many in southern regions.

When mosquito surveillance programs in northern Western Australia and the Northern Territory detect Murray Valley encephalitis virus, this triggers mosquito control measures and public health messaging. Communities across northern Australia are seasonally reminded of the risk associated with this virus and mosquito bites.

But many tourists, both local and international, also travel across the endemic regions and have been among the cases reported over recent years.

Murray Valley and Japanese encephalitis are different

Murray Valley and Japanese encephalitis are closely related but different viruses. They are likely spread by the same type of mosquitoes, cause similar disease, and infect similar animals. But there are key differences.

Japanese encephalitis is major health concern in Asia, Southeast Asia and the Western Pacific. After a fatal case in the Northern Territory in February 2021, an unprecedented outbreak of Japanese encephalitis occurred in southeastern Australia beginning early 2022.

By the time the epidemic ended in winter 2022, there had been more than 40 documented human cases, including seven deaths.

Japanese encephalitis virus is now likely to be endemic in northern Australia. Feral pigs are suspected as an important animal host but water birds are crucial too.

There is a safe and effective vaccine against Japanese encephalitis that is locally available. Health authorities are now working on how best to recommend its use against potentially annual activity in northern Australia and occasional outbreaks in southern regions.

But vaccines are not currently available for Murray Valley encephalitis. Scientists are investigating whether the Japanese encephalitis vaccine may provide cross-protection for Murray Valley encephalitis but there is more research to be done.

Unfortunately, for a disease so rarely reported a specific vaccine for Murray Valley encephalitis may never be available.

Is there a greater risk of Murray Valley encephalitis this year?

When there is more water, there are typically more mosquitoes.

Record-breaking rainfall and extensive flooding has occurred across northern Australia this year mainly due to cyclones. This rainfall has created ideal conditions for mosquito breeding and increased mosquito numbers.

This is likely to escalate the health risks. But the spread of mosquito-borne disease also depends the interaction of waterbirds, mosquitoes and people, which can be influenced by a range of factors.

There have been two confirmed fatal cases of Murray Valley Encephalitis in Alice Springs.

The virus has also been detected in mosquito surveillance programs in the Top End of the Northern Territory and northern Western Australia.

The historical period of Murray Valley encephalitis virus transmission risk is from February to July, peaking March to May.

The cold weather of winter will limit mosquito activity so a spread now to southern states is unlikely. The risk in coming spring and summer will be highly dependent on rainfall but predictions are for hot and dry conditions.

How to protect yourself from Murray Valley encephalitis

With no vaccine or specific treatment for the disease, reducing mosquito bites is the only way to protect yourself from the virus.

When outdoors, wear long-sleeved shirts, long pants and covered shoes when possible.

Avoid areas where mosquitoes are active, especially at times when they’re most active such as dawn and dusk.

When camping, ensure you’re using insect netting or mosquito nets.

Applying insect repellents to all exposed areas of skin will provide the best protection. Formulations containing products that effectively keep mosquitoes from biting, such as diethyltoluamide, picaridin and oil of lemon eucalyptus, are safe and affordable.

Cameron Webb and the Department of Medical Entomology, NSW Health Pathology and University of Sydney, have been engaged by a wide range of insect repellent and insecticide manufacturers to provide testing of products and provide expert advice on medically important arthropods, including mosquitoes. Cameron has also received funding from local, state and federal agencies to undertake research into various aspects of mosquito and mosquito-borne disease management.

Bart J. Currie does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.