Japanese encephalitis has claimed a second life in NSW and been detected in Brisbane. What is it?

Source: The Conversation (Au and NZ) – By Cameron Webb, Clinical Associate Professor and Principal Hospital Scientist, University of Sydney

A second man has died from Japanese encephalitis virus in New South Wales on March 6, the state’s health authorities confirmed on Friday. Aged in his 70s, the man was infected while holidaying in the Murrumbidgee region.

This follows the death of another man in his 70s in Sydney last month, after holidaying in the same region in January.

Japanese encephalitis virus has also been detected for the first time in mosquitoes collected in Brisbane’s eastern suburbs, Queensland health authorities confirmed on Saturday.

With mosquito activity expected to increase thanks to flooding rains brought by Ex-Tropical Cyclone Alfred, it’s important to protect yourself from mosquito bites.

What is Japanese encephalitis virus?

Japanese encephalitis is one of the most serious diseases that spreads via mosquitoes, with around 68,000 cases annually across Southeast Asia and Western Pacific regions.

The virus is thought to be maintained in a cycle between mosquitoes and waterbirds. Mosquitoes are infected when they feed from an infected waterbird. They then pass the virus to other waterbirds. Sometimes other animals, and people, can be infected.

Pigs are also a host, and the virus has spread through commercial piggeries in Victoria, NSW and Queensland. (But it poses no food safety risk.)

Feral pigs and other animals can also play a role in transmission cycles.

What are the symptoms?

Most people infected show no symptoms.

People with mild cases may have a fever, headache and vomiting.

In more serious cases – about one in 250 people infected – people may have neck stiffness, disorientation, drowsiness and seizures. Serious illness can have life-long neurological complications and, in some cases, the infection can be life-threatening.

There’s no specific treatment for the disease.

When did Japanese encephalitis get to Australia and why is it in Brisbane?

Outbreaks of Japanese encephalitis had occurred in the Torres Strait during the 1990s. The virus was also detected in the Cape York Peninsula in 1998.

There had been no evidence of activity on the mainland since 2004 but everything changed in the summer of 2021–22. Japanese encephalitis virus was detected in commercial piggeries in southeastern Australia during that summer.

This prompted the declaration of a Communicable Disease Incident of National Significance. At the time, flooding accompanying the La Niña-dominated weather patterns and a resulting boom in mosquito numbers, and waterbird populations, was thought responsible.

The virus has spread in subsequent years and has been detected in the mosquito and arbovirus surveillance programs as well as detection in feral pigs and commercial piggeries in most states and territories. Only Tasmania has remained free of Japanese encephalitis virus.

Human cases of infection have also been reported. There were more than 50 cases of disease and seven deaths in 2022.

Cases of Japanese encephalitis have already been reported from Queensland in 2025.

Due to concern about Japanese encephalitis virus and other mosquito-borne pathogens, health authorities around Australia have expanded and enhanced their surveillance programs.

In Queensland, this includes mosquito monitoring at a number of locations, including urban areas of southeast Queensland. Mosquitoes collected in this monitoring program tested positive for Japanese encephalitis virus, promoting the current health warnings.

Why is its detection in Brisbane important?

Up to now, scientists have thought the risk of Japanese encephalitis was likely greatest following seasons of above-average rainfall or flooding. This provides ideal conditions for waterbirds and mosquitoes.

But the activity of Japanese encephalitis virus over the summer of 2024–25 has taken many scientists by surprise. Before Ex-Tropical Cyclone Alfred arrived, there had been somewhat dry conditions with less waterbird activity and low mosquito numbers in many regions of eastern Australia.

However there has still been widespread Japanese encephalitis virus activity in Victoria, NSW and Queensland.

To date, Japanese encephalitis virus activity hasn’t extended to the coastal regions of southeast Queensland. The detection of the virus in suburban Brisbane may require authorities to rethink exactly where the virus may turn up next. Authorities are ramping up their surveillance to see just how widespread the virus is in the region.

Health authorities and scientists are also trying to understand how the virus moved from western areas of the state to the coast and what drives virus transmission in different regions.

There is currently no evidence the virus is active in coastal regions of northern NSW.

What can people do to protect themselves?

Avoiding mosquito bites is the best way to reduce the risk of Japanese encephalitis virus.

Cover up with long-sleeved shirts and long pants for a physical barrier against mosquito bites.

Use topical insect repellents containing DEET, picaridin, or oil of lemon eucalyptus. Be sure to apply an even coat on all exposed areas of skin for the longest-lasting protection.

Ensure any insect screens on houses, tents and caravans are in good repair and reduce the amount of standing water in the backyard. The more water there is around your home, the more opportunities for mosquitoes there are.

A safe and effective vaccine is available against Japanese encephalitis. Each state and territory health authority (for example Queensland, NSW, Victoria) have specific recommendations about access to vaccinations.

It may take many weeks following vaccination to achieve sufficient protection, so prioritise reducing your exposure to bites in the meantime.

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 management of various medically important arthropods.

Andrew van den Hurk has received funding from local, state and federal agencies to study the ecology of mosquito-borne pathogens, and their surveillance and control. He is an employee of the Department of Health, Queensland government.

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