Australia's Summer Season Increases Brain Swelling Risk

With the arrival of Australia's summer season, New South Wales (NSW) Health recently urged people to protect themselves from mosquito bites after earlier-than-expected detections of mosquito-borne viruses that can cause brain swelling.

While one mosquito-borne disease is vaccine-preventable, the other is not.

According to the NSW press release on December 4, 2025, the first detection of Japanese Encephalitis virus (JEV) in mosquitoes for Australia's 2025-2026 summer season was recently detected from the Horsham Rural City Council region in Victoria, located northwest of the city of Melbourne.

Currently, 60 Local Government Areas in NSW are at risk of the mosquito-transmitted JEV.

Another mosquito-borne virus, Kunjin virus, has also been detected in Western NSW at Cowra, through a sentinel chicken blood sample as part of NSW Health's routine surveillance.

NSW Health's Director of Health Protection, Dr. Stephen Conaty, commented in a press release, saying these detections so early in the summer season are concerning. 

"Mosquito numbers will likely increase with warmer weather, and we remind everyone in NSW to protect themselves against mosquito bites, which can cause diseases such as Japanese Encephalitis and Murray Valley Encephalitis, and infection with Kunjin virus, Ross River virus, and Barmah Forest virus," Dr. Conaty added.

"There is a safe, effective, and free vaccine (JEspect / Ixiaro) to protect against Japanese Encephalitis (JE)."

"This vaccine is free to anyone who lives or routinely works in these 60 LGAs west of the dividing range and in northern NSW, and for people who work in some other high-risk occupations."

JE vaccine is available in Australia through General Practitioners, Aboriginal health services, and pharmacists. These providers may require a few days' notice to order the vaccine and may charge a consultation fee.

Additionally, a booster dose may be recommended for some people with an ongoing risk of exposure, one to two years after completing their primary course.

Since millions of international travelers visit NSW, the U.S. CDC recommends the JEV vaccination to mitigate health risks when in Australia's Murray River and the Outer Torres Strait Islands in 2025.

Currently, there is no vaccination or specific treatment for Kunjin virus in Australia. The best way to avoid infection is to avoid being bitten by mosquitoes, which are more active between dusk and dawn, says NSW Health.