‘Silent transmission’: Warning as potentially deadly mosquito-borne diseases emerge in Victoria, NSW

Aussies have been warned to take extra care this summer after two mosquito-borne viruses were detected in Victoria and NSW in the past week.

Health experts issued a warning to residents in Victoria after Japanese encephalitis was detected in a mosquito trap in Horsham.

This is the first recorded report of the mosquito-borne virus for the 2025-2026 season.

“Anyone can be bitten by mosquitoes, but people living, working or spending time outdoors in northern Victoria – especially near rivers – may be at higher risk,” Victorian chief health officer Caroline McElnay said.

Since the first detection, the virus has been identified in NSW, with authorities confirming 60 local government areas were now considered higher risk of the potentially-deadly disease.

Another mosquito-borne virus, Kunjin, was detected in Cowra in Western NSW on November 20 during a routine health check through a sentinel chicken blood sample.

“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,” NSW Health director of health protection Stephen Conaty said.

What is Japanese encephalitis?

Japanese encephalitis virus (JEV) is a disease caused by a virus spread through mosquito bites.

Similar to other flaviviruses like dengue fever, it is not spread from human-to-human.

“The mosquitoes pick up the virus from infectious animals - mostly pigs, horses, and large wading birds,” UNSW Sydney epidemiologist Kishor Kumar Paul told NewsWire.

JEV occurs widely across southern and eastern Asia, but is also found across Australia.

Typically, JEV becomes prominent in the country’s southeast from mid-spring to late autumn, but it can be found year-round in northern Australia.

While infection is rare, the virus is capable of causing serious and potentially life-threatening illness.

About one in 250 cases can lead to a severe brain infection, which can be fatal.

Dr Paul said the disease was rare but there was “evidence of silent transmission”.

“About one in 10 persons in some high-risk cities in NSW had antibody against JEV in their blood, indicating historic infection,” he said.

What are the symptoms of JEV?

Due to the nature of the virus, most people who have been bitten by an infected mosquito don’t develop symptoms at all, but some may experience mild self-limiting symptoms.

It’s the rare cases where the real trouble can begin.

“In a small number of people, JEV infection progresses to severe brain inflammation, known as Japanese encephalitis,” Dr Paul said.

“Encephalitis patients can present with headache, high fever, vomiting, convulsion, sudden behaviour change, reduced consciousness level, weakness, or paralysis of limbs.”He said there was an increased risk of death if patients develop encephalitis.

“Encephalitis patients have high mortality rates,” he said.

“Some of the survived patients suffer from continuation of some degree of neurological disorder.”

What is Kujin virus?

Much like JEV, Kunkin virus — a subtype of the West Nile virus — is also spread by mosquito bites.

Kunjin is spread when a mosquito bites an infected waterbird or horse.

It is not spread from person-to-person, though is more likely to infect to those living in areas with rivers, wetlands, highly irrigated or flooded areas.

The mosquito is commonly found around the Murray Darling basin areas of NSW during summer and early autumn.

“Kunjin virus is regularly detected in sentinel chicken or in mosquitoes during summer, but human cases are rare,” Dr Paul told NewsWire.

What are the symptoms of Kunjin?

The symptoms of Kunjin virus are milder than JEV, with most cases undetected by the person infected.

“Most individuals infected with Kunjin virus do not develop any symptoms, like JEV, or mild self-limiting illness without realising they have been infected,” Dr Paul told NewsWire.

“If symptomatic, Kunjin infection is milder than JEV infection.”

Symptoms of Kunjin can include a fever, enlarged lymph nodes, a rash, swollen and aching joints, headache and fatigue.

“Progression to encephalitis is less common for Kunjin compared to JEV,” he said.

Is there a cure for the mosquito-borne viruses?

No, there is no treatment available to cure either JEV or Kunjin viruses.

However, Dr Paul explained there were several methods to prevent the spread of the Japanese encephalitis virus, including a free vaccine.

“Safe and effective vaccine is available to prevent JEV infection,” he said.

A booster dose may be recommended to those with an ongoing risk of exposure.

Unfortunately, there is no vaccine available for Kunjin or other mosquito-borne viruses in Australia.

Instead, residents have been urged to protect themselves with repellents and wearing loose-fitting and light-coloured clothing when outside.

Typically, mosquitoes are more active during dusk and dawn, and can breed in areas with trapped water, such as old tyres and empty pots around the home.

Residents have been told to regularly check areas for still water where mosquitoes could breed.

NSW Health recommends people use repellents that include EET, picaridin, or oil of lemon eucalyptus, which should be reapplied regularly, particularly after swimming.