‘Silent transmission’: Warning as potentially deadly mosquito-borne diseases emerge in Victoria, NSW
Australians have been warned to be extra careful this summer after two mosquito-borne viruses were detected in Victoria and NSW last week.
Health experts have issued a warning to Victoria residents after Japanese encephalitis was detected in a mosquito trap in Horsham.
This is the first recorded report of mosquito-borne virus for the 2025-2026 season.
“Anyone can be bitten by mosquitoes, but people who live, work or spend time outdoors in northern Victoria, particularly near rivers, may be at higher risk,” Victoria’s chief health officer Caroline McElnay said.
Since initial detection, the virus has been identified in NSW, with authorities confirming 60 local government areas are considered to be at higher risk of the potentially fatal disease.
Another mosquito-borne virus, Kunjin, was detected during a routine health check using a sentinel chicken blood sample at Cowra in western NSW on 20 November.
“Mosquito numbers are likely to increase as the weather warms and we remind everyone in NSW to protect themselves against mosquito bites that can cause diseases such as Japanese encephalitis and Murray Valley encephalitis, as well as infection with Kunjin virus, Ross River virus and Barmah Forest virus,” said Stephen Conaty, health protection manager at NSW Health.
What is Japanese encephalitis?
Japanese encephalitis virus (JEV) is a disease caused by a virus spread through mosquito bites.
It does not spread from person to person, unlike other flaviviruses such as dengue fever.
“Mosquitoes pick up the virus from infectious animals, mostly pigs, horses and large walking birds,” UNSW Sydney epidemiologist Kishor Kumar Paul told NewsWire.
JEV is common in southern and eastern Asia, but is also found in Australia.
Typically JEV is prominent in mid-spring to late autumn in the south-eastern part of the country, but can be found year-round in northern Australia.
Although infection is rare, the virus can cause serious and potentially life-threatening illness.
About one in 250 cases can lead to a serious brain infection that can be fatal.
Dr Paul said the disease was rare but there was “evidence of silent transmission”.
“About one in 10 people in some high-risk cities in NSW had antibodies to JEV in their blood, indicating historic infection,” he said.
What are the symptoms of JEV?
Due to the nature of the virus, most people bitten by an infected mosquito develop no symptoms, although some may experience mild, self-limiting symptoms.
It is in rare cases that real problems can begin.
“In a small number of people, JEV infection progresses to severe brain inflammation known as Japanese encephalitis,” Dr Paul said.
“Encephalitis patients may present with complaints of headache, high fever, vomiting, convulsions, sudden behavioral changes, decreased level of consciousness, weakness or paralysis in the limbs.” He said that the risk of death increases if patients develop encephalitis.
“Morality rates are high in encephalitis patients,” he said.
“A proportion of surviving patients suffer from some degree of continued neurological impairment.”
What is Kujin virus?
Much like JEV, Kunkin virus, a subtype of West Nile virus, is spread by mosquito bites.
Kunjin is spread when a mosquito bites an infected waterfowl or horse.
It does not spread from person to person, but is more likely to infect those who live in areas with rivers, wetlands, overwatered or flooded areas.
The mosquito is commonly found in the Murray Darling basin areas of NSW during the summer and early autumn.
“Kunjin virus is regularly detected in sentinel chickens or mosquitoes in summer, but cases in humans are rare,” Dr Paul told NewsWire.
What are the symptoms of Kunjin?
Symptoms of Kunjin virus are milder than JEV, and most cases go undetected by the infected person.
“Most people infected with Kunjin virus do not develop any symptoms such as JEV or self-limiting mild disease without realizing they are infected,” Dr Paul told NewsWire.
“If symptomatic, Kunjin infection is milder than JEV infection.”
Symptoms of Kunjin include fever, enlarged lymph nodes, rash, swollen and aching joints, headache, and fatigue.
“Progression to encephalitis is less common in Kunjin compared to JEV,” he said.
Is there a cure for mosquito-borne viruses?
No, there is no treatment available to cure JEV or Kunjin viruses.
However, Dr Paul explained that there are various methods available to prevent the spread of Japanese encephalitis virus, including a free vaccine.
“A safe and effective vaccine is available to prevent JEV infection,” he said.
A booster dose may be recommended for individuals who remain at risk of exposure.
Unfortunately, there is no vaccine against Kunjin or other mosquito-borne viruses available in Australia.
Instead, residents were asked to protect themselves with repellents and wear loose-fitting, light-colored clothing outside.
Typically, mosquitoes are more active at dusk and dawn and can breed in areas around the home where water is trapped, such as old tires and empty flower pots.
Residents are told to regularly check areas of standing water where mosquitoes may breed.
NSW Health recommends people use repellents containing EET, picaridin or lemon eucalyptus oil, which should be reapplied regularly, especially after swimming.
