There could be more Japanese Encephalitis Virus (JEV) infections in the Victorian community than previously thought, according to a survey conducted by the state's Department of Health.
More than 800 people in northern Victoria participated in the survey, which included a questionnaire and blood work.
Results show evidence of previous infection in about one-in-30 participants, suggesting there may be more people in the community who are asymptomatic with the virus.
"What we've found is about 3 per cent of people have a blood test that shows that they've been infected with JEV," Deputy Chief Health Officer Associate Professor Deborah Friedman said.
"They haven't been ill, which is very fortunate, but it indicates that the spread of infection is fairly reasonable over a reasonably short period of time."
Dr Friedman said she was confident those people acquired the virus locally within the last two years, given all were Australian-born and had never been to a country where JEV was endemic.
Thirteen cases reported last season
During the last mosquito breeding season, which typically runs from November to April for inland areas, 13 people were reported as being infected.
The survey was conducted to help health authorities determine how widespread JEV had been across Victoria.
"We presumed that [infections] would be in the areas that were close to the Murray River because they tend to have a worse problem with mosquito populations between October to March," Dr Friedman said.
"But we didn't actually know, apart from those 13 cases of people who were unwell, how many other people developed infection but had no symptoms.
"Those 13 people that we know about might only represent 1 per cent of the people infected."
Only one in about 100 people will develop symptoms from JEV, which include fever, headache, vomiting, neurological symptoms, weakness and movement disorders, but in some case it can result in death.
Vaccine eligibility expanded
The current mosquito season in Victoria has only recorded one JEV case among humans so far.
"We are aware that there are some patients in hospital that are being tested currently for JEV, but it's not something that can be diagnosed on a single test," Dr Friedman said.
"It typically takes a multitude of tests, and the interpretation of these tests is fairly complicated."
She said despite extensive surveillance this season, JEV had not been detected among mosquitoes in any local government area.
"We know it's there; we just haven't detected it," Dr Friedman said.
Vaccine eligibility expanded
The eligibility criteria for vaccinations against JEV has been expanded to include seven new local government areas.
People in Greater Bendigo, Northern Grampians, Hindmarsh, Horsham, Buloke, Yarriambiack and West Wimmera are now eligible and vaccination remains free for those most at risk.
The Victorian government has about 20,000 doses available, which will increase in coming months.
There have also been detections of other mosquito-borne viruses, including Murray Valley encephalitis.
Some viruses cannot be vaccinated against and health officials say the best protection is to avoid mosquito bites.
"Wear long, loose-fitting, light-coloured clothing, use insect repellents, clear stagnant water from around homes or properties, and avoid the outdoors when mosquitoes are observed, especially at dusk and dawn," Dr Friedman said.
