Vaccinations for Japanese encephalitis virus (JEV) are ramping up in communities on both sides of the NSW-Victoria border, after a new study found about one in 11 people surveyed had been infected.
The NSW Health study collected blood samples from volunteers in Corowa, Temora, Griffith, Dubbo and Balranald.
Out of 917 people who were tested, 80 people showed antibodies indicating past infection with JEV that would not have been acquired in another country.
"We knew there'd be more cases we didn't know about, but this work was really important in showing just the extent of it and how spread it is across the region as well," April Roberts-Witteveen, manager of infectious disease surveillance at the Murrumbidgee and Southern NSW Public Health Unit, said.
"The results show that Japanese encephalitis is well and truly in our mosquito populations locally."
More people at risk
JEV is a virus spread to humans by infected mosquitoes, and for less than one per cent of people it can cause a potentially fatal inflammation of the brain.
Locally acquired cases of JEV had never previously been identified in NSW or Victoria, but between mid-January and the end of February this year, 23 people were clinically diagnosed in NSW and Victoria.
Three people died.
As so many infections are asymptomatic, it can be difficult to fully understand how far the virus has travelled, and who has been impacted.
Ms Roberts-Witteveen said that was why surveys like this one were key.
"For every one person we know about, there's 100 to 250 other people who have been infected but have mild or no symptoms," she said.
"This study helped us define who is most at risk."
Eligibility for the JEV vaccine is heavily dictated by where people live and work, primarily tracing a line along the Murray River and surrounding waterways.
Along with people who work with pigs and mosquitoes, those who are over 50, spend more than four hours per day outside and live in a relevant local government area are now recommended to get the jab.
North of the border, those areas are Albury, Berrigan, Carrathool, Edward River, Federation, Greater Hume, Griffith, Lockhart, Murray River and Temora.
South of the Murray, they are Campaspe, Gannawarra, Greater Shepparton, Indigo, Loddon, Mildura, Moira, Swan Hill, Wodonga and Towong.
"We're just really calling on everyone who meets those eligibility criteria to make an appointment with their GP and get that vaccination," Ms Roberts-Witteveen said.
"We don't want to see people really unwell this season and vaccine is a really effective way to prevent serious illness from JEV."
Victoria follows suit
In Victoria, a similar survey is underway, but the government decided to expand eligibility early.
Victoria's Deputy Chief Health Officer, Associate Professor Deborah Friedman, said the government wanted to protect at-risk people ahead of the warmer months.
"We have more vaccine available which means we can think in a much broader scope than what we were able to before," she said.
"We know through looking at data in Victoria, and also through looking at the data from NSW, that people aged 50 years and older were at much higher risk of getting infected with Japanese encephalitis during the last mosquito season."
Associate Professor Friedman said while there were a number of factors that determined the life cycle of JEV, a wet spring and summer provided the ideal conditions for mosquitoes.
"It's likely that we'll have the same conditions that we had earlier this year and therefore there's no reason why we wouldn't necessarily see cases of Japanese encephalitis infection," she said.
But she said health authorities were in a better position this year, due to the substantial increase in surveillance data.
As well as these population surveys, mosquito tracking information is collected every fortnight, increasing to every week in the coming months.
She said the department was watching the situation closely and working with its counterparts in NSW.
"When and if we need to make additional changes to vaccination eligibility, we'll certainly do that, and we also are always aligning with NSW for all of our border communities," she said.
"We have to make sure that we don't have any discrepancy in eligibility of people who might live only a few kilometres apart on different sides of the border."
Your blood sample is needed
Meanwhile, Victoria's Department of Health is conducting its own serosurvey for JEV, to better understand the spread in the northern parts of the state.
Collaborating with Ovens and Murray, Goulburn Valley and Loddon Mallee public health units, the department was initially asking people already attending pathology to add another vial of blood for the study.
But now it wants to increase the sample size, encouraging as many people as possible to go to their local pathology centre and take part.
Jody Bellette from the Ovens and Murray public health unit said it was particularly focusing on younger people who may not routinely go in for blood tests.
"We'd like to see people that are active on the river; people that are hunting and fishing; are out at dawn and dusk," Ms Bellette said.
"So anyone across any age group, just to broaden the catch of data that we'll be able to then use to determine where and who is getting the vaccine. "
