QIMR Berghofer Medical Research Institute-led research has found more than 80 per cent of koalas in the Moreton Bay region, in southeast Queensland, have been infected with Ross River virus in their lifetime.
The researchers say the findings indicate that humans living in these areas could also be at high risk of contracting the mosquito-borne virus.
The study was conducted in collaboration with veterinary and ecological consultancy Endeavour Veterinary Ecology at Toorbul, which specialises in koala conservation and monitoring.
The lead researcher from QIMR Berghofer's Mosquito Control Laboratory, Dr Brian Johnson, said more than 220 koalas were tracked for three years around Kippa Ring and Murrumba Downs, north of Brisbane.
"Our study is the largest survey of marsupial serum conducted in relation to Ross River virus. Numerous marsupials and bird species are known to be reservoirs for the virus, which can be passed on to humans via the bite of an infected mosquito. Our survey helps us understand the risk of Ross River virus to koalas - particularly among animals living in degraded coastal systems where there are large freshwater mosquito populations," Dr Johnson said.
"We found levels of Ross River virus exposure in koalas in this region that were far higher than those reported in other surveys of koala populations. Unsurprisingly, the older the animal, the more likely it was to have been exposed.
"Their increased risk of Ross River virus appears directly related to their confinement to the edges of permanent wetlands where there are large numbers of Culex annulirostris, a major mosquito carrier of Ross River virus."
Study partner Endeavour Veterinary Ecology collared, tracked and monitored the two koala populations and routinely collected blood samples.
The QIMR Berghofer team then analysed the blood samples for the presence of Ross River virus antibodies.
Veterinarian Dr Amy Robbins, from Endeavour Veterinary Ecology, said koala colonies were being pushed into coastal wetland areas because their historical, natural habitats in eucalypt forests were being developed.
"Koala numbers in many areas of Australia have declined sharply in response to habitat loss, disease and the effects of climate change, and our study reveals they are also exposed to mosquito-borne diseases which may exacerbate these effects," Dr Robbins said.
Dr Johnson said residential developments in southeast Queensland are continuing to encroach on these coastal wetland areas and the study shows that humans moving into these areas are also likely to be exposed to Ross River virus.
"Our study findings demonstrate that the careful study and sampling of wildlife populations can provide valuable insights to animal conservation and public health in rapidly changing urban environments," Dr Johnson said.
The researchers hope to build on the study by expanding the areas surveyed across state borders and to use a range of marsupials as sentinels. They say that will help them better understand the environmental and ecological risks of virus exposure to wildlife across region and further predict exposure risks to humans.
While several mosquito species captured in the monitoring areas are known to transmit Ross River virus, Culex annulirostris is believed to be the most likely disease spreader among koalas because it tends to seek hosts above the ground.
Ross River virus is Australia's most medically important mosquito-borne disease with approximately 5000 cases reported annually. According to Queensland Health data, more than 1500 cases are reported annual within the state, with the majority occurring within the southeast. Cases often peak between February and May each year.
The research was significantly supported by the Queensland Government (Department of Transport and Main Roads), as part of the Moreton Bay Rail project.
The research findings are published in the Nature journal Scientific Reports.
