Australia’s bird flu battle is entering a critical phase, with scientists monitoring not only where H5N1 is spreading, but also whether the virus could change after encountering influenza viruses already circulating among Australian birds.

H5N1 is an RNA virus made up of eight genetic segments. When different influenza viruses infect the same host, these segments can sometimes be exchanged through a process called reassortment. Scientists say this could potentially create a virus better adapted to certain bird species although there is currently no evidence that a concerning reassortment has occurred in Australia.

CSIRO researchers are using whole-genome sequencing to track these changes. So far, sequencing suggests H5N1 has entered mainland Australia at least six or seven separate times, with the detected introductions believed to have originated from Heard Island, around 4,100 kilometres south-west of Perth. Researchers caution that the true number could be higher because undetected introductions would not appear in the data.

More concerning is evidence that the virus is now spreading between some Australian birds. Highly similar viral genomes found in crested terns and silver gulls in South Australia and Victoria indicate local transmission rather than entirely separate introductions.

The threat is particularly serious for species already struggling with small populations, habitat pressures or other environmental challenges. Scientists warn that an outbreak could have an “extinction-level impact” on some vulnerable species.

Vaccination presents another challenge. CSIRO trials have involved native birds, but current injectable vaccines require two doses, making large-scale vaccination of wild populations impractical. Instead, vaccination may need to be targeted towards specific threatened species that can be captured and treated.

As of August 11, Australia had recorded 236 confirmed detections of H5 bird flu in wild birds, including 163 in South Australia and 58 in Victoria.

For scientists, the race is now not simply to stop a virus but to understand how it might change before Australia’s most vulnerable wildlife pays the price.