Fans of photogenic koalas have something to cheer about, after years of bad news. The endangered marsupials are now eligible for a vaccine against a disease that has taken a major toll on them.
Australia’s veterinary medicine regulator, the Australian Pesticides and Veterinary Medicines Authority, has approved the use of a vaccine against chlamydia. The sexually transmitted bacterial infection is widespread in koalas, causing urinary problems, blindness, infertility and even death.
The new vaccine, “was the answer to reducing the rapid, devastating spread of this disease, which account for as much as half of koala deaths across wild populations,” said Peter Timms, a microbiologist at Australia’s University of the Sunshine Coast (USC) who led work to develop the vaccine.
The breakthrough comes none too soon. Koalas, with their teddy-bear-cute looks and contrastingly raucous roars, might be media darlings. But they are also imperiled by habitat loss to logging and farming, increasingly intense wildfires, attacks by wild and domestic dogs, and road traffic, as well as disease. In 2022, in the wake of devastating fires in eastern Australia, the Australian government decided their status had worsened enough that the animals should be listed as “endangered” in three of the five states where they are found.
A vaccine has been in development for more than a decade for a bacteria that is thought to have first reached koalas from the feces of livestock brought by early English colonists. The lengthy timeline underscores the challenges of developing a vaccine for wild animals. First the researchers had to develop a vaccine, one that in this case uses a protein from the bacteria’s membrane to trigger an immune response. Then they had to test it on wild koalas to see how well it worked.
Over the course of ten years, the researchers vaccinated 165 wild koalas, then compared their long-term health to unvaccinated animals. In a 2024 study, the scientists found that vaccinated koalas had a 64% reduction in disease-related deaths and 25% reduction in the likelihood that an infection could cause symptoms of the disease.
In addition, “the average age at which koalas are likely to develop chlamydial disease was pushed back more than three years, from five to over eight years old, crucially protecting them during their main reproductive years,” said Samuel Phillips, a USC microbiologist involved in the research.
Vaccinating the animals, which spend much of their lives perched high in eucalyptus trees, could be a long, arduous effort. Researchers told the Guardian they expect to have 500 doses ready by early next year, which will be used on animals already receiving medical care. There are an estimated 224,000 to 524,000 koalas in the wild.
Computer models showed that vaccinating 20% of the animals in a group could stabilize their population. But no one is saying this is a standalone solution to the koala’s plight. “When vaccination is combined with multiple koala conservation strategies, including addressing traffic, wild and domestic dogs and importantly, retaining and restoring habitat—local population declines can be reversed,” Phillips said.
Meanwhile, add another potential difficulty for some koalas: inbreeding triggered by a lack of genetic diversity. In separate research, scientists just reported finding concerning levels of inbreeding among a group of koalas that have long been regarded as a success story.
In the 1920s, between 18 and 24 koalas were transplanted to Kangaroo Island off Australia’s southern coast, near Adelaide. It was part of a campaign to rescue the species, which was being decimated by hunting, habitat loss and disease on the mainland. In the short term, it worked remarkably well. Today there are more than 48,000 koalas on the island, and they are free of chlamydia.
But that legacy comes with a price. When scientists compared the genomes of 74 Kangaroo Island koalas to two different groups from the mainland, they found strong evidence that the islanders were much more closely related. Their genomes contained more sections where the genes inherited from one parent were identical to the other, and those sections were also at least twice as long, the scientists reported in a new paper in Molecular Ecology. That increases the risk that a genetic condition might translate into a physical problem, because an individual is more likely to have matching pairs of recessive genes.
The scientists found some evidence of deformities that could have a link to these genetic problems, including deformed reproductive organs and jaws. With less genetic diversity, the population may also be less equipped to adapt to emerging problems, such as climate change, warned Katie Gates, a scientist at Flinders University in Adelaide who was involved in the research.
“Kangaroo Island koalas are a successful conservation story in terms of population size,” said Gates. “But their genetic health tells a different story.”
One possible solution: Bringing new immigrants from the mainland to introduce new genes to the mix, a strategy known as genetic rescue. They just need to make sure they don’t bring any diseases with them.
Gates, et. al. “Conservation Arks: Genomic Erosion and Inbreeding in an Abundant Island Population of Koalas.” Molecular Ecology. Sept. 13, 2025.
Phillips, et. al. “Immunisation of koalas against Chlamydia pecorum results in significant protection against chlamydial disease and mortality.” npj Vaccines. Aug. 6, 2024.
Images: ©University of the Sunshine Coast
