A t wildlife hospitals across eastern Australia, many koalas brought in have distinctive brown patches on their bottoms – a sign of urinary incontinence and a painful inflamed bladder. Others have red, weeping eyes, pointing to a bacterial disease that can cause blindness in the endangered marsupials.
Chlamydia infection, also known as chlamydiosis, is the most significant disease affecting koalas. The culprit is most commonly the bacterium Chlamydia pecorum, which is spread via mating or close contact.
“Relatively inevitably, infected females will become infertile, and quite rapidly,” says Mark Krockenberger, a professor of veterinary pathology at the University of Sydney.
“In the animals that are heavily diseased with chlamydia, they can also be quite emaciated too … more boney rather than the robust kind of koala we’re used to seeing,” adds Dr Freya Russell, a postdoctoral research fellow at the Queensland University of Technology (QUT).
Infection rates vary geographically, but in some koala populations the prevalence of chlamydia is as high as two in three. Populations in northern regions tend to be worse affected than those in the southern states.
Researchers have struggled for years to devise a solution to bring koala chlamydia rates down. Two separate chlamydia vaccines have been in development in Queensland for more than a decade. One was approved by the veterinary medicines regulator last year, but questions have been raised about its efficacy. What will it take to save the iconic animal from one of its key existential threats?
The problem with antibiotics
Since 2022, koalas in Queensland, New South Wales and the Australian Capital Territory have been listed as endangered by the Australian government. Chlamydia is only one of a handful of threats facing the emblematic species, along with land clearing, habitat fragmentation, vehicle strikes and other disease such as koala retrovirus, which causes an Aids-like illness.
In humans, chlamydia infections – caused by Chlamydia trachomatis – are easily treatable, usually requiring a single dose of antibiotics.
In koalas, antibiotic treatment is less straightforward – and often less successful. “We can kill the chlamydia … but the problem is that in the process, you can easily kill the koala as well,” Krockenberger says.
The animals are highly dependent on gut microbes to digest toxic eucalypt leaves. Antibiotics – which kill bacteria – result in a “shift in the abundance of good microbes that help the koalas digest eucalypt”, says Dr Lyndal Hulse, a postdoctoral research fellow at the University of Queensland.
“When you get that shift in those microbes … that’s when the koalas get into trouble – they have weight loss, they have diarrhoea, and the worst case scenario is that they go on to dysbiosis [a microbial imbalance] which can prove to be sometimes fatal.”
On top of that, some chlamydia strains have developed antibiotic resistance.
“There are very few effective vaccines against chlamydia,” Krockenberger says. None exists to treat the infection in humans. “There’s one in sheep that has variable efficacy, and that’s about it.”
Russell is part of a team at QUT developing a two-dose vaccine that contains proteins found on the outer membrane of chlamydia bacteria. In trials, they have vaccinated more than 500 koalas in the Currumbin region of the Gold Coast, with promising results, such as a reduction in chlamydia-related admissions to the local wildlife hospital.
But the researchers have not yet conducted placebo-controlled studies, which are the gold standard for determining how effectively a new vaccine or drug works. “When we first started doing trials … the population was rapidly declining,” Russell says. “Our team decided it wasn’t really ethical to be doing placebo controls in this heavily diseased population” – as it amounted to withholding treatment they believed to be effective from sick animals.
Given the difficulties of catching wild koalas to deliver a booster shot 30 days after an initial jab, the researchers have developed an implant that they insert at the time of first vaccination, which degrades over a few weeks to deliver the second dose.
“It’s completely biodegradable and non-toxic,” Russell says. “We’ve found in a captive study at Currumbin that it had the same efficacy when compared to the two-shot vaccine in stimulating antibody responses.”
The delayed-release implant vaccine is now being trialled in wild koalas, and has not yet been approved by the Australian Pesticides and Veterinary Medicines Authority (APVMA).
‘Silly’ data error
A second protein-based vaccine, available as a single-dose, has been developed by University of the Sunshine Coast (UniSC) researchers and was approved by the APVMA in September 2025.
Prof Keith Chappell, a molecular virologist at the University of Queensland, a third Queensland institution, has since questioned the efficacy of the vaccine. Last month, he took issue with a 2024 paper the researchers published in npj Vaccines, which had found that the vaccine significantly lowered disease incidence and resulted in a 64% reduction in chlamydia deaths over a 10-year period.
In a pre-print – which is yet to be peer reviewed – Chappell and colleague Dr Michaela Blyton said they had identified an error in data analysis “which resulted in survival from birth to time of vaccination incorrectly being attributed to vaccination”. After correcting for the error, they wrote, “there was no significant difference in the survival or disease rate between vaccinated and unvaccinated koalas”.
They argued that the vaccine’s use by veterinarians “should not be recommended”, advocating for “the continued development and testing of an improved vaccine formulation”.
Speaking with the Guardian, Chappell described the data error as a “silly mistake”. Commenting on the approval of the vaccine by the APVMA, Chappell said: “The regulatory system has failed … There should be an independent review of science to make sure that it’s done according to best practice.”
“I’ve had … setbacks as well,” says Chappell, who was a co-developer of the molecular clamp Covid vaccine, which stalled when trials returned false positive HIV results. “It made me realise the importance of high quality, transparent research.”
In a statement, an APVMA spokesperson said it “does not accept that issuing the permit represents a regulatory failure”.
“The permitted claim – that the vaccine ‘may aid in the reduction of clinical signs of the disease’ – is qualified and limited and does not mean the vaccination prevents chlamydial disease or mortality in koalas,” it said.
“Any concerns must be assessed against the permitted claim and the complete body of relevant evidence, not solely against interpretations of a single publication. The APVMA will assess credible new scientific information relevant to the permit and determine whether regulatory action is warranted.”
In a statement, a UniSC spokesperson said: “We welcome scientific scrutiny of our work, and disagreement between researchers is not unusual in any active area of science.
“What is being debated here is one aspect of a much larger research program. It does not invalidate more than a decade of peer-reviewed science or the substantial evidence supporting the continued development of a vaccine to help protect koalas from one of their most significant disease threats.”
The UniSC vaccine was also studied last year in a randomised controlled trial. That study, which involved Sydney researchers, followed for 12 months three populations of wild koalas in NSW, which were either vaccinated or given a placebo treatment.
The inclusion of a control group was so researchers could be “100% sure that whatever you’re looking at is not just by chance”, says Dr Valentina Mella, a senior lecturer in animal behaviour and conservation at the University of Sydney, and a collaborator on the project.
The study found that vaccination did stimulate an immune response in the koalas, but “did not translate to any effect on chlamydiosis”.
“The immune system was doing something, but that response was actually not enough to do anything useful to stop the disease,” Mella says. It was a disappointing result: “We wanted a very strong effect of the vaccine and we didn’t get it.”
The study concluded it was “possible that vaccination has a more modest effect and could benefit koala populations with a lower disease prevalence or be useful in conjunction with additional management strategies”.
Prof Peter Timms, a microbiologist at UniSC who has led the vaccine’s development, says: “We’ve published 29 papers in the whole vaccine story and they’ve all been reviewed scientifically by experts. We publish everything with the attitude that we’re there to share it, and do it openly and transparently.”
“We’re very confident that what we’ve got is a strong health benefit, but it can keep being improved,” he adds. “It’s a journey, right? There’ll be ups and downs.”
