At wildlife hospitals across eastern Australia, many koalas brought in have distinctive brown patches on their bottoms \u2013 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.<\/p>\n
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.<\/p>\n
\u201cRelatively inevitably, infected females will become infertile, and quite rapidly,\u201d says Mark Krockenberger, a professor of veterinary pathology at the University of Sydney.<\/p>\n
\u201cIn the animals that are heavily diseased with chlamydia, they can also be quite emaciated too \u2026 more boney rather than the robust kind of koala we\u2019re used to seeing,\u201d adds Dr Freya Russell, a postdoctoral research fellow at the Queensland University of Technology (QUT).<\/p>\n
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<\/a>.<\/p>\n 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<\/a> 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?<\/p>\n The problem with antibiotics<\/p>\n 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<\/a>, which causes an Aids-like illness.<\/p>\n In humans, chlamydia infections \u2013 caused by Chlamydia trachomatis \u2013 are easily treatable, usually requiring a single dose of antibiotics.<\/p>\n In koalas, antibiotic treatment is less straightforward \u2013 and often less successful. \u201cWe can kill the chlamydia \u2026 but the problem is that in the process, you can easily kill the koala as well,\u201d Krockenberger says.<\/p>\n The animals are highly dependent on gut microbes to digest toxic eucalypt leaves. Antibiotics \u2013 which kill bacteria \u2013 result in a \u201cshift in the abundance of good microbes that help the koalas digest eucalypt\u201d, says Dr Lyndal Hulse, a postdoctoral research fellow at the University of Queensland.<\/p>\n \u201cWhen you get that shift in those microbes \u2026 that\u2019s when the koalas get into trouble \u2013 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.\u201d<\/p>\n On top of that, some chlamydia strains have developed antibiotic resistance.<\/p>\n \u201cThere are very few effective vaccines against chlamydia,\u201d Krockenberger says. None exists to treat the infection in humans. \u201cThere\u2019s one in sheep that has variable efficacy, and that\u2019s about it.\u201d<\/p>\n Russell is part of a team\u00a0at 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<\/a> to the local wildlife hospital.<\/p>\n Koalas at Currumbin on the Gold Coast. Photograph: Dave Hunt\/AAP<\/p>\n But the researchers have not\u00a0yet conducted placebo-controlled studies, which are the gold standard for determining how effectively a new vaccine or drug works. \u201cWhen we first started doing trials \u2026 the population was rapidly declining,\u201d Russell says. \u201cOur team decided it wasn\u2019t really ethical to be doing placebo controls in this heavily diseased population\u201d \u2013 as it amounted to withholding treatment they believed to be effective from sick animals.<\/p>\n 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.<\/p>\n \u201cIt\u2019s completely biodegradable and non-toxic,\u201d Russell says. \u201cWe\u2019ve found in a captive study<\/a> at Currumbin that it had the same efficacy when compared to the two-shot vaccine in stimulating antibody responses.\u201d<\/p>\n 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).<\/p>\n \u2018Silly\u2019 data error<\/p>\n A second protein-based vaccine<\/a>, 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.<\/p>\n 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<\/a>, which had found that the vaccine significantly lowered disease incidence and resulted in a 64% reduction in chlamydia deaths over a 10-year period.<\/p>\n In a pre-print<\/a> \u2013 which is yet to be peer reviewed \u2013 Chappell and colleague Dr Michaela Blyton said they had identified an error in data analysis \u201cwhich resulted in survival from birth to time of vaccination incorrectly being attributed to vaccination\u201d. After correcting for the error, they wrote, \u201cthere was no significant difference in the survival or disease rate between vaccinated and unvaccinated koalas\u201d.<\/p>\n They argued that the vaccine\u2019s use by veterinarians \u201cshould not be recommended\u201d, advocating for \u201cthe continued development and testing of an improved vaccine formulation\u201d.<\/p>\n Speaking with the Guardian, Chappell described the data error as a \u201csilly mistake\u201d. Commenting on the approval of the vaccine by the APVMA, Chappell said: \u201cThe regulatory system has failed \u2026 There should be an independent review of science to make sure that it\u2019s done according to best practice.\u201d<\/p>\n