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Biology students often learn about diversity and connection across the animal kingdom. From the elongated bills of tropical toucans, to complex neural networks of cephalopods, organisms are unified by common elements, such as genetic code and respiratory pathways. However, a rather overshadowed but equally strong commonality is the prevalence of sexually transmitted Chlamydia. Over 600 animals can contract variant forms of chlamydia, ranging from cats and mice to oysters; in a disturbing way , chlamydia connects us all. The disease also varies in severity, and is threatening to decimate Koala populations in South Australia(1). 

A chlamydia infection involves an infestation of Chlamydia trachomatis bacteria, which reproduce similarly to many viruses. They exploit existing molecular machinery in animal cells, using them as a means to reproduce their own proteins and nucleic acids. The widespread presence and high transmission rate of chlamydia can be attributed to absent or very mild symptoms: the disease is often coined a “silent infection” (2). The apparent lack of symptoms renders the STI harmless to many; however, this actually contributes to the maliciousness of the disease, as it is frequently transmitted undetected. If the disease persists for many years in humans, it is known to cause ectopic pregnancies, pelvic inflammatory disease, and many other symptoms which promote infertility. In Koalas, many similar ___ diminish fecundity, however, over a much shorter timespan. Thus, the disease is far more pernicious and threatening in koala populations.

Because of anatomical differences in the reproductive tract, ovaries of the koala are more susceptible to damage from a variant form of the Chlamydia trachomatis bacteria. Chlamydia-induced cysts often develop on koala ovaries, quickly rendering them infertile. Humans can be treated using a single-administered antibiotic, however, treatment for the critically endangered koalas is far more complex. Conservationists in Australia often administer antibiotics to the infected, but these also act on the microbiome within the koala’s gut. Many resident bacteria are essential for digesting compounds of the eucalyptus leaf: the exclusive component of the koala’s diet. The absence of such degrading bacteria ultimately results in the accumulation of indigestible tannins, which are highly toxic when absorbed into the bloodstream (4).

The dire need to limit chlamydia transmission in human populations, combined with the desperate efforts to save koala populations, has left biomedical pathologists such as Peter Timms at the University of the Sunshine Coast in an unconventional yet fortunate situation. An ideal model organism has arisen to trial a potential chlamydia vaccine on. Animals such as mice are typically used in biomedical trials, however, because the strain that koalas are infected by is so similar to that of humans, it is likely that a suitable vaccine will be akin and transferable between both species (5). Furthermore, successful results in vaccine trials will benefit the clinical models as well as humanity. 

Such clinical trials are underway in Australia, with substantial success. While the one-time vaccine has not proven to eliminate transmission, infection rates of vaccinated koala populations have been halved (5). Although this vaccine is unlikely to eliminate chlamydia, these figures boast a promising move in the direction of herd immunity. Ken Beagly at the University of Technology Queensland is even applying the same principles of this vaccine to manufacture a human version (5).

Beyond lighting up the faces of local Australians, koalas have provided much for humanity.  The social stigma surrounding STIs prevents discussion and discourages testing, which together with minimal symptoms has allowed a clandestine manifestation of chlamydia. Thankfully, these marsupials don’t experience the same taboo complexes we do with regards to STIs, and will benefit by being the model of chlamydia vaccine trials. Humankind is scrambling every day for more vaccines to treat a growing and evolving list of infections, and we should consider ourselves lucky as Koalas have given us a clear path to chlamydia prevention.




  1. Gross, Rachel. How Koalas With an STD Could Help Humanity. The New York Times, July 2020.
  2. O’Farrel, N., et al. Chlamydia - CDC Facts Sheet. Centres for Disease Control and Prevention. October, 2016.
  3. Woods, Michael. Chlamydia: The Silent STD. Beth Israel Lahey Health Winchester Hospital. 2016.
  4. Chong, Rowena, et al. Marsupial Gut Microbiome. Frontiers in Microbiology, May 2020. V.
  5. Klein, Alice. Chlamydia vaccine for koalas slows spread of deadly disease. New Scientist, April 2017.