The link between a human sexually transmitted infection and Australia’s koala often raises questions. The idea that humans contracted chlamydia from these marsupials is a misconception that simplifies a complex biological story spanning millions of years. Exploring the facts reveals the distinct nature of the diseases in each species and their shared, but distant, origins.
The Two Types of Chlamydia
The term “chlamydia” describes infections in both humans and koalas, but the bacteria responsible are different species. In humans, the infection is caused by Chlamydia trachomatis, a common sexually transmitted infection. While treatable, many cases are asymptomatic and can lead to complications like infertility if untreated. It is transmitted through sexual contact and is exclusive to humans.
Koalas, on the other hand, are afflicted by a different bacterium called Chlamydia pecorum. This infection is harmful to koala populations, causing blindness, urinary tract infections called “dirty tail,” and widespread infertility. The disease contributes significantly to population decline and is a threat to their survival.
The Evolutionary Connection
The reason for the similar names is that both C. trachomatis and C. pecorum descended from a common bacterial ancestor. This shared lineage explains why they function as intracellular parasites that replicate inside the host’s cells. Millions of years of evolution have led these bacteria down separate paths in a process of co-evolution, where a pathogen adapts specifically to its host. This specialization means C. trachomatis is adapted to humans, while C. pecorum is adapted to koalas. While another species, Chlamydia pneumoniae, can infect both, the sexually transmitted forms are caused by distinct bacterial species.
Addressing the Transmission Myth
The direct answer is no, humans did not get chlamydia from koalas. The risk of zoonotic transmission—the transfer of disease from animals to humans—is exceptionally low. Because C. pecorum is highly adapted to the unique biology of koalas, it does not establish infections in humans. The cellular environments and immune systems of the two species are simply too different for the bacterium to thrive in a human host.
Interestingly, the story of chlamydia in koalas may involve a species jump from another source. A prominent theory suggests koalas acquired C. pecorum from livestock, such as sheep or cattle, introduced to Australia by settlers. This shows that chlamydial bacteria can cross between animal species but reinforces that the jump from koala to human is not the origin of the human STI.