As early as the late 1930s, disease had become a suggested factor in koala population collapse in Queensland, though it is hard to distinguish hunting, habitat destruction or fire regimes from the equation. Not until 1944 was there an effort to translocate the creatures to try and reestablish their populations. Koalas remain largely absent in the eastern regions of Australia to this day. 

 

Koala populations are declining, though it remains unclear what the role infectious disease plays in this loss, particularly those with chlamydia. In the literature of ecology, it is sometimes assumed that "disease" is synonymous with "infection". However, numerous pathogens may infect an individual without causing any significant clinical disease. This is conveyed in the case of the koala, where complexities associated with the bacteria and its effects may only weaken them and not ensure death.

 

Infected individuals may have lower fecundity or survival odds than uninfected koalas, but individual infection severity may not have the same effects when studying a population as a whole. Some researchers believe that the endemic chlamydial infection is a key agent in the decline, while others associate multiple factors that may play a more significant role in local extinctions. Koalas are affected by a variety of pathogens and parasites, but the spontaneous nature of these infections means that mass morality is not definite.

 

Chlamydia is a genus of intracellular bacteria in the Chlamydiaceae Family that infect koalas and may cause pneumonia, blindness, sterility, reduced reproductive output and increased mortality. There are two identified species, Chlamydia pneumoniae and Chlamydia pecorum, with the latter being accepted as more pathogenic. Generally Chlamydia is recognized as an STI, but infection can be transmitted through direct contact with the eyes and reproductive tract of infected individuals. Mother koalas can also pass the bacteria to their off-spring through pap-feeding. It remains unclear which route has more relative importance in transmission, and the effects on individual koalas differs. While koalas have coexisted with these bacteria for thousands of years, recent genome evidence has identified a newer strain of C. pecorum that was shared with livestock introduced into the area, implying that post-European colonization has played a key role in the introduction of harmful pathogens. 

 

Some infected koalas have no clinical signs of disease yet have the highest intensities of infection. Stress in any creature increases its susceptibility to pathogenic infection and reduced tolerance. An individual that is resistant to infection can be exposed to the bacteria and never develop an infection themselves, whereas a tolerant individual may develop an infection but has limited signs of clinical disease. This tolerance, however, does diminish the fitness of the individual.

 

For presumptive uninfected koalas, proper quarantine standards are to be expected to be followed, especially when veterinary care is required. Keeping koalas in care away from those who come from different regions of Australia is crucial for successful treatment and recovery. Some more isolated populations have low genetic diversity due to inbreeding and the introduction of a new strain could be detrimental to their already threatened survival because they lack the tolerance; populations with high infection but low prevalence of disease suggests that coevolution is already underway between koalas and the chlamydia bacteria.

 

Recent vaccination field trials have failed to significantly reduce new infections in vaccinated koalas, though it has been reported that the viral load of their infections lessened greatly. It is important to note that there is well over 50 years of research in an effort to create an effective vaccine for human strains of chlamydia, and has yet to produce one that is safe. The administering of a vaccine may have negative consequences in itself when you consider the stress that is induced from the capture and handling of wild koalas. Antibiotics are known to be used extensively in care facilities but have serious side effects due to the dysbiosis of the microflora of the koala gastrointestinal tract. 

 

A major problem with the data that has since been collected studying the impact of chlamydial infection in koalas lacks long-term monitoring. The conservation of koala populations leaves us with two main questions: (1) is it possible to reduce the disease impact chlamydia has on koalas and (2) will reduction of disease improve the viability of koalas enough to promote a health recovery of threatened populations.