The koala, an iconic marsupial of Australia, faces numerous threats to its survival, including habitat loss, bushfires, and vehicle strikes. However, a bacterial infection known as chlamydiosis represents the greatest disease threat to the species across much of its range. Caused by an obligate intracellular bacterium, this widespread disease significantly contributes to the koala’s current vulnerable conservation status. The long-term health and reproductive viability of wild koala populations are severely compromised by this pathogen. In some regions, chlamydiosis may account for as much as half of all koala deaths, making its management a primary focus for conservation efforts.
Prevalence and Geographic Variability
There is no single percentage to describe how many koalas have Chlamydia; the infection rate varies widely depending on the specific location and population. Wild populations in undisturbed habitats, such as Kangaroo Island or French Island, have demonstrated infection rates as low as zero percent. Conversely, koalas living in highly stressed, fragmented, or urbanized environments often show high prevalence rates.
In regions like Southeast Queensland, the infection rate can range from 29% to over 70% of the population. Some isolated populations in South Australia have also recorded high rates, nearing 88%. Northern populations (Queensland and New South Wales) tend to experience more severe clinical disease compared to their southern counterparts. This wide epidemiological range highlights the complexity of the disease, where the health of the habitat plays a direct role in the pathogen’s distribution.
The Pathogen and Its Health Consequences
Chlamydiosis in koalas is primarily caused by the bacterium Chlamydia pecorum, which is considered the more virulent and prevalent species, though Chlamydia pneumoniae is also detected in some cases. The bacteria target mucosal surfaces, leading to severe inflammation in three main areas of the koala’s body.
Ocular Disease
One of the most visible consequences is ocular disease, known as keratoconjunctivitis, which causes inflammation of the eyelids and eye tissue. This can lead to a mucopurulent discharge, pain, and in advanced stages, permanent blindness. Blindness prevents the koala from climbing or finding food.
Urogenital Disease
The urogenital tract is also severely affected, resulting in urinary tract infections and cystitis. This internal inflammation often presents externally as the “wet bottom” syndrome, characterized by a stained, wet rump due to incontinence and constant urine leakage.
Reproductive Impairment
Reproductive tract disease is a particularly devastating consequence, especially for female koalas. Infection of the uterus and ovaries can cause fibrosis, cysts, and often leads to permanent infertility. Infected males can also develop prostatic and testicular changes that impact their ability to reproduce. This reproductive impairment significantly hinders the ability of affected populations to recover their numbers.
Ecological and Genetic Factors Driving Infection Rates
The high susceptibility of koalas to Chlamydia is influenced by external and internal factors. Environmental stress is a major contributor, as the chronic stress caused by habitat fragmentation, high population density, and encroaching urbanization suppresses the koala’s immune system. When a koala’s natural forest habitat is cleared or disturbed, the resulting stress makes the animal far more vulnerable to both contracting the infection and progressing to clinical disease.
The koala’s unique biology also plays a role, particularly its genetic makeup and immune response. Research links specific Major Histocompatibility Complex (MHC) class II genes to susceptibility to chlamydial disease. These genes regulate the immune system, and variation affects how effectively a koala fights the infection. Furthermore, many northern koala populations suffer from co-infection with Koala Retrovirus (KoRV), which compromises the immune system and worsens chlamydiosis severity.
Transmission primarily occurs through sexual contact between adults. However, young joeys can also contract the infection from their mothers, either during birth or through the consumption of “pap,” a specialized maternal fecal material. This combination of environmental stressors and genetic factors drives the high infection rates observed in vulnerable populations.
Current Management and Research Efforts
Managing chlamydiosis currently involves treating clinically affected koalas with antibiotics, such as chloramphenicol or doxycycline. While antibiotics can clear the bacterial infection, they carry a risk: these medications disrupt the gut microbes necessary for digesting eucalyptus leaves, potentially leading to starvation. Furthermore, antibiotic treatment often fails to reverse permanent tissue damage, such as scarring of the reproductive tract.
The most promising solution is a recently approved single-dose vaccine developed by the University of the Sunshine Coast. This vaccine, based on the Major Outer Membrane Protein (MOMP) of C. pecorum, reduces the likelihood of koalas developing clinical symptoms during breeding age. Long-term trials have shown that the vaccine decreases mortality from the disease in wild populations by at least 65%. Its approval for use in wildlife hospitals and in the field simplifies widespread rollout. Alongside vaccination, habitat restoration and conservation are necessary preventative strategies to reduce environmental stress and support the health of wild koalas.