The koala, an iconic Australian marsupial, is facing a severe health crisis driven by a bacterial infection that significantly drives population decline. While the public often references koala “gonorrhea,” the underlying issue is actually chlamydiosis. This devastating infection severely compromises the health and reproductive success of individual koalas. It contributes to the species being listed as endangered in parts of Australia, which is already vulnerable due to habitat loss and climate change.
The True Cause: Chlamydia in Koalas
The infection plaguing koalas is caused primarily by the bacteria Chlamydia pecorum, though Chlamydia pneumoniae also plays a role in some cases. This is distinct from Neisseria gonorrhoeae, the bacterium responsible for human gonorrhea, despite the similar urogenital symptoms. The C. pecorum strain is the most pathogenic species affecting these marsupials.
This bacterial infection often circulates through koala populations at high rates, sometimes affecting up to 90% of individuals in certain areas. Koala chlamydia is caused by strains related to those found in livestock, unlike the human strain (C. trachomatis). A koala can be infected without showing visible signs, potentially remaining a subclinical carrier for years before the disease progresses.
Symptoms and Severe Health Impacts
The physical consequences of chlamydiosis often progress from mild infection to debilitating disease. One recognizable symptom is keratoconjunctivitis, a painful inflammation of the eye causing swelling and discharge. This eye infection can lead to scarring and permanent blindness, impairing the koala’s ability to find food and escape predators.
The bacteria also target the urinary tract, causing painful infections of the bladder and urethra. A clinical sign of this urogenital distress is a visible brown staining of the fur around the rump, referred to as “wet bottom,” resulting from urinary incontinence. The infection also causes severe inflammation and scarring within the reproductive tracts. In females, this damage leads to cysts and irreversible fibrosis, resulting in permanent infertility.
Transmission and Ecological Factors
The primary method of spread within adult koala populations is through direct venereal contact during mating. The infection is also frequently passed from mother to joey when the young animal consumes “pap,” a specialized form of feces necessary for establishing gut microbes. Exposure to infected material is a common route for young koalas to acquire the bacteria early in life.
The progression from infection to severe disease is often linked to environmental and host stress factors. Koalas facing competition, drought, or human encroachment experience chronic stress that suppresses their immune systems. This immune suppression allows the latent Chlamydia infection to become active and progress to full-blown disease. The genetic vulnerability of koalas, including low genetic diversity, also contributes to their poor ability to fight off the infection.
Conservation and Treatment Efforts
Treating chlamydiosis in koalas presents significant challenges for veterinarians and conservationists. Standard treatment involves a lengthy course of antibiotics, requiring the koala to be held in captivity. A major drawback is that these broad-spectrum antibiotics disrupt the specialized gut bacteria koalas rely on to digest their eucalyptus diet. This disruption can lead to starvation and death, even if the bacterial infection is cured.
The most promising long-term solution is the development of a vaccine to prevent permanent damage. A single-dose vaccine targeting Chlamydia pecorum has been developed and approved for use in wildlife hospitals and in the field. Clinical trials show the vaccine reduces the rate of infection and disease progression, significantly lowering mortality in wild populations. While treatment saves individual lives, vaccination is the focus for achieving population stability by preventing infertility.