Koala populations in Australia face a severe health crisis due to a widespread bacterial infection, often transmitted during mating. While the term “STD” is usually reserved for human illnesses, this infection is caused by a highly prevalent and serious bacterial pathogen. This pathogen impacts koala welfare, fecundity, and survival across large parts of their range. The disease leads to debilitating symptoms that significantly compromise the long-term viability of the species, driving population declines, particularly in Queensland and New South Wales.
The Primary Infectious Agent
The disease is primarily caused by the bacterium Chlamydia pecorum, though Chlamydia pneumoniae also plays a role in some cases. C. pecorum is the more significant agent, causing severe clinical disease at mucosal sites. This bacteria is distinct from the Chlamydia trachomatis strain that affects humans, but both belong to the same family of obligate intracellular pathogens.
Infection rates are highly variable, sometimes reaching 85% in free-living groups. C. pecorum causes inflammation and subsequent fibrosis, or scarring, in affected tissues. Many koalas can be infected without showing visible symptoms, becoming silent carriers. The infection often remains subclinical until stress or other factors trigger the progression to full-blown illness.
Health Consequences for Koalas
The chlamydial infection attacks mucosal surfaces, severely compromising a koala’s ability to survive. One common outcome is severe ocular disease, beginning as keratoconjunctivitis. This inflammation of the eye’s surface can progress to cause discharge, swelling, and corneal ulcerations. In severe cases, this results in temporary or permanent blindness, preventing koalas from foraging and avoiding predators.
The infection also targets the urogenital tract, causing cystitis and urethritis (inflammation of the bladder and urethra). A painful symptom is urinary incontinence, which manifests externally as a brown-stained, wet rump, commonly termed “wet bottom.” This continuous wetness leads to painful skin ulceration around the rump area.
In female koalas, the infection causes inflammation and scarring in the reproductive tract, including the uterus, oviducts, and ovaries. This damage often results in cysts and anatomical changes like hydrosalpinx, physically preventing conception. The most significant consequence is widespread infertility or sterility, affecting up to 57% of females in highly infected populations. Male koalas also suffer reproductive tract damage, contributing to population-wide breeding failure.
Understanding Disease Transmission
The spread of Chlamydia pecorum occurs through several distinct pathways, explaining the high prevalence rates. Sexual transmission is a primary route, passing the bacteria between males and females during mating. Direct contact facilitates the exchange of infectious bodily fluids.
The bacteria also spreads through mother-to-joey (vertical) transmission. This can occur while the joey is in the pouch, potentially through contact with the mother’s infected urogenital secretions. A significant route is when the young joey consumes “pap,” a specialized form of feces essential for inoculating the joey’s gut with necessary microbes.
Environmental transmission is also a factor, as the bacteria can be shed in urine and feces, contaminating shared habitat and food sources. Koalas frequently share trees, meaning contact with contaminated surfaces or infected excretions can lead to new infections. This multi-faceted strategy allows the pathogen to persist and circulate widely.
Conservation Implications and Treatment Efforts
The high rates of sterility caused by chlamydial infection present a major hurdle to koala conservation. The disease directly undermines the ability of populations to reproduce and recover. The loss of reproductive capacity significantly contributes to population decline and localized extinctions. The national Koala Disease Risk Assessment identifies chlamydial disease as a top priority due to the severe threat it poses.
Current veterinary management relies on antibiotics, such as chloramphenicol, to treat infected koalas. This method is complicated because koalas require a high dose, which disrupts the highly specialized gut microbiota necessary for digesting their sole diet of toxic eucalyptus leaves. This disruption can lead to malnutrition and, in some cases, starvation, posing a significant risk to the animal’s survival despite the treatment. Furthermore, antibiotics often fail to prevent reinfection and are less effective against chronic, advanced-stage disease characterized by extensive scarring.
To overcome antibiotic limitations, researchers developed and approved a single-dose vaccine targeting C. pecorum. This breakthrough offers a preventative measure that can be administered to healthy koalas in the field and those in care, reducing their likelihood of developing symptomatic chlamydia during breeding age. Clinical trials demonstrated that the vaccine can reduce disease-related mortality in wild populations by at least 65%. This vaccination initiative is a critical intervention intended to be used alongside habitat preservation efforts to stabilize declining koala numbers.