The decline of Australia’s koala population is inextricably linked to chlamydiosis, an infectious disease that has reached epidemic proportions across many regions. This disease, caused by a bacterial infection, represents a significant conservation threat to the iconic marsupial. While the pathogen is a common cause of sexually transmitted infections in humans, the koala chlamydia crisis involves different bacterial strains. Understanding the nature of this bacterial infection is fundamental to grasping why it poses such a serious obstacle to the koala’s long-term survival.
The Pathogen and Its Devastating Impact on Koalas
The chlamydiosis affecting koalas is primarily caused by the bacterium Chlamydia pecorum, with the species C. pneumoniae also found in some populations. These bacteria are obligate intracellular parasites, meaning they must live inside host cells to replicate. This makes them particularly difficult to eliminate once an infection is established, and the resulting disease can manifest in three major, interconnected ways.
One of the most outwardly visible signs is ocular disease, known as keratoconjunctivitis, where the eyes become red, inflamed, and swollen. This infection can lead to a persistent discharge and, in advanced cases, permanent blindness, preventing the koala from being able to climb or find food. The infection also commonly targets the urinary tract, causing cystitis and nephritis. This chronic urinary tract infection leads to incontinence, giving rise to the common description of an affected animal as having a “wet bottom.”
The most catastrophic effect of the disease is the severe reproductive tract infection, which drives population decline. In female koalas, the infection can cause cysts and scarring in the uterus and ovaries, leading to permanent infertility. Male koalas also suffer from reproductive tract disease, but the resulting sterility in females is the primary biological reason the infection poses such a threat to the species’ ability to reproduce.
Understanding the Spread: Initial Introduction and Current Transmission
The question of how koalas originally acquired chlamydia involves a likely cross-species transmission event, known as spillover. DNA sequencing suggests that the koala pathogen is genetically similar to strains found in livestock, particularly sheep and cattle. This introduction may have occurred in the early 1800s, coinciding with the arrival of European settlers. The Chlamydia pecorum strain likely jumped from these non-native reservoirs into the koala population, possibly through environmental contamination of shared habitats, as livestock shed the bacteria in their feces.
Once the infection was established, it began to spread rapidly through the koala population using several transmission routes. Sexual contact is the most common and efficient way the disease is passed between adult koalas during mating. Young koalas can also be infected vertically, from the mother to the joey. Infection often occurs when joeys are weaned onto “pap,” a specialized form of the mother’s feces. Pap is rich in microbes necessary for the joey to digest eucalyptus leaves. If the mother is infected, her pap can transmit the chlamydia bacteria, leading to an early-life infection that often develops into severe disease later on.
Why Koalas Are Uniquely Vulnerable
The severity of chlamydiosis is largely due to a combination of the koala’s unique biology and external environmental stressors. Koalas possess a specialized diet of toxic eucalyptus leaves, relying on a finely tuned gut microbiome to process them. Treating the infection requires broad-spectrum antibiotics, which simultaneously devastate the koala’s essential gut flora. The loss of these critical microbes interferes with the koala’s ability to digest its only food source, sometimes leading to starvation. This creates a difficult treatment dilemma, where the cure can be as deadly as the disease itself.
The koala’s immune system is frequently compromised by chronic stress, making it susceptible to severe, chronic infection. Factors such as habitat loss, fragmentation, and relocation force koalas into closer proximity, increasing the rate of transmission while simultaneously increasing stress hormones. The specialized nature of the koala’s diet and its metabolic requirements also compound the challenge posed by this persistent bacterial invader.
Co-infection with the Koala Retrovirus (KoRV) increases the severity of chlamydiosis in many populations. KoRV is a virus that suppresses the koala’s immune function, making them less capable of fighting off the Chlamydia bacteria and more likely to develop severe symptoms, such as widespread inflammation and cancers. This combination of an immune-suppressing virus, a specialized diet, and pervasive environmental stress makes the koala vulnerable to the destructive power of chlamydiosis.