Chlamydiosis represents a major threat to Australia’s koala populations, driving a conservation crisis across the continent. This bacterial infection has reached epidemic proportions in many wild habitats, leading to severe health outcomes for individual animals. In some northern populations, the prevalence of infection can approach 90%, severely compromising the future viability of the species. The disease’s impact on reproduction and survival ranks it among the greatest non-environmental causes of death for koalas in the wild.
The Specific Pathogen
The primary agent responsible for the koala epidemic is the bacterium Chlamydia pecorum, an obligate intracellular organism. This strain is distinct from Chlamydia trachomatis, the species most commonly associated with human infections. While a less pathogenic species, Chlamydia pneumoniae, has also been detected in koalas, C. pecorum is the dominant cause of severe chlamydiosis in the marsupials.
The distinction between the koala and human strains is important for understanding the disease’s pathology and preventing misunderstandings about transmission risk. C. pecorum infects various livestock species, but there is no confirmed evidence of its transmission directly from koalas to humans. Research focuses on this specific pathogen because its high prevalence and virulence are linked to the decline in koala numbers.
The Source of Infection
Scientists theorize the initial introduction of Chlamydia pecorum into koalas was a historical cross-species event, known as a spillover. The prevailing hypothesis points toward contact with European livestock, such as cattle, sheep, and pigs, which were introduced to Australia during the period of European settlement. Genetic sequencing demonstrates that certain C. pecorum strains found in koalas share a close genetic relationship with those found in these domestic animals.
Ancestral koala populations either had no exposure to this virulent strain or were infected with less pathogenic variants. The arrival of European livestock carrying C. pecorum provided an opportunity for the bacteria to “jump” hosts, likely in the 19th or early 20th century. Historical records from that era report koalas exhibiting symptoms of eye disease, which align with chlamydial infection.
The exact mechanism of this initial spillover is not definitively known, but it may have occurred through shared environmental resources. Koalas could have encountered the bacteria via contaminated water sources or soil that was also used or fouled by infected livestock. The introduction of this novel strain into a host species with little or no adaptive immunity set the stage for the current widespread epidemic.
Transmission Within Koala Populations
Once established, the disease spreads efficiently among koalas through several primary routes. The most common mechanism of spread between adults is venereal transmission, or sexual contact during mating. The pathogen is present in the urogenital secretions of both infected males and females, ensuring its passage during reproduction.
Vertical transmission also plays a significant role in infecting the next generation of koalas. An infected mother can pass the bacteria to her joey either during birth as it passes through the birth canal or later while the joey is developing in the pouch.
Joeys also consume a specialized form of the mother’s feces called ‘pap’ during the weaning process to acquire necessary gut microbes. If the mother is infected, this pap can be a route for oral-fecal transmission of C. pecorum to the young koala. Furthermore, the bacteria can survive outside the host in environmental discharge, meaning an uninfected koala may contract the disease by coming into contact with contaminated urine or other discharges on tree trunks or in the soil.
Clinical Consequences and Population Decline
The effects of chlamydiosis on an individual koala are severe, targeting three main organ systems. Ocular infection leads to conjunctivitis, causing swollen and inflamed eye tissue often accompanied by a purulent discharge. Chronic inflammation results in corneal scarring, ultimately causing complete blindness in the affected koala.
Infection of the urinary tract causes cystitis, a painful inflammation of the bladder that leads to incontinence. This condition is commonly referred to as “wet bottom” due to the constant staining and wet fur around the koala’s rump. The persistent pain and discomfort can lead to secondary infections and an inability to forage effectively.
The most significant impact stems from reproductive tract infections, which often remain subclinical but cause irreversible damage. In females, inflammation of the reproductive organs, known as salpingitis, causes scarring and blockage of the oviducts, resulting in permanent infertility. Males can also suffer from inflammation of the testes and epididymis, which similarly impairs their reproductive function. This widespread infertility is directly responsible for dramatic declines in koala breeding rates and population viability.