The koala is an iconic Australian marsupial facing significant threats to its survival. While many factors contribute to their decline, disease is a primary driver of population loss. These health challenges are complex, often intertwining with external pressures that compromise the animals’ natural resilience. Understanding this interplay between pathogens and environmental conditions is fundamental to appreciating the crisis they face.
Chlamydial Infections
Chlamydial infections represent a major health crisis for koalas, primarily caused by the bacterium Chlamydia pecorum. This pathogen causes debilitating conditions that severely impact koala populations. Transmission occurs both through sexual contact between adults and from an infected mother to her joey. While some koalas can carry the bacteria without showing signs of illness, many develop severe, life-altering symptoms.
The clinical signs of chlamydiosis are distressing and often fatal if left untreated. Ocular infections cause severe inflammation of the conjunctiva, leading to discharge and, in advanced cases, permanent blindness, making it impossible for the koala to find food. Urinary tract infections are another common manifestation, causing inflammation of the bladder wall. This condition, often called “wet bottom,” results in incontinence and can be very painful.
Perhaps the most devastating impact of chlamydia on a population level is its effect on the reproductive system. In females, the infection can cause inflammation and cysts in the reproductive tract, leading to widespread infertility. With infertility rates in some Queensland populations recorded at over 50%, the disease effectively halts the ability of these groups to replenish their numbers.
Koala Retrovirus
Another significant pathogen affecting koalas is the Koala Retrovirus (KoRV). This retrovirus is unique because it integrates itself directly into the koala’s genetic code. Scientists have identified different forms of the virus, including endogenous types like KoRV-A, which are inherited from parent to offspring as part of their DNA. Other exogenous forms are spreading through the population like a typical infectious agent.
The primary consequence of a KoRV infection is severe immunosuppression, which weakens the koala’s immune system and leaves the animal highly susceptible to other pathogens. This compromised immune state means a koala is less capable of fighting off diseases that a healthy animal might otherwise endure.
This state of weakened immunity also makes koalas more vulnerable to developing various forms of cancer. KoRV is strongly associated with an increased incidence of lymphoma and leukemia, which are often fatal. The retrovirus disrupts the normal functioning of the immune system, which is responsible for identifying and destroying cancerous cells.
Influence of Environmental Stress
The diseases impacting koalas do not operate in a vacuum; their effects are magnified by persistent environmental stress. Factors such as habitat destruction and fragmentation are primary sources of this chronic stress. As eucalyptus forests are cleared for urban development and agriculture, koalas are forced into smaller, isolated territories with fewer resources. This displacement increases competition for food and forces them to travel across dangerous areas, such as roads and backyards.
These constant pressures have a direct physiological impact, as chronic stress weakens a koala’s immune system and makes them less able to fight off infections. An animal that might have carried a low-level, asymptomatic chlamydial infection can suddenly develop a life-threatening disease when subjected to the stress of losing its home. This connection explains why disease outbreaks are often more severe in populations living in degraded or fragmented habitats.
The result is a devastating cycle where habitat loss increases stress, which in turn weakens immune function and exacerbates the severity of diseases like chlamydia and the effects of KoRV. Encounters with vehicles and domestic dogs add to this burden, causing not only direct injury and death but also contributing to the underlying stress that makes koalas sick.