The term “koala AIDS” refers to the Koala Retrovirus (KoRV), a virus that impacts koala populations across Australia. It functions in a way that is comparable to the human immunodeficiency virus (HIV), as it weakens the animal’s immune system, making it vulnerable to other infections and diseases. However, KoRV is a distinct gammaretrovirus that cannot be transmitted to humans.
How Koalas Contract and Develop the Virus
Koala Retrovirus is transmitted through two primary pathways. The first is vertical transmission, where the virus exists as an endogenous retrovirus within the koala’s genetic code. In this form, the virus has integrated into the germline cells—the egg and sperm—and is passed directly from a mother to her joey during development. This process of becoming a permanent part of the host genome is known as endogenization.
The virus also spreads horizontally as an exogenous retrovirus, meaning it is transmitted between individual koalas. This can occur through close contact, such as fighting, mating, or other social interactions. Researchers believe this horizontal transmission is a more recent development in the virus’s evolution.
Once a koala is infected, the retrovirus uses an enzyme called reverse transcriptase to create a DNA copy of its own RNA genome. This viral DNA then inserts itself into the host’s cells, including immune cells. By integrating into the DNA of these cells, KoRV disrupts their normal function, leading to a state of immunodeficiency. This compromised immune system leaves the koala unable to effectively fight off other pathogens.
Resulting Illnesses and Symptoms
A compromised immune system caused by Koala Retrovirus leaves koalas highly susceptible to a range of opportunistic infections and cancers. The animal does not die from KoRV itself, but from secondary diseases that take hold due to the koala’s inability to mount an effective immune response. This condition is formally known as Koala Immune Deficiency Syndrome, or KIDS. The presence of a high viral load is often correlated with the progression of these diseases.
One of the most common secondary infections is chlamydia. While chlamydia can infect healthy koalas, an animal with a KoRV-weakened immune system is unable to control the infection, leading to severe disease. The resulting chlamydiosis can cause blindness, pneumonia, and painful urinary tract infections that lead to a condition known as “wet bottom,” characterized by urine staining on the rump. Ultimately, these infections often result in infertility, impacting the koala’s ability to reproduce.
KoRV is also linked to the development of cancers, particularly lymphomas and leukemias. The retrovirus’s ability to insert its genetic material into the host’s DNA is a mutagenic event, meaning it can disrupt the normal regulation of cell growth and division. This insertional mutagenesis is thought to be a trigger for the formation of malignant tumors. In some captive koala populations, cancers and related immune deficiency disorders have been responsible for a high percentage of deaths.
Threat to Koala Survival
The widespread prevalence of Koala Retrovirus, combined with the severe secondary illnesses it facilitates, poses a substantial threat to the survival of the species. The virus is a contributor to koala population decline, a fact that played a role in the decision to list koalas as an endangered species in Queensland, New South Wales, and the Australian Capital Territory. The combination of immunodeficiency, chlamydiosis, and cancer creates a health crisis that puts pressure on vulnerable populations.
Currently, there is no cure or vaccine for KoRV. The situation is further complicated by the fact that the virus is, in many populations, an endogenous retrovirus embedded within the koala genome itself. This makes it incredibly difficult to manage or eradicate, as it is passed down from one generation to the next.