Koala Immune Deficiency Syndrome

Koala Immune Deficiency Syndrome (KIDS) is caused by the Koala retrovirus (KoRV). An infection with this syndrome results an AIDS-like immunodeficiency that leaves infected koala (Phascolarctos cinereus) more susceptible to infectious disease and cancers.

Koala retrovirus is closely related genetically to gibbon-ape leukemia virus (GaLV), feline leukemia virus (FeLV), and porcine endogenous retrovirus (PERV)[1].

The Koala retrovirus is thought to be a recently introduced exogenous virus that is also integrating into the koala genome (becoming endogenous). Thus the virus can transmit both horizontally (from animal to animal in the classic viral sense) and vertically (from parent to offspring as a gene).

Koala retrovirus was initially described as a novel endogenous retrovirus found within the koala genome and in tissues as free virions. Analysis showed that KoRV is an active replicating endogenous retrovirus that can also produce infectious virions.

The analysis also showed that KoRV was closely related to the highly pathogenic gibbon ape leukemia virus (GALV).

Some 80% of all deaths of captive koalas in Queensland (Australia) from leukemia, lymphoma, malignant tumours, and immune deficiency disorders is attributable to the virus. The virus is considered a threat that could lead to extinction of koalas in Queensland within 15 years

Research has also shown that some populations of koalas, particularly an isolated colony on Kangaroo Island do not appear to have the endogenous form of the retrovirus. This suggests that the virus gene sequence is a rather new acquisition for the koala genome.

Prevalence of KoRV (and KIDS) in Australian koala populations suggests a trend spreading from the north down to the south of Australia.[ Northern populations are completely infected, while some southern populations are still free.

In 2013, an exclusively exogenous subtype of KoRV was identified and termed Koala retrovirus-B (KoRV-B), with the endogenous form of KoRV referred to as Koala retrovirus-A (KoRV-A)[2]. KoRV-B will likely remain exogenous and more pathogenic than KoRV-A, because the deleterious effects it causes in its hosts will not be selected against to the extent they would in a virus capable of integrating into the germ line. So far, nine subtypes of KoRV have been isolated (KoRV-A to KoRV-I).

Currently, no vaccine or effective treatment is available for KoRV or its associated neoplastic diseases.

[1] Kayesh et al: Koala retrovirus epidemiology, transmission mode, pathogenesis, and host immune response in koalas (Phascolarctos cinereus): a review in Archives of Virology - 2020
[2] Xu et al: An exogenous retrovirus isolated from koalas with malignant neoplasias in a US zoo in PNAS- 2012