Retroviral particles were first identified in tissues from a leukaemic koala in 1988. The virus was successfully isolated from Queensland (QLD) koalas and the full genome was sequenced in 2000. Koala retrovirus (KoRV) is the only known exogenous retrovirus undergoing a process of active endogenisation in its host at the present time. KoRV is 100% prevalent within QLD koalas and the virus has endogenised in that population, with high proviral (DNA) loads reported. These populations also have high levels of chlamydiosis (40%) and neoplasia. The scenario is different in South Australia (SA) where KoRV-free animals are present, and apparently both endogenous and exogenous forms of the virus exist. There is anecdotal evidence that the picture of population disease associated with KoRV infection differs between these two geographic areas however the source and relevance of these differences is unknown. The aim of this project is to characterise these differences and to explore possible reasons, whether they are host or viral factors. Viruses from both areas will be characterised, proviral and viral (RNA) load will be quantified and compared to pathology and the host response to infection will be characterised using transcriptome analysis. It is hypothesised that high viral load is associated with disease advancement because, as the viral load becomes high, the chance of insertional mutagenesis and subsequent tumour induction increases. Virus will be cultured to investigate any sequence variation between virus isolates from SA and QLD koalas. Transcriptome analysis will identify differences in expression of genes associated with the immune response between QLD and SA populations and between KoRV-infected and uninfected koalas. The transcriptome study may also provide information about characteristics of KoRV in the koala genome. Preliminary results will be presented. It is anticipated that this information will identify factors that influence development of KoRV associated disease, information that can hopefully be used to design koala management programs and disease prevention measures.