Temporal dynamics of koala retrovirus plasma RNA load in relation to faecal glucocorticoid metabolites and Chlamydia infection

Koala retrovirus (KoRV) is endemic throughout northern koala populations that are currently in steep decline. We have previously found a strong association between KoRV plasma RNA loads and the risk of secondary diseases, including chlamydiosis. However, it is unclear whether (1) KoRV loads are elevated in sick koalas due to the expansion of leucocyte populations; and/or (2) KoRV induces immunosuppression, increasing susceptibility to disease; and/or (3) KoRV and secondary diseases are related through a third variable such as the physiological stress response. Here we assess the temporal dynamics of KoRV load over a year, and in relation to Chlamydia, to explore the causal direction of their relationship. We also investigated co-variation in faecal glucocorticoid metabolites (FGMs: cortisol and corticosterone) with KoRV load and Chlamydia. The results of this study provide compelling evidence that KoRV load does not respond to a change in disease status and instead that koalas with consistently high KoRV loads are more likely to develop chlamydiosis potentially through immunosuppression.In addition to the characterisation of temporal patterns in KoRV load and faecal glucocorticoid metabolites, this study has revealed spatial variation between south-east Queensland and the north coast of NSW. Both KoRV load and faecal glucocorticoid metabolite levels were lower for koalas in the NSW cohort than the QLD cohort. The reason for these regional differences requires further investigation but may be related to environmental factors, the koalas` genetics and/or KoRV subtype profiles. In a previous study, we identified two new KoRV subtypes, L and M, in NSW that have not been detected in QLD to date. In this study, these subtypes were again detected only in NSW by amplicon sequencing of the KoRV env gene from cell-free plasma, while KoRV-F was found in the QLD cohort at high prevalence. Further the average proportion of plasma load that could be attributed to KoRV-A was higher for the NSW cohort than the QLD cohort. With the known association between KoRV subtype profiles and KoRV load and the observed association between average KoRV load and stress levels, these variations in subtype profiles between the regions could play an important role in determining regional differences in koala health and should be considered in conservation management of the species.