Relationships among faecal glucocorticoid metabolites, koala retrovirus and disease across a gradient of human associations

Abstract Koalas are now under threat of extinction from anthropological environmental change and the impacts of koala retrovirus (KoRV) induced immunosuppression and chlamydiosis, with these disease and environmental threats potentially interconnected through the effects of physiological stress. In this study, we investigated the associations among glucocorticoid metabolites (FGMs: cortisol, corticosterone and tetrahydrocortisol), KoRV load, chlamydiosis and two koala herpes viruses (PcGHV-1 and 2) across a landscape gradient of human modification/activity on the mid-north coast of NSW, Australia. Contrary to previous studies, FGM levels were not significantly correlated with any disease measures, which may be related to small sample sizes and baseline differences between individuals in hormone concentrations due to proposed intrinsic and extrinsic factors identified here. Also contrary to previous studies, KoRV load was not significantly correlated with Chlamydia score or body condition, potentially due to spatial variation in chlamydia prevalence. KoRV load was significantly correlated with adjusted tetrahydrocortisol levels and covaried with faecal corticosterone metabolites across the region. We found that koalas on private properties in rural/agricultural areas had significantly higher faecal corticosterone metabolite levels than koalas captured in peri-/urban areas, with the same trend seen for faecal cortisol metabolites. Koalas from peri-/urban areas had lower Chlamydia scores than those in national parks and koalas further from population centres had higher KoRV loads. Together these findings suggest that koalas may have the capacity to cohabitate with humans in the peri-/urban space, where sufficient suitable habitat is provided, despite the presence of conspicuous threats. Furthermore, the lower rates of disease observed in peri-/urban koalas suggests that wildlife hospitals can potentially make a meaningful difference to local wildlife population health.