Relationships among environmental stress, koala retrovirus and disease across a gradient of human associations

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 environmental factors, glucocorticoid metabolites (FGMs: cortisol and corticosterone), KoRV load, chlamydiosis and two recently identified koala herpes viruses (PhaHV-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 we suggest may be the result of acute stress and chronic stress acting antagonistically on FGMs in koalas. Also contrary to previous studies, KoRV load was not significantly correlated with Chlamydia pecorum infection, chlamydiosis or body condition, potentially due to spatial variation in chlamydia prevalence. Although, KoRV-M was significantly positively correlated with C. pecorum infection (qPCR positivity), associated pathologies and body condition. KoRV load and FGMs were not significantly correlated on the individual level but they did appear to be co-distributed across the region. We found that koalas on private properties in rural/agricultural areas had significantly higher faecal corticosterone levels than koalas captured in peri-/urban areas, with the same trend seen for faecal cortisol. Koalas from peri-/urban areas had a lower incidence of chlamydiosis than those on rural properties or 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 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 make a meaningful difference to wildlife population health.