The relationships of viral and protozoal co-infections to Chlamydia pecorum infection and chlamydiosis outcomes in northern koalas (Phascolarctos cinereus)

Several infectious agents concurrently infect wild koalas and so, as for similar agents in other species, co-infection interactions could affect disease presentation and clinical outcomes. This study determines the frequency of circulating and mucosal Chlamydia pecorum infections along with phascolarctid herpesvirus (PhaHV), Koala retrovirus (KoRV), and trypanosome infections in 115 wild koalas admitted to wildlife hospitals in the Australian states of Queensland and New South Wales. C. pecorum, PhaHV, trypanosomes, and KoRV (endogenous subtype A and exogenous subtype D) were detected in 61.1%, 68.9%, 63.3% and 100% of the individuals sampled, respectively. The co-infection relationships identified generate hypotheses for the observed variation in disease presentations in that they resemble co-infection interactions that drive the variations in presentation and response to treatment for chlamydiosis in other species, including humans. Among koalas with chlamydiosis, PhaHV-1 mucosal shedding positively predicted euthanasia on admission, and accounting for Trypanosome irwini infection status improved the model quality. Additionally, in female koalas, the detection of mucosal PhaHV-1 and greater KoRV proviral pol loads were equal predictors of chlamydial reproductive disease. While the detection frequency of C. pecorum, PhaHV-1, PhaHV-2, and T. gilletti in circulation were low, cases with circulating C. pecorum and without mucosal C. pecorum shedding or clinical chlamydiosis were observed presenting an important consideration for future diagnostic testing. This study serves as a basis for investigating co-infection interaction pathways through mechanistic studies to determine their effect on pathogenesis of chlamydiosis, improve our understanding of host-pathogen-environment dynamics impacting the koala, and identify novel intervention and screening methods.