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

## TL;DR

This study examines how viral and protozoal co-infections affect Chlamydia pecorum infection and disease outcomes in northern koalas.

## Contribution

The study identifies specific co-infection interactions that influence clinical outcomes and diagnostic limitations in koalas with chlamydiosis.

## Key findings

- PhaHV-1 mucosal shedding is linked to higher euthanasia rates in koalas with chlamydiosis.
- KoRV proviral loads and PhaHV-1 detection equally predict chlamydial reproductive disease in female koalas.
- Circulating C. pecorum can occur without mucosal shedding or clinical disease, suggesting current diagnostic methods miss some infections.

## Abstract

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.

High frequency of euthanasia of koalas in care highlights a need for better understanding of the pathogenesis of infectious disease to better inform clinical interventions and population disease management. This study detected koala retrovirus (KoRV), phascolartid herpesvirus (PhaHV), Chlamydia pecorum, and trypanosomes in 115 northern koalas admitted to wildlife hospitals and examined the data for associations between co-infection, clinical presentation, and admission outcomes. Co-infections were common, and each infectious agent and various co-infections differed in their association with clinical manifestation of disease. Our findings indicate that PhaHV-1 and KoRV are equally important infectious agents that may contribute to the diversity of clinical presentations in the koala. Detection of circulating C. pecorum occurred independent of mucosal C. pecorum shedding or disease in some koalas. Current C. pecorum screening only identifies mucosal shedding, hence a small portion of Chlamydia infected koalas are not identified currently. Further investigation of the potential mechanisms by which infectious agents interact with each other, and the host, is required to understand pathogenesis and improve disease mitigation and treatment methods.