Vaccination against chlamydiosis: An effective disease management tool in wild koala populations?

Abstract

The koala, Phascolarctos cinereus, is classified as endangered following population declines due to numerous threats, one prominent threat being chlamydiosis, caused by Chlamydia pecorum infection. This has necessitated the exploration of effective management solutions to address chlamydiosis in wild koala populations. Vaccination offers the most promise to deliver immediate outcomes. Currently there is limited evidence that vaccination is efficacious in wild koala populations affected by chlamydiosis. The central aim of this thesis was to assess the suitability vaccination as a management tool to address chlamydiosis in wild koala populations, with the overarching goal of enhancing conservation management for the koala. This thesis evaluated the ability of two vaccines to reduce chlamydial shedding, prevent infection, and minimise the development and progression of chlamydiosis in diverse populations, in two field-based blinded randomised placebo-controlled trials. The vaccines examined in this thesis did not significantly reduce chlamydial shedding, prevent infection, or prevent or mitigate chlamydiosis when compared to a placebo-control cohort. The conclusion reached from this body of work is that these vaccines are not effective as standalone tools for managing chlamydiosis. It is hypothesised that these vaccines did not stimulate an appropriate immune response to confer beneficial effects. This prompted investigation of the immunological profiles of koalas naturally infected with C. pecorum and comparisons were made to vaccine-elicited immune responses. The findings revealed differences between diseased, infected and non-diseased, infected koalas, offering insights into vaccine trial outcomes. The findings of this thesis contribute knowledge that will allow for advancements in vaccine efficacy and highlight prospective areas for future research, essential for the use of vaccination as a management strategy to address chlamydiosis within wild koala populations.