A Holistic Evaluation of Health Indicators in the Koala, Phascolarctos cinereus: Complex Host-Pathogen- Environment Relationships in Rehabilitation and in the Wild.

Abstract

This thesis demonstrates the greater depth of understanding of complex wildlife diseases that can be gained by application of a holistic multifactorial approach; in this case investigation of host-pathogenenvironment interactions in disease of the koala (Phascolarctos cinereus), an iconic and endangered Australian marsupial. The multifactorial nature of disease complicates efforts to understand epidemiology, pathogenesis, and disease risk, and develop treatment or risk mitigation strategies. In wildlife disease investigations, small sample sizes, lack of medical histories, broad case definitions and discipline-oriented research often limit the variables that can be included in studies, oversimplifying relationships and sometimes restricting findings to the study context. As a result, while many host, pathogen, and environmental factors have been identified as significant to koala health, their interconnections and relative importance remain unclear. This fragmented understanding hampers the identification of reliable indicators of koala health and disease. Currently, diagnosis, prognosis and definition of disease outcomes in research are largely based on clinical signs, while the complex interactions of coinfections, host responses and underlying stress physiology, all of which may impact fitness and eventual disease and treatment outcomes, remain unseen. The integrated approaches used in this thesis to capture the complexity of disease systems are founded conceptually on two key frameworks: the Damage-Response Framework and the Host-Pathogen- Environment (HPE) interaction (Casadevall & Pirofski, 2003; Fidel et al., 2020; Guégan et al., 2024; McNew, 1960; Scholthof, 2007). Analysis of the large number of variables involved was facilitated by development of novel analytical approaches that deepened our understanding of complex disease threats facing the koala, and thereby provided new knowledge to inform management of koalas in the clinic and in the wild.