Studies into the diagnosis, treatment and management of chlamydiosis in koalas

Abstract

Koalas are an iconic Australian marsupial species that attract much public sympathy and support. Despite several thousand koalas being presented to wildlife rehabilitation facilities annually for treatment of traumatic injuries (primarily motor vehicle strikes and dog predation) and disease (principally chlamydiosis), little information exists regarding the success of treatments or whether rehabilitated animals survive in the wild after release. This thesis examines several aspects of the diagnosis, treatment and management of the most important infectious disease of koalas, chlamydiosis, and provides an evidence base for rational diagnostic and treatment decisions in the rehabilitation setting. Experimental work commences in Chapter 2 with a study of the admission records of a large koala rehabilitation facility (the Koala Hospital of the Koala Preservation Society of NSW) showing that traumatic presentations and those relating to clinical chlamydiosis were most common, with motor vehicle collisions apparently a significant and increasing threat to survival of the local koala population. The implications of these findings are discussed with reference to measures aimed at maintaining a viable population of wild koalas in Port Macquarie and for logistic planning at the Koala Hospital. Initial studies in this thesis confirmed that koalas with chlamydiosis are frequently treated at wildlife rehabilitation facilities. Despite the commonness of this disease, there is a lack of rigorous scientific studies examining frequently used treatments. Chapter 3, a retrospective review of medical records of a cohort of koalas admitted for treatment for chlamydiosis, revealed that diagnostic and treatment decisions were frequently based on clinical signs alone and treatment choices and durations were inconsistent with those used to successfully treat chlamydiosis in other species. Despite this, treated animals were frequently released and many survived in the wild. Chapter 4 outlines general methods common to the clinical work undertaken in Chapters 5, 7 and 9. Antibiotic treatment with drugs commonly used to treat chlamydiosis in other species (erythromycin, oxytetracycline) has led to wasting and death in koalas. A pilot study, presented in Chapter 5, found that, similarly, more modern forms of these drugs (doxycycline and azithromycin) cannot be used safely in koalas, leading to the author’s decision to investigate, in detail, the efficacy of the less conventional anti-chlamydial drugs, the fluoroquinolones. Studies of marsupial pharmacokinetics are uncommon and, prior to this thesis, there were no published studies of pharmacokinetics in koalas. The author’s investigations, using a modified agar diffusion assay (Chapter 6) and high performance liquid chromatography (Chapter 7), found the absorption of enrofloxacin and marbofloxacin by the oral route in koalas was extremely poor and suggested absorption rate limited disposition pharmacokinetics. In combination with plasma protein binding of approximately 50%, the concentrations of enrofloxacin and marbofloxacin achieved in plasma were not considered likely to inhibit the growth of chlamydial pathogens in vivo. In Chapter 8 the author explored the apparent contradiction between the failure to achieve appropriate plasma concentrations of fluoroquinolones to treat chlamydiosis and the apparent efficacy of these drugs reported in historical medical records. Methods to monitor clinical signs by clinical scoring and chlamydial load using real-time polymerase chain reaction were developed during the study. The results of these studies showed that clinical signs were poorly sensitive in determining the presence of chlamydial organisms in koalas; all fluoroquinolone treatment regimes led to a dramatic reduction in Chlamydophila pecorum load during treatment; and clinical signs improved in many animals. Importantly, however, pathogen load rebounded after withdrawal of treatment, indicating that most animals failed to clear infections. These findings have implications for the diagnosis, and treatment of chlamydial disease in koalas and for the subsequent return of fluoroquinolone treated animals to the wild. The findings and limitations of these studies are presented in general terms in Chapter 9 and recommendations for future studies are proposed.