Analysis of morbidity and mortality of wild koalas in south-east Queensland using passive surveillance data

Abstract

The koala is an iconic marsupial threatened by habitat loss, trauma, droughts, and diseases. The Australian state of Queensland was estimated to hold the largest koala population during the 1990s; however, this number has nearly halved in two decades, with declines of up to 80%, particularly in SouthEast Queensland (SE QLD). Despite this alarming reduction, the relative contribution of threats causing this decline in SE QLD had not been quantitatively assessed. In addition, information on the spatialtemporal distribution of morbidity and mortality cases is lacking for SE QLD, along with competing injury and disease risks. To address this issue, a retrospective epidemiological study from cases submitted to hospitals (1997-2013) was performed. The analysis included N=20,250 records, highlighting that most koalas arrived dead (48%), followed by other being euthanized (35%), and a smaller number being released (17%). Trauma by motor vehicles, urogenital chlamydiosis and low body conditions were the leading causes of admission and frequently co-occurred. Of concern was that 37% of koalas were injured but otherwise healthy, and that urogenital chlamydiosis, which causes infertility, overwhelmingly affected females. Multinomial logistic regression models identified year of admission, age class, sex and season as risk factors significantly influencing koala outcomes for main clinical syndromes assessed. Exploratory space-time permutation scans detected significant clusters of trauma, chlamydiosis and low body condition in specific local government areas. In this study, passively-acquired data provided insight into the complex interplay of threats impacting koalas in SE QLD. Although hospitals have been providing specialised care to koalas for decades, there is no standard nomenclature to record admissions, complicating the estimation of causes of mortality and the incidence of comorbidities. Additionally, autopsies are frequently not conducted in a systematic manner, hampering the identification of comorbidities and disease interactions in the SE QLD population decline. A literature search for the commonly documented morbidity and mortality causes for koalas was used to categorise standardised nomenclature. This was followed by a large scale autopsy survey (2013-2016) including N=519 koala autopsies from carcasses derived from main SE QLD hospitals. Demographically, there was a higher number of mature koalas of breeding age admitted with infections and trauma compared to young/subadult or senescent koalas. Trauma by motor vehicles, urogenital and ocular