Decline causes of Koalas in South East Queensland, Australia: a 17-year retrospective study of mortality and morbidity

Abstract

Koala populations are in catastrophic decline in certain eastern Australian regions. Spanning from 1997-2013, a database derived from wildlife hospitals in southeast Queensland with N = 20,250 entries was classified by causes of morbidity and mortality. A total of 11 aetiologies were identified, with chlamydiosis, trauma, and wasting being most common. The clinical diagnosis at submission varied significantly over the observation period. Combinations of aetiologies were observed in 39% of koalas submitted, with chlamydiosis frequently co-occurring. Urogenital (cystitis 26.8%, bursitis 13.5%) and ocular (conjunctivitis 17.2%) chlamydiosis were the most frequently diagnosed representations of the infection. Approximately 26% of submissions comprised koalas involved in vehicle accidents that were otherwise healthy. Age and sex of the koala as well as season and submission period were compared for the case outcomes of 'dead on arrival', 'euthanized', or 'released' for the four most common clinical diagnoses using multinomial logistic regression models. Exploratory space-time permutation scans were performed and overlapping space-time clusters for chlamydiosis, motor vehicle traumas and wasting unveiled high risk areas for koala disease and injury. Our results suggest that these aetiologies are acting jointly as multifactorial determinants for the continuing decline of koalas.

Figure 1. Total counts and proportions of aetiologies Chlamydia-like signs, trauma caused by motor vehicles, and wasting occurring in koalas submitted to wildlife hospitals in SEQLD from 1997–2013 (No. of occurrences of aetiologies = 41,606).

Figure 2. Locations of significant clusters of aetiologies as determined by the space-time permutation scans.

(a–c) The area comprised within the rectangle includes the South East Queensland study area for the space-time permutation. The following map inserts display the significant clusters by the space-time permutation scans: (a) trauma by motor vehicle, (b) Chlamydia-like signs, (c) wasting. Each cluster is labelled with its p-value, number of observations, and time frame. Maps generated using ESRI ArcGIS Desktop v10.2.1. Centre points for each cluster generated using SaTScan^TM v8.0. SaTScan^TM is a trademark of Martin Kulldorff. The SaTScan^TM software was developed under the joint auspices of (i) Martin Kulldorff, (ii) the National Cancer Institute, and (iii) Farzad Mostashari of the New York City Department of Health and Mental Hygiene.

Figure 3. Proportion of top four koala diagnoses (Trauma by motor vehicle, Chlamydia-like signs, clinical syndrome combination of Chlamydia-like signs & wasting, and trauma by animal attacks) affecting koalas submitted to wildlife hospitals in SEQLD from 1997–2013 with a CI 95% (No. koalas with diagnosis = 20,250).

Figure 4. Outcomes (dead on arrival, euthanized, released) for koalas submitted to wildlife hospitals by year of submission between from 1997–2013 (No. koalas with clinical syndromes diagnosed = 20,250).

The following figures display the (a) Proportion of outcomes by year of submission and (b) Counts of outcomes by month of submission.