Causes of mortality in koalas autopsied at the University of Melbourne.

Objective: To determine the causes of mortality in koalas autopsied at the University of Melbourne based on the retrospective analysis of autopsy reports.

Methods: The autopsy reports of 239 koalas examined at the University of Melbourne from 1970 to 2023 were reviewed to determine which of the comorbidities present was the primary cause to which death could be attributed.

Results: Infectious disease (62 cases) including presumptive chlamydiosis (38 cases), emaciation (38 cases) and trauma (35) were the leading cause of mortality. Death was attributed to other causes in 89 cases, whereas the cause was not definitively determined in 39. Twenty-six different causes of death were identified overall. Presumptive chlamydiosis was the greatest single disease to cause mortality, primarily due to the increased likelihood of leading to euthanasia. Traumatic causes included motor vehicle accidents (16 cases), animal attacks (all from dogs; 5 cases) and malicious acts (1 case). Emaciation resulting in death was attributed to senescence (9), sarcoptic mange (7), starvation (6) and dental disease (6). Sarcoptic mange caused mortality, with or without emaciation, in 16 cases. There was one case each of congenital abdominal hernia, cardiomyopathy, burns and ivermectin toxicosis, and two of oxalate nephropathy. Neoplasms were the cause of mortality in 12 koalas and included five female reproductive tract neoplasms and, importantly, only two lymphomas, in contrast with previous studies in which lymphoma is the most prevalent neoplasm.

Conclusion: This is the most comprehensive data analysis of mortality amongst Victorian koalas, and one of the few autopsy studies on koalas. Infectious disease including presumptive chlamydiosis, emaciation and trauma were the leading causes of mortality amongst koalas submitted for autopsy at this institution.