Causes of mortality in koalas autopsied at the University of Melbourne

Abstract

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.

The koala, Phascolarctos cinereus, is an arboreal marsupial species endemic to Australia. ¹ , ² The species range includes scattered areas of Queensland, New South Wales (NSW) and the Australian Capital Territory (ACT) in the north; and Victoria and South Australia (SA) to the south. ² In addition to several documented morphological differences, ² the conservation status also differs between northern and southern koala populations: as of February 2022, the combined populations of Queensland, NSW and the ACT were listed as endangered. ² Habitat fragmentation and loss, climate change, fire, infectious disease and predation by introduced species have all been cited as contributing factors in the koala population decline. ¹ , ³ This is in stark contrast to regions of Victoria where overabundance has necessitated translocation efforts and fertility control. ⁴ , ⁵

Victorian koalas are unique even within their species: a history of intense hunting for the fur trade throughout the 19^th century significantly reduced genetic diversity in the state's population. ⁵ Even though Victorian koalas are not listed as endangered like their northern counterparts, their distribution across the state is inconsistent, with some areas densely populated (to the point of requiring intervention) and other areas arousing concern for declining numbers, such as the South Gippsland and Ballarat regions. ¹ , ⁵ , ⁶ Bushfires have also affected the Victorian koala population. ⁵ , ⁶

Literature suggests that motor vehicle accidents are the most significant noninfectious cause of injury and death in koalas. ⁷ There is also evidence that chlamydiosis is the most common infectious cause of morbidity and death. ³ , ⁸ It is also speculated that retroviral infection plays a significant role in morbidity and mortality, especially in Queensland and NSW where the prevalence of the virus is 100%, compared with Victoria, with only 73% prevalence. ³

Relatively few studies have examined major causes of mortality, and those that exist largely focus on Queensland and NSW populations. ⁹ , ¹⁰ , ¹¹ , ¹² These studies state that road accidents, chlamydiosis and predation are the biggest causes of morbidity and mortality in northern populations. ⁹ , ¹⁰ , ¹¹ , ¹² There is a significant lack of research into Victorian koala mortality. Only one study of similar nature has been undertaken on Victorian koalas. ¹³ This 1983 study reviewed the autopsy reports of 44 koalas submitted to the University of Melbourne from1975 to1980 and found motor vehicle accidents and clinical syndromes consistent with what we now know to be chlamydiosis to be the most prevalent morbidities. ¹³

This study aimed to investigate the causes of mortality in the Victorian koala population based on the findings of autopsies performed at the University of Melbourne. As the only academic institution with a veterinary school in the state of Victoria, the institution has been able to perform significant numbers of autopsies on relatively under‐investigated Victorian populations of koalas over many decades. To the authors' knowledge, this is the most comprehensive data analysis of mortality amongst Victorian koalas. Results provide insights into comparative pathology aspects and into certain diseases affecting koalas and may also be used as a point of comparison to koala populations of other states.

Materials and methods

Archival case retrieval

The University of Melbourne archives were searched for all koala autopsy reports produced from 1970 (start of records) to 31^st December 2021. Data extracted from the retrieved reports included: year of autopsy, age, sex, nutritional status, preservation state (fresh or autolysed), geographic location of origin, diagnoses offered (presumptive or definitive), manner of death (euthanased or deceased), recorded cause of death, gross and microscopic abnormalities described, and ancillary test results.

Source of the animals examined

Most animals were submitted in recent years as part of the Wildlife Health Victoria Surveillance programme led by one of the coauthors (P. Whiteley). Animals submitted were either found dead or moribund in the wild. If found moribund in the wild, the animals were either euthanased or were brought to a rescue centre, vet clinic or zoo and assessed. Most of these were euthanased the same or following day, in which case death was attributed to the condition that led to them being in a moribund state. A small number were submitted after planned culling in the wild. There was one report of a joey raised in human care (likely rescued as a neonate) that did not thrive and died while in human care of an undetermined cause. We found no other animal that was submitted from a zoo that was raised and died in captivity.

Determination of cause of death

The extracted data were tabulated and examined to determine which of the comorbidities was the primary cause of death in the context of each case examined.

Results

A total of 239 autopsy reports were found. The koalas originated from a wide range of regions across Victoria (Figure 1). The number of koala autopsies performed per decade was as follows: 32 autopsies in the period 1970–1979, with only one performed by 1975, 12 autopsies in the period 1980–1989, 7 during 1990–1999, 4 during 2000–2009, 158 during 2010–2019 and 22 in the 2020–2021 period. Only 55 koala autopsies were performed in the first 40 years to 2010.

Figure 1.

Map of Victoria showing the geographic origin and the number of koalas per area autopsied at the University of Melbourne from 1970 to 2023.

Of these, 50.6% were female (n = 121), 43.5% male (n = 104) and 5.9% had no sex recorded (n = 14); 14.6% (n = 35) of koalas were juvenile, 64.9% (n = 155) were adult and 20.5% (n = 49) had no age recorded; 57.3% (n = 137) of koalas had been euthanased, and where this was the case, the reason for euthanasia was determined to be the cause of death; 31% (n = 74) were found dead, and a further 11.7% of reports (n = 28) did not record the manner of death.

Twenty‐six different causes of death were identified, with an additional category of reports where no clear primary cause of death could be determined.

These 26 causes were grouped into four major categories: trauma, emaciation, presumptive chlamydiosis and ‘Other’, miscellaneous causes of mortality (Table 1). Presumptive chlamydiosis (15.9%), emaciation (15.9%) and trauma (14.6% of cases) were the leading defined causes of mortality. In 16.3% of cases, a cause of death could not be determined, meaning that 83.6% of autopsy reports provided a definitive diagnosis. Presumptive chlamydiosis was the greatest single disease aetiology to cause mortality, primarily due to the increased likelihood to lead to euthanasia.

Table 1.

Frequency and demographic breakdown of major causes of koala mortality per sex, age and manner of death category in Victoria from 1970 to 2023 in 239 koalas autopsied at the University of Melbourne from 1970 to 2023

Cause of mortality	Frequency	Proportion %	Sex				Age			Manner of death
			F	M	NR	Juv	Adult	NR	Died	Eu	NR
Trauma	35	14.6	20	13	2	8	22	5	12	21	2
Emaciation	38	15.8	19	16	3	5	25	8	15	21	2
Presumptive chlamydiosis	38	15.8	22	15	1	3	26	9	12	22	4
Other	89	372	46	40	3	13	57	19	20	53	16
Not determined	39	16.3	15	19	5	6	25	8	15	20	4
Total	239	100	121	104	14	35	155	49	74	137	28

F, Female; M, Male; NR, Not recorded; Juv, Juvenile; Eu, Euthanased.

Causes of trauma (n = 35) included motor vehicle accidents (n = 16), animal attacks (n = 5), falls (n = 2), a malicious act (n = 1) and were otherwise undetermined in 11 cases. All animal attacks were caused by dogs. The malicious act was gunshot wounds inflicted on an approximately 5‐month‐old male joey found near a river without other koalas nearby. The main finding was an air gun pellet lodged in the leptomeninges of the cerebellum and caudal cerebrum causing extensive malacia; pieces of skin and tufts of fur were forced into the cranial cavity; there were no other noteworthy findings. Causes of emaciation (n = 39) in koalas included senescence (n = 9), parasitism (n = 7), starvation (n = 6) and dental disease (n = 6) and were otherwise undetermined in 11 cases. Sarcoptes scabiei was the only parasitic cause of emaciation identified (n = 7).

Koalas that were classified as either having died or been euthanased due to presumed chlamydiosis (n = 39) had several consistent autopsy findings including cystitis, the so‐called ‘wet bottom’ due to chronic urinary incontinence and urine scalding, hydroureter, chronic nephritis, renal fibrosis, bursitis and keratoconjunctivitis.

In 89 cases in which death was attributed to causes other than emaciation, trauma or presumptive chlamydiosis, these causes included infectious disease other than chlamydiosis (n = 22), renal disease not attributed to chlamydiosis (n = 10), blindness not attributed to chlamydiosis (n = 8), parasitism other than cases in which parasitism resulted in emaciation (n = 10), neoplasia (n = 12), monitoring programme (n = 7) mechanical gastrointestinal disease (n = 6), lameness other than caused by trauma (n = 4), orphaned (n = 4), congenital abnormalities (n = 1), burns (n = 1), hypothermia (n = 1), haematopoietic disease (n = 1), cardiac disease (n = 1) and toxicosis (n = 1).

Infectious diseases or conditions other than presumptive chlamydiosis included: aspiration pneumonia (n = 4), bronchopneumonia (n = 2), pleuropneumonia (n = 1), chronic enteritis (n = 2), cholangiohepatitis (n = 1), hepatitis (n = 2), nephritis with Staphylococcus aureus isolated (n = 2) and severe dermatitis (n = 1); as well as pyometra (n = 3), peritonitis (n = 2), one of which followed pyometra rupture, focal embolic suppurative nephritis (n = 1) and pyelonephritis (n = 1), the latter conditions in which chlamydiosis cannot be entirely ruled out. Numerically, more female koalas were diagnosed with infectious disease as a cause of death than males; however, this was not statistically significant (P = 0.1, χ² test).

Neoplasms identified included female reproductive neoplasms (n = 5), squamous cell carcinoma (n = 2), lymphoma (n = 2), unidentified mammary mass (n = 1), rectal fibroleiomyoma (n = 1) and a round cell tumour of the ileum, not otherwise classified (n = 1).

The single case that was euthanased for a congenital condition was a juvenile female koala with a nonstrangulated abdominal hernia (Figure 2). Mechanical gastrointestinal diseases reported included gastrointestinal impaction (n = 3), small intestinal strangulation (n = 1), rectal prolapse and intussusception (n = 1) and rupture of the caecum (n = 1). Renal diseases or conditions included renal failure (n = 3), oxalate nephropathy (n = 2), renal calculi obstruction (n = 2), chronic fibrosis (n = 2) and pigmentary nephropathy (n = 1). Regarding haematopoietic disease, a single case of anaemia was interpreted to be due to an immune‐mediated haemolytic anaemia or immune‐mediated thrombocytopaenia type process. There were four cases of juvenile koalas being euthanased after being orphaned. Seven reports were from koalas that had been euthanased as part of population monitoring programmes. Lameness not attributed to trauma‐related deaths included severe arthritis (n = 3) and a case of degenerative arthropathy (n = 1). The single case of cardiac disease was a koala found to have hypertrophic cardiomegaly. The single case attributed to toxicosis was a koala that received an overdose of ivermectin to treat severe multifocal chronic dermatitis due to sarcoptic mange. Of the koalas that died or were euthanased as a result of parasitism, there were nine cases of sarcoptic mange and one case of facial myiasis. There were 16 cases in total in which the cause of death can ultimately be attributed to sarcoptic mange (seven and with and nine without emaciation respectively).

Figure 2.

Congenital focally extensive nonstrangulated caudal abdominal hernia in a juvenile female koala weighing 5.8 kg. The nutritional state was good. There was a 6 × 7.5 × 10 cm with a 17 cm circumference soft, black mass on the caudal abdomen, containing 55 cm of nonstrangulated caecum with 1.8 ml of brown, clear fluid, although there was a clear constriction point on the caudal third of the caecum. The distended stomach led to cranial displacement of the diaphragm and lungs. Also see ‘F62629, Nonstrangulated caudal abdominal hernia’, submitted by Panayiotis Loukopoulos. Accessible at: https://davisthompsonfoundation.org/image‐detail?image=F62629

Discussion

This study utilised koala autopsy reports produced by a single institution from a period of over 50 years (1970–2023). Though multiple studies have investigated causes of morbidity and mortality in northern koala populations, ⁹ , ¹⁰ , ¹¹ , ¹² , ¹⁴ , ¹⁵ this is the largest study into mortality drivers for Victorian koalas, and one of the largest overall for koalas. Limitations of this study include the inherent drawbacks of retrospective studies, particularly when dealing with wildlife, the variability in the sources of the animal submissions, the large period examined (53 years) and the somewhat surprisingly low number of koalas autopsied in that same period of time.

Trauma and emaciation were identified as leading causes of mortality in this study, each contributing approximately one sixth of total mortalities, although some may argue that emaciation is part of the process of death and not the underlying cause. Similar studies into koala mortality from both Victoria, ¹³ South Australia ¹⁶ and NSW, ¹⁴ have likewise found trauma to be the most common cause of mortality in their respective koala populations. While some studies from Queensland also reported that trauma is the leading cause of koala mortality, ¹⁰ others found that in south‐east Queensland chlamydiosis‐related signs were responsible for 52% of mortality, with trauma only the second most common cause at 15%. ¹¹ A South Australian study on autopsy findings ¹⁶ found that trauma and oxalate nephrosis were the predominant causes of death and/or disease in koalas from the Mount Lofty Ranges region. Death was attributed to oxalate nephrosis in only two cases in our study, which however did not examine morbidity; it is similarly not clear in how many of the South Australian cases mortality was attributed to this condition.

A significant portion (43%) of all trauma‐related deaths in this study was determined to be the result of motor vehicle accidents. Existing studies agree that motor vehicle accidents are the greatest contributor to traumatic deaths. ¹⁰ , ¹¹ , ¹² , ¹³ , ¹⁴ The next most significant cause of trauma was animal attack, a finding that is also in agreement with existing literature. ¹⁰ , ¹¹ , ¹³ , ¹⁴ All animal attacks in this study were caused by dogs.

The proportion of koalas autopsied in which emaciation was determined to be the cause of death (15.5%) is overall similar to what has been found in two similar Queensland studies by Gonzalez‐Astudillo et al, where 14.3%, ¹¹ and 7.7%, ¹⁰ of deaths were attributed to wasting. Within the category of emaciation, cases of undetermined cause represented the most common subcategory, closely followed by senescence and parasitism. Interestingly, in their 1983 study on causes of mortality and morbidity in Victorian koalas, ¹³ Obendorf did not include emaciation or wasting as a major cause of koala death, although they did describe a set of clinical signs which they referred to as ‘koala stress syndrome’. This syndrome contributed to 23.6% of mortality in their report and encompassed such nonspecific antemortem findings as ‘depression, anorexia and a precipitous decline in metabolic function’, ¹³ that may refer to emaciation.

Presumptive chlamydiosis was one of the three most common mortality causes together with trauma and emaciation. It is well documented that chlamydiosis disease has impacted koala populations throughout Australia, ⁸ , ¹⁷ commonly presenting as urogenital disease, ocular and rhinitis‐pneumonia syndromes. ¹¹ Urogenital disease can result in infertility and deterioration and death or euthanasia. With ocular presentations, chronic conjunctivitis may eventually result in blindness and thus inability to forage and eventual starvation. In this study, blindness was determined as the cause of death in eight koalas, of which seven were euthanased. It was not possible to definitively conclude that these cases of blindness were caused by chronic ocular chlamydiosis; however, the possibility cannot be entirely excluded, meaning that the true proportion of deaths due to chlamydiosis may have been higher than stated here but also possibly underestimated due to counting all ‘wet bottoms’ as presumptive urogenital chlamydiosis. Furthermore, cases of pneumonia and some forms of renal and urogenital disease described in the category of ‘Other’ causes of mortality may also have been attributable to chlamydiosis; however, the autopsy report findings were not specific enough to include these as chlamydiosis cases with any confidence. Chlamydiosis was only reported here when it was assessed to be the primary cause of death. The prevalence of chlamydiosis or any other disease in Victorian koalas is not described here.

The findings of this paper are generally in agreement with existing literature from Queensland, where chlamydiosis is either the leading cause of mortality, ¹¹ or a close second to trauma‐related death, ¹⁰ and the most significant infectious cause of mortality. ¹⁰ , ¹¹ , ¹⁶ While older papers from NSW, ¹⁴ and Victoria ¹³ may not describe chlamydiosis specifically, they do describe a high incidence of urogenital tract disease and keratoconjunctivitis, which are highly suggestive of chlamydiosis.

Studies from south‐east Queensland have also suggested that female koalas are more frequently diagnosed with chlamydiosis than males and are more likely to be euthanased because of the disease. ¹⁰ , ¹¹ In this study, 55.6% of koalas autopsied with a diagnosis of presumptive chlamydiosis were female, whereas 41.7% were male, a ratio consistent with the existing literature.

Aside from these three major categories of mortality—trauma, emaciation and presumptive chlamydiosis—there were several miscellaneous causes of death that comprised smaller contributions to overall mortality.

Infectious diseases other than chlamydiosis were found to be the cause of 10% of overall mortality. The specific diseases identified are similar to those described by Backhouse et al. ¹⁵

Neoplasia contributed to 5% of overall mortality. Reproductive tract neoplasms were the most frequent diagnosis, followed by only two cases of lymphoma and a single round cell tumour. This may contrast with existing studies which generally agree that lymphoma and round cell tumours are the most prevalent neoplasms in koalas. ¹⁴

It is possible that the low incidence of lymphoma found in this study, as compared with studies of northern populations, reflects the difference in prevalence of Koala retrovirus (KoRV) between geographic regions and subpopulations. KoRV is more prevalent in Queensland and New South Wales populations, where it is believed to be endogenous, than in southern populations.^18,19,20 On the Victorian mainland, KoRV was found to have a prevalence of only 72.2%, with a lower prevalence on Victorian islands at 26.6%,²⁰ and is therefore postulated to remain exogenous in Victorian koala populations. ¹⁸ KoRV can have an immunosuppressive effect on koalas and may lead to increased incidence of neoplasia, especially lymphoma and leukaemia. ¹⁸ , ¹⁹

The autopsies in this study were undertaken over a period of 52 years. Individual variation between pathologists, and changes in understanding of koala pathology over the years, as well as the primary purpose of the submission, may affect the accuracy of results. Many koala carcasses were also in a poor preservation state. Detecting, preserving and transporting carcasses for postmortem examination is a challenge in any wildlife study, and unfortunately, advanced decomposition and freezing and thawing artifacts may obscure lesions potentially present. Some autopsies were also undertaken as part of research into specific disease processes (e.g., studies on koala ectoparasites, potentially leading to overrepresentation of sarcoptic mange) or as part of population welfare management programmes—introducing a degree of bias to the data. It is also necessary to consider that some causes of mortality are far more visible to the public and are therefore more likely to be submitted for diagnostic investigation. Car accidents, for example, are a highly visible cause of koala death. Koalas that are killed in road accidents may be more likely to be submitted for autopsy, potentially leading to an overrepresentation of trauma as a cause of koala mortality.

Despite these limitations, the findings of this study are valuable as the most comprehensive analysis of Victorian koala mortality to date.

Conflicts of interest and sources of funding

The authors declare no conflicts of interest or sources of funding for the work presented here.

Wilson, L. , Whiteley, P. , Stent, A. , Stockman, F. , Elloy, F. , Hobbs, EC. , Astudillo, VG. , Jubb, KVF. and Loukopoulos, P. , Causes of mortality in koalas autopsied at the University of Melbourne. Aust Vet J. 2025;103:500–505. 10.1111/avj.13451

Data availability statement

The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.