Koala retrovirus, a recent discovery in Australian koalas, is endogenised in 100% of northern koalas but has lower prevalence in southern populations, with lower proviral and viral loads, and an undetermined level of endogenisation. KoRV has been associated with lymphoid neoplasia, e.g., lymphoma. Recent studies have revealed high complexity in southern koala retroviral infections, with a need to clarify what constitutes positive and negative cases. This study aimed to define KoRV infection status in Mount Lofty Ranges koalas in South Australia using RNA-seq and proviral analysis (n = 216). The basis for positivity of KoRV was deemed the presence of central regions of the KoRV genome (gag 2, pol, env 1, and env 2) and based on this, 41% (89/216) koalas were positive, 57% (124/216) negative, and 2% inconclusive. These genes showed higher expression in lymph node tissue from KoRV positive koalas with lymphoma compared with other KoRV positive koalas, which showed lower, fragmented expression. Terminal regions (LTRs, partial gag, and partial env) were present in SA koalas regardless of KoRV status, with almost all (99.5%, 215/216) koalas positive for gag 1 by proviral PCR. Further investigation is needed to understand the differences in KoRV infection in southern koala populations.

To determine Phascolarctid gammaherpesviruses (PhaHV) infection in South Australian koala populations, 80 oropharyngeal swabs from wild-caught and 87 oropharyngeal spleen samples and swabs from euthanased koalas were tested using two specific PCR assays developed to detect PhaHV-1 and PhaHV-2. In wild-caught koalas, active shedding of PhaHV was determined by positive oropharyngeal samples in 72.5% (58/80) of animals, of which 44.8% (26/58) had PhaHV-1, 20.7% (12/58) PhaHV-2 and 34.5% (20/58) both viral subtypes. In the euthanased koalas, systemic infection was determined by positive PCR in spleen samples and found in 72.4% (63/87) of koalas. Active shedding was determined by positive oropharyngeal results and found in 54.0% (47/87) of koalas. Koalas infected and actively shedding PhaHV-1 alone, PhaHV-2 alone or shedding both viral subtypes were 48.9% (23/47), 14.9% (7/47) and 36.2% (17/47), respectively. Only 45.9% (40/87) were not actively shedding, of which 40.0% (16/40) of these had systemic infections. Both wild-caught and euthanased koalas actively shedding PhaHV-2 were significantly more likely to be actively shedding both viral subtypes. Active shedding of PhaHV-2 had a significant negative correlation with BCS in the euthanased cohort, and active shedding of PhaHV-1 had a significant positive relationship with age in both wild-caught and euthanased cohorts.

In the wake of increasingly frequent bushfires emerging as a threat to wildlife worldwide, koalas have notably been the most rescued species in Australia. However, our understanding of burns and their severity in koalas is limited; hence, this study investigated the histopathological features and depth of burns in koala skin, as well as the presence of smoke-induced respiratory tract damage. In four bushfire-affected koalas that had been euthanised on welfare grounds, skin burns in various body regions were scored based on clinical appearance as superficial, partial thickness, or full thickness. Histological sections of affected regions of skin were assessed as Grades I–IV and showed that furred regions on the ear margins and dorsum were histologically more severe, at Grade III, compared with the clinical score. There was a similar finding for footpad burns, which were the most common body region affected. In the respiratory tract, pulmonary oedema and congestion were evident in all koalas. Overall, the results highlight that cutaneous burn lesions on furred and palmar/plantar surfaces can have higher severity based on the burn depth than is clinically apparent. Therefore, there is a need to consider this when developing treatment plans and establishing prognosis for burnt koalas at triage, as well as that a high likelihood of pulmonary oedema exists.

Pneumonia has been reported in both free-ranging and captive koalas and a number of causative agents have been described. Between 2016 and 2019, 16 free-ranging and 1 captive koala (Phascolarctos cinereus) from the Mount Lofty Ranges of South Australia were identified with pyogranulomatous lobar pneumonia, which involved the left caudal lobe in 14/17 (82%) cases. Within lesions, numerous gram-positive or gram-variable, non-acid-fast filamentous bacteria were observed in association with Splendore-Hoeppli phenomenon. Culture yielded growth of anaerobic bacteria, which were unidentifiable by MALDI-TOF-MS (matrix-assisted laser desorption ionization-time of flight mass spectrometry) analysis in 5/5 cases. Sequencing of the bacterial 16S rRNA gene identified a novel Actinomyces species in 4 samples, confirming a diagnosis of pulmonary actinomycosis. Concurrent examination of resin lung casts from healthy koalas suggested greater laminar flow of air to the left caudal lung lobe in koalas. Actinomyces spp. have been reported as commensals of the oral microbiome in other species, and an association with similar pulmonary lesions in other species. Considering the predilection for involvement of the left caudal lung lobe, aspiration is suggested as the likely cause in some cases of pulmonary actinomycosis in koalas. Pulmonary actinomycosis has not been previously described in koalas and further work needs to be undertaken in order to classify this organism within the Actinomyces genus.

The pathogenesis of koala retrovirus (KoRV) has been explored in various contexts, yet its role in tumorigenesis remains incompletely understood. Unlike acute transforming retroviruses, KoRV lacks a viral oncogene but may contribute to oncogenesis via indirect mechanisms. However, the relationship between KoRV and telomere length, as a potential indicator of telomerase activity, has not been examined. This study investigates the effect of KoRV infection on telomere length in 47 samples from Southern Australian koalas in a novel telomere length quantification method. Telomere lengths of 30 KoRV-negative samples were compared to those of 17 KoRV-positive samples using the Absolute Human Telomere Length Quantification qPCR kit (ScienCell Research Laboratories, California, USA). The telomere length in KoRV-infected WBCs was significantly longer than the uninfected ones (t = −2.059, p-value = 0.045). In line with this, telomere length correlated positively with proviral load (r = 0.421, p-value = 0.003), further linking viral burden to telomere elongation. Furthermore, the effect of age on telomere length differed by infection status (β = −5329.7, p-value = 0.0038); KoRV-positive individuals exhibited longer telomeres at a younger age but experienced more rapid telomere attrition over time compared to KoRV-negative individuals. These results suggest KoRV promotes telomerase elongation ability and modulates age-related telomere dynamics, potentially contributing to subsequent cellular immortality and oncogenesis. These pathways may overlap with other retroviruses, where telomerase dysregulation contributes to their oncogenic potential. This study provides new insights into KoRV pathogenesis and DNA quantification methodology, which could be valuable for future research by identifying predictive markers for tumour progression and potential therapeutic targets in affected koalas.