ONCOLOGY OF THE SUGAR GLIDER ( PETAURUS BREVICEPS ): A COMPREHENSIVE REVIEW

Sugar gliders ( Petaurus breviceps ) are increasingly being kept as pets. Despite their popularity, relatively little is known about the oncology of this species. To summarize the current body of knowledge, a literature search was performed to identify published cases. Only a small number of cases was identified that described cancer in sugar gliders. The identified case reports described several tumour types, ranging from mammary (adeno) carcinoma to haemangiosarcoma. In the majority of cases, no metastases were reported. In several cases, the primary tumour was removed using a surgical approach. Pain management also was a cornerstone of the treatment. The prognosis was often poor, with only two animals out of eight being alive at the time of publication. It is clear that further research is required to get more insight into the epidemiology and characteristics of cancer in this species.


INTRODUCTION
The sugar glider (Petaurus breviceps) is a small, nocturnal marsupial originally from New Guinea and Australia (Campbell et al., 2019;Varriale et al., 2019). These animals are kept as an exotic pet in several countries such as Italy (Varriale et al., 2019) and the United States (Campbell et al., 2019). Therefore, several studies have investigated important aspects of husbandry such as nutrition (Dierenfeld, 2009) and behavioural aspects (Mallick et al., 1994). Work has also been published on medical aspects such as common emergencies (Lennox, 2007;McLaughlin and Strunk, 2016), common procedures (Johnson-Delaney, 2006), surgical procedures (Morges et al., 2009;Miwa and Sladky, 2016) and diagnostic approaches (Evans and Souza, 2010). Cusack and colleagues (2016) published a case of bilateral hydronephrosis, while Johnson-Delaney and Lennox (2017) investigated reproductive disorders. Other conditions, where case reports or series were published, include pulmonary hyalinosis (Sokol et al., 2017), zoonoses (Pignon and Mayer, 2011), toxoplasmosis (Barrows, 2006) and cancer (Gentz et al., 2003;Sokol et al., 2017;Son et al., 2021). Several cases of neoplasia in sugar gliders have been described. Gentz and colleagues (2003) described 33 cases of neoplastic lesions of different histological types. In a more recent retrospective study, three cases of cancer in gliders were identified (Sokol et al., 2017). These studies did, however, not describe the case characteristics, such as symptoms and treatment in detail. Relevant details have been described in single case reports (Churgin et al., 2015;Chen et al., 2018). To this date, no review has been published. Therefore, this paper aims to provide a comprehensive review of the current knowledge of cancer in the sugar glider.

SEARCH STRATEGY AND LITERATURE INCLUSION
The Medline database (through PubMed) was searched using the keywords "Sugar glider*" (103 results) and "Petaurus" (85 results). Additionally, Google Scholar was searched using "Sugar glider tumor" (724 results), "Sugar glider cancer" (1970 results), "Petaurus tumor" (520 results) and "Petaurus cancer" (453 results). The titles of the publications in the search results were screened for potential eligibility for inclusion. The abstracts of these selected papers were read and the papers were included if they (1) described cancer in at least one sugar glider, (2) they mentioned at least basic information about the animal, (3) mentioned at least basic information about the disease and (4) mentioned at least basic information about the treatment and outcome. Relevant information was then extracted from the publication.
Two case series (Gentz et al., 2003;Sokol et al., 2017) described several cases retrospectively, but did not meet the inclusion criteria because they did not report any information on the treatment pursued. No other case series were identified. Additionally, no studies investigating any risk factors for neoplastic disease in sugar gliders were identified. Finally, no prospective trials were identified that assessed potential treatment strategies.

CASE CHARACTERISTICS AND TREATMENT
The ages of the animals at presentation ranged from 4 (Hough et al., 1992) to 15 years (Lindemann et al., 2016), while the average age was 8.5 years. Three out of the eight animals were male. Little additional consistent information about the animals was mentioned. Two cases of mammary (adeno)carcinoma were reported (Keller et al., 2014;Churgin et al., 2015). Other types of cancer ranged from lymphosarcoma (Hough et al., 1992) to hemangiosarcoma (Rivas et al., 2014). In the majority of cases, no metastases were reported. Symptoms were very variable and depended on the type and location of a tumour. Likewise, the treatment also varied significantly according to the specific case. However, surgical removal of the primary tumour was performed in several cases (Keller et al., 2014;Rivas et al., 2014;Chen et al., 2018). In several cases, antibiotic treatments were also initiated. It is also clear that pain management is very important.
Unfortunately, in five cases (Hough et al., 1992;Marrow et al., 2010;Keller et al., 2014;Churgin et al., 2015;Lindemann et al., 2016), the animal was eventually euthanized at the time the case was published. In one further case, the animal died (Son et al., 2021). In only two cases, the glider was alive without signs of cancer (Rivas et al., 2014;Chen et al., 2018

DISCUSSION AND CONCLUSION
A relatively small number of case reports was identified about cancer in sugar gliders. This means that there is a clear lack of external and internal validity in the current body of literature.
There was a wide range of ages at presentation. However, with an average of 8.5 years, it could be stated that it seems that age is a potential risk factor for cancer in gliders. There was a relative equal distribution between males and females. However, based on these limited cases, it is very difficult to make sound conclusions about potential risk factors and epidemiology of the condition. There were several different histological types of cancer described in the case reports. Previous case series have reported more cases and could provide a better overview of the most common types of cancer that occur in sugar gliders (Gentz et al., 2003;Sokol et al., 2017). In most cases, there were no metastases reported. However, it is not clear which mechanisms play a role in metastazing in this species and which types may be predisposed to metastasize. Several treatment strategies were utilized depending, amongst others, on the type and location of the primary tumour. Based on the current results, it does not seem feasible yet to develop standardized, evidence-based treatment guidelines for specific cancer types. It is, however, clear that surgical excision is an important component of several treatment schedules. Unfortunately, most of the animals were euthanized or died, while only two animals were alive at the time of publication of their case. This confirms that further research is required to assess and establish therapeutic strategies. Further research could start with a better reporting of cases. An important role could be played by communities of owners. These communities are very active in discussing health problems with their pets. They could be approached to assist in the recording of cases. Additionally, retrospective studies could be conducted of cases already seen in veterinary practices. Finally, prospective clinical trials should be conducted to establish effective and save therapeutic strategies.
Relatively few case reports were identified regarding cancer in sugar gliders. There was a large variation in the age at which the animals were diagnosed and in the tumour types that were described. In several of the published cases, surgical removal of the primary tumour was performed. This was often complemented with pain management. Unfortunately, prognosis was very poor, since only two out of eight animals were alive at the time of publication. Further research is required to get more insight into the epidemiology and characteristics of cancer in this species.