KOALAS
Koalas are susceptible to neoplasia, in particular lymphoid malignancies (Canfield et al. 1987a; Blanshard and Bodley 2008; Gillett 2014). The incidence varies considerably between populations and geographical locations.
A review of over 20 000 presentations to three wildlife hospitals in Qld found that 1% of koalas had detectable neoplasia at the time of presentation (Gonzalez-Astudillo et al. 2017). Of 279 (managed and free-ranging) koala necropsies in the ARWH (1998-2018), 20.4% had neoplastic disease at death. Previous studies report incidences at necropsy of 7.3-56% (Canfield et al. 1987a; Canfield 1990; Gillett 2014; Gonzalez-Astudillo et al. 2019). In a cohort of koala retrovirus (KoRV) positive koalas neoplasia was found at necropsy in 13.4% of Qld based koalas (9/67) and in 5.4% of SA based koalas (5/92) (Fabijan et al. 2020).2.1 Lymphoid neoplasia
Koalas are highly susceptible to haemopoietic neoplasia, particularly lymphoid neoplasms. In the ARWH (19982018), 16.1% of all koalas had lymphoid neoplastic disease (ARWH 2018). One managed population in south-eastern Qld reportedly had an 80% incidence of haemopoietic neoplasia at necropsy (Hanger et al. 2000). KoRV positive koalas from Qld and SA had an incidence of lymphoid neoplasia at necropsy of 7.5% (n = 5/92) and 4.3% respectively (n = 4/92). Lymphoid neoplasia was only observed in koalas with high KoRV loads in these cohorts (Fabijan et al. 2020).
Both viral infection, primarily KoRV (see Chapters 33 and 34), and genetic factors have been considered as possible predisposing factors. Northern (NSW, Qld) koalas are considered to have an increased incidence of lymphoid neoplasia in comparison with southern koalas (Gillett 2014).
Lymphoid neoplasia in koalas is typically a lymphosarcoma with a predilection for extension to the circulation (i.e. leukaemic lymphosarcoma).
Lymphosarcoma in koalas has been categorised into four morphologic types: leukaemic; multicentric; alimentary; and focal (Spencer and Canfield 1996). Of 45 cases of lymphosarcoma in koalas in the ARWH (1998-2017), the majority were leukaemic (n = 23) or multicentric (n = 19) (ARWH 2018).2.1.1 Clinical signs
Lymphoid neoplasia can be highly variable in presentation and disease progression can be acute or chronic (Spencer and Canfield 1996; Blanshard and Bodley 2008; see Chapter 33). Most commonly the signs are non-specific (loss of body condition and weight, lethargy, inappetence), though
Table 18.2. Outcome of surgical treatment of mammary neoplasia in marsupials
| Neoplasm | Case details | Age at surgery (longevity in managed care) |
| Tasmanian devil (Sarcophilus harrisii) | ||
| Adenoma, complex1a | Excisional biopsy. Recurrence of a mass at the same location occurred 3 yr later. Euthanased due to an unrelated illness. | 4 yr (7-8 yr)2 |
| Carcinoma-in-situ1b | Excisional biopsy, inclusive of nipple. 12 mo following surgery developed masses in the contralateral mammary gland. Excisional biopsy and diagnosed as mammary hyperplasia. Died 20 mo after the 1st mass was excised from an unrelated illness and no sign of mammary neoplasia or metastasis | 5 yr |
| Carcinoma-in-situ1c | Mass excised. 7 mo later euthanased for unrelated illness | 6.5 yr |
| Carcinoma, bilateral1d | Bilateral complex carcinomas and a unilateral papillary cystadenoma were excised with pouch ablation. Euthanased 8 mo following surgery due to weight loss and hind limb ataxia. Metastasis of carcinoma to the liver was identified and presumed to be from a mammary primary. | 7 yr |
| Eastern ring-tailed possum (Pseudocheirus peregrinus) | ||
| Carcinoma1e | Solid carcinoma excised by marginal excision. 1 mo later contralateral mammary nodule noted to be present and growing: pouch ablation performed and de novo mass diagnosed as adenomyoepithelioma. Euthanased 2 yr following surgery for unrelated illness with no evidence of recurrence. | 7 yr (7-8 yr)3 |
| Red kangaroo (Osphranter rufus) | ||
| Adenocarcinoma, metastatic1f | Mammary mass noted, excised with local lymph node. Metastasis to node confirmed by histopathology. Euthanased 10 d after surgery. | 9 yr (25 yr)4 |
1ARWH 2018 case nos a11792.1; b11127.1; c11637.1; d12162.1; e8777.3; f11279.2; 2Holz 2008; 3Collins 1973; 4Weigl 2005
Table 18.3. Recommended diagnostic tests for lymphoid neoplasia in koalas (Phascolarctos cinereus)
| Diagnostic sample | Technique | Findings indicative of lymphosarcoma |
| Blood sample | Blood smear Total blood count | Presence of lymphoblasts or other abnormal WBCs, relative lymphocytosis Lymphocytosis (relative or actual), non-regenerative anaemia |
| Lymph node/mass fine needle aspirate | Cytology | Presence of abnormal lymphoid cells in large numbers Interpretation may require input from an experienced cytopathologist, as lymphoid cells in reactive lymph nodes can be difficult to differentiate from lymphoid neoplasia in some cases |
| Pleural or peritoneal fluid | Cytology | Presence of abnormal lymphoid cells and/or lymphocytosis |
Iymphadenomegaly or detectable masses at any location should be considered highly suspicious. Given the frequency of this disease in koalas, lymphoid neoplasia should be excluded in any adult koala with unexplained, non-specific illness.
2.1.2 Diagnostics
The two most important diagnostic samples are blood and lymph node or mass aspirates (Blanshard and Bodley 2008) (Table 18.3). Approximately 50% of affected koalas have circulating neoplastic cells (leukaemic lymphosarcoma) that are detectable on a blood smear or reflected in a WBC count. Cytological examination should be performed of fine needle aspirates from any clinically accessible enlarged lymph nodes or masses, or from free thoracic or abdominal fluid (see Chapter 33).
2.1.3 Treatment and management
Treatment is rarely attempted and the only reports of successful treatment are limited to surgical resection of focal masses in two cases (Hanger 2003; Blanshard and Bodley 2008).
Most of koalas with lymphoid neoplasia are diagnosed in end-stage disease or at necropsy. In 41 cases of koalas with clinical histories that were reviewed for descriptions of treatment (ARWH 1998-2017, n = 40 and (Kido et al. 2012, n = 1), 20 (49%) were diagnosed at necropsy, including all cases of focal (n = 3) and alimentary lymphosarcoma (n = 1); 12 koalas (29%) were euthanased at first clinical presentation based on a clinical diagnosis or strong suspicion of lymphoid neoplasia and 5 koalas (12.2%) were given supportive care only and died on average 14 d later (range 3-35 d). One animal had surgical excision of a cervical mass, but died 7 d later and had lymphoid leukaemia at necropsy (ARWH 2018 case no. 6065.1). One animal diagnosed clinically with lymphoid leukaemia was treated with prednisolone, but died 15 d later (ARWH 2018 case no. 2477.2).
It is likely that early diagnosis is an important element in successful treatment by surgical resection, given that the majority of lymphoid neoplasms are multisystemic or leukaemic by the time of death (93.3% of the 45 ARWH cases) (ARWH 2018). The rate and method of progression of this disease is unknown. High doses of prednisolone have been reported to temporarily alleviate clinical signs (Hanger 2003), though there are no reports of clinically significant remission.
Other chemotherapeutic regimens have rarely been attempted. A koala with leukaemic lymphosarcoma was treated with cytarabine, prednisolone and L-asparagi- nase, but rapidly deteriorated and died (Connolly 1999).2.2 Serosal malignancies
Koalas are susceptible to mesothelioma-like neoplasms of the abdominal and/or thoracic serosal layers. The proliferations are usually multinodular and often bicavitary, and demonstrate various proportions of fibrous, myxoid and mesothelial tissues. In 1990, a review of eight cases of proliferative serosal disease (including some previously diagnosed as mesothelioma, fibrosarcoma and chronic peritonitis) led to the assessment that all were consistent with mesothelioma and that they represented a single disease entity (Canfield et al. 1990a). In the past 12 yr (2008-20) two further cases have been reported by Fabi- jan et al. (2020) and the ARWH (ARWH 2018 case no.
6503.1). A further seven cases were reported by Gillett (2014), but there is potential overlap with cases from Canfield et al. (1990a) and ARWH (2018) because Gillett (2014) used data from various Australian organisations, including the ARWH. Four cases of serosal myxosarcoma have been reported (Astudillo et al. 2015; Gonzalez- Astudillo et al. 2019). These proliferations were classified as myxosarcoma based on immunohistochemistry. Whether these serosal malignancies are collectively representative of a single entity or not is unclear. Descriptions of the clinical signs in these cases are not reported, although most describe emaciation at necropsy. Prognosis is poor and there are no reports of successful treatment.
2.3 Craniofacial osteochondroma
This is a morphologically benign, though clinically significant, neoplasm of cartilage and bone that appears to be a specific neoplasm of koalas. These proliferations most commonly affect the bones of the face (Canfield et al. 1987b; Hulst et al. 2015; ARWH 2018 case nos 3621.1, 11930.1; Gonzalez-Astudillo et al.
2019; Fabijan et al. 2020). Similar proliferations have been seen in other anatomical locations such as the pelvis (Hanger and Loader 2014) and the ribs (Gonzalez-Astudillo et al. 2019; Fabijan et al. 2020). Both managed and free-ranging animals have been reported with this neoplasm from NSW, Qld and SA.The proliferations are expansile and typically grow around rather than into associated normal tissues (Fig. 18.2). They generally become clinically significant when the masses begin to compress soft tissue structures (such as the globe), obstruct and expand in the nasal cavities or disrupt the temporomandibular joint, leading to malocclusion and degenerative joint disease. In one case, an osteochondroma originated within the calvarium, compressing the brain (T Portas pers. comm.).
Surgical excision has been the only known successful treatment thus far; however, the prognosis is still considered poor and resection is considerably restricted by location and size of the lesion (Canfield et al. 1987b).
2.4 Other reported neoplasms in koalas
A range of other benign and malignant neoplasms have been reported in koalas (Tables 18.4 and 18.5).
3.