Brushtail Possum (Trichosurus vulpecular) with lymphosarcoma and leukaemia

Registry Case ID TARZ-6926.1 (Prosector: Jane Hall, Pathologist: Dr Cheryl Sangster)

CASE HISTORY:

An adult, female brushtail possum (weight 2.456 kg) was rescued from a residential laundry by WIRES and brought to Taronga Wildlife Hospital for examination. The possum had been living in the laundry for 1 month and residents had been feeding it cat food, milk, bread and some fruit.  It had a wound to the right fore leg and was having trouble breathing.  Fur was missing over most of the body and tail. The possum was euthanased due to poor prognosis for survival.

GROSS FINDINGS:

External findings: Lesion at right elbow approx 4-5 cm in length, mid ventral thoracic area 2.5cm in length, and left side of neck 5cm in length.  There is notable thickening of ventral skin. The base of the tail shows crusty exudative dermatitis with fur loss. Left shoulder 1.5 x 1.5cm bruise. Superficial wounds on head, hips, ventrum, limbs, and tail. Almost complete alopecia apart from some thickened, abnormal fur over dorsal back from below the neck to just cranial to tail. There is sparse fur over the tail.  The pelvic bones are prominent.  The animal is in poor body condition.

Hydration: dehydrated

Fat Deposits: nil

Muscle Mass: emaciated

Internal Findings:  No visible lesions except heart ventricular ratio of 5:1, left to right. There is severe hardening of the aortic wall. Axillary and inguinal lymph nodes are enlarged and wet, with a consistent appearance.  There is marked swelling of axillary subcutaneous tissue. The stomach contains pasty brown material and green vegetable material. There are formed pellets in the colon.

HISTOLOGY:

Tissues are in good condition. The following tissues exhibit no significant histological changes: Heart (no histological changes, though left ventricle was hypertrophied grossly), duodenum (E), colon (F), brain (F, G), stomach (G)

All slides:  A homogeneous population of small lymphocytes with round, darkly basophilic nuclei and minimal eosinophilic cytoplasm are found in interstitial areas of numerous organs, including: subepithelial tissues of the trachea and oesophagus (B), salivary gland (B), skeletal muscle endomysium and perimysium (B), thyroid gland (B), renal pelvic fat (C), adrenal gland (C), perivascular connective tissue in the lung (D, E), portal triads and sinusoids of the liver (D), splenic red pulp (D), submucosa of the bladder (E)

(Image: oesophagus, trachea, thyroid gland 2x H&E)

Lymph nodes (B, C, D):  Lymph nodes are enlarged by the presence of increased numbers of small lymphocytes.  These cells are present in the sinuses and lymphoid follicles, which are still recognizable throughout the tissue.  The cells extend beyond the capsule of the structures and into the interstitium of the surrounding adipose tissue.

Skin (C, E): Immense numbers of neoplastic lymphocytes are expanding the dermis, surrounding and effacing adnexal structures.  The lymphocytes extend to the basal surface of the epithelium and occasionally cross into the squamous epithelial layers.  Large numbers of lymphocytes are dissecting between myocytes of the subcutaneous muscle layer and are percolating through the interstitium of the subcutaneous fat.

(Image: Brushtail possum skin with lymphosarcoma 4x H&E)

(Image: Brushtail possum skin with lymphosarcoma 20x H&E)

Kidney (A):  Some thin-walled tubules within the medulla are filled with basophilic crystalline material.  Small foci of lymphoplasmacytic inflammation are present in the interstitium of the cortex and pelvis.

Aorta (B):  Extensively, central to peripheral portions of the tunica media have lost cellular detail, have a smudgy appearance and are rimed by basophilic granular material (necrosis and mineralization).

(Image: Brushtail possum aorta with metastatic mineralisation 10x H&E)

DIAGNOSIS:

Cutaneous lymphosarcoma with multisystemic systemic involvement and leukaemia

Aortic mineralization – segmental, chronic and severe

Left ventricular hypertrophy

Alopecia – extensive, chronic and severe (secondary to neoplasia)

COMMENTS:

This possum was suffering from advanced lymphosarcoma involving multiple tissues, but primarily affecting the skin.  The close association with epidermis suggests this is a T-cell epitheliotropic lymphosarcoma, also known as mycosis fungoides in domestic animals.  The severe alopecia demonstrated by this animal was almost certainly a result of neoplastic effect on cutaneous adnexal structures.  Interestingly, the aorta of this animal was markedly mineralized.  Lymphosarcoma is well known for inciting hypercalcaemia of malignancy, a secondary effect of which can be aortic mineralization.  The left ventricular hypertrophy likely resulted from decreased elasticity of the aorta and therefore increased load on the ventricular muscle.