Amelanotic Melanoma in a New Zealand White Rabbit (Oryctolagus cuniculus)

Abstract

A 3.5-year-old intact male double-transgenic New Zealand white rabbit (Oryctolagus cuniculus), apoA-I and LCAT (apolipoprotein and lecithin:cholesterol acyltransferase), was presented with a discrete, raised facial mass (0.5 Â 1.0 Â 1.0 cm). The mass was surgically excised, with reoccurrence to the same site 88 days later. A second surgical excision was performed, and the rabbit died 3 weeks later from respiratory distress. At necropsy, multiple varying-sized masses were observed in the ventral mandibular region and throughout the lungs, pleura, and diaphragm. On histopathology, the masses were composed of moderately anisocytotic and anisokaryotic polygonal to spindloid cells with moderate finely granular, lightly eosinophilic cytoplasm, having round to oval nuclei with one to several nucleoli and finely stippled chromatin. Mitotic figures were frequent. Lymphatic and venous invasion were noted with neoplastic cells metastasized to the submandibular lymph nodes, lungs, liver, and adventitial surface of the aorta. Fontana-Masson stain was negative for melanin, thereby necessitating immunohistochemistry and transmission electron microscopy. Positive staining with MART-1 (a melanocyte protein marker) combined with transmission electron microscopy revealing type II melanosomes confirmed the diagnosis of an amelanotic melanoma.

Cutaneous malignant melanoma in two rabbits (<i>Oryctolagus cuniculus</i>)

“…The types of skin tumors reported in rabbits ( Oryctolagus cuniculus ) include myxomatosis, fibroma, fibrosarcoma, papilloma, basal cell carcinoma, squamous cell carcinoma, sebaceous gland carcinoma, and lymphoma; melanocytic tumors are extremely rare, with only 10 cases reported to date [ 7 , 8 , 17 , 18 ]. In a previous analysis with 190 different tumors in rabbits [ 17 ], malignant melanoma accounted for only eight cases (4.2%).…”

“…In a previous analysis with 190 different tumors in rabbits [ 17 ], malignant melanoma accounted for only eight cases (4.2%). Out of the five cases of malignant melanoma, whose outcome information (including euthanasia) were available, metastasis was confirmed in two, indicating high malignancy of these tumors [ 7 , 17 , 18 ]. However, owing to the limited number of cases reported in rabbits, a comprehensive understanding of the occurrence site, pathological characteristics, immunological phenotype, mitotic index, and proportion of Ki-67-positive cells associated with clinical outcome is lacking.…”

“…There are only 10 reported cases till date, of malignant melanoma in rabbits: seven with lymphatic vessel invasion and two with metastases to distant sites. Five cases resulted in malignant melanoma-related death (including euthanasia), suggesting that the melanomas in rabbits were highly malignant [ 7 , 17 , 18 ]. Consistent with these previous reports, both the rabbits in the present study died due to metastasis [ 7 , 17 , 18 ].…”

“…Five cases resulted in malignant melanoma-related death (including euthanasia), suggesting that the melanomas in rabbits were highly malignant [ 7 , 17 , 18 ]. Consistent with these previous reports, both the rabbits in the present study died due to metastasis [ 7 , 17 , 18 ]. In veterinary medicine, malignant melanoma occurs frequently in dogs, most commonly in the oral cavity, followed by the skin [ 15 ].…”

“…In cats, malignant melanomas are most commonly detected in the eyes and are reported to metastasize frequently [ 10 ]. In rabbits, malignant melanomas tend to occur in the auricle, eyelid, head, extremities, or groin; metastasis and death are associated with tumors occurring on the head and groin [ 7 , 17 , 18 ]. In the present study, malignant melanomas were observed in the skin of the scrotum and eyelid; in both cases, the tumors metastasized, resulting in death.…”