A 10-year-old, male, captive-bred southern black rhinoceros (Diceros bicornis minor) presented with a several-day history of a firm to slightly fluctuant, black cutaneous mass on the dorsal midline. Fine needle aspirate cytology was consistent with a melanocytic neoplasm. Recumbent anesthesia was achieved with 1 mg etorphine and 120 mg azaperone (ZooPharm, Fort Collins, CO, USA; IM). The mass was excised via a partial thickness skin incision with 2 cm circumferential margins and the site was left to heal by second intention. Histopathology confirmed melanoma with no evidence of vascular invasion. Five months later, no local recurrence of the tumor was present, but a second comparable nodule was found on the inner thigh. This smaller mass was similarly excised and submitted for histopathology.
To the authors’ knowledge, this is the first report of a melanoma in a black rhinoceros. A case of interdigital melanoma has been reported in a greater Asian one-horned rhinoceros (Rhinoceros unicornis).4 In veterinary medicine, melanoma diagnosis typically carries a grave prognosis, and the tumors tend to recur after excision.5 They are frequently resistant to therapeutics, can be very aggressive clinically, and have a predisposition to rapidly metastasize to regional lymph nodes.2 However, certain presentations have a more favorable prognosis; 90% of cutaneous melanomas in grey horses are benign at presentation, with slow progression to malignancy in approximately two-thirds of cases3 and less than 5% of canine cutaneous melanomas are malignant1. Although histologically classified as malignant, thus far, the tumor behavior in this case has not appeared to be aggressive.
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