A Spontaneous Melanocytic Iris Melanoma in a Rabbit ( <i>Oryctolagus cuniculus</i> Linnaeus, 1758)

This report describes the clinical, gross, histopathological, immunohistochemical, and therapeutic aspects of a spontaneous melanocytic iris melanoma in a rabbit, representing the first documented case of this condition in the species in the semiarid region of the state of Rio Grande do Norte, Brazil. A 2-year-old, mixed-breed female rabbit was referred to the Wildlife Department presenting an inadequate diet based on food intended for human consumption. According to the anamnesis, the patient had previously received ocular treatment but later developed bleeding in the right eye. On ophthalmic clinical examination, there was active hemorrhage and ocular rupture, leading to an indication for surgical enucleation. A transpalpebral enucleation was performed, and the eyeball was fixed in 10% buffered formalin and submitted for histopathological evaluation. Histopathological analysis revealed a moderately cellular, unencapsulated, and poorly demarcated neoplasm of the iris and ciliary body, composed of spindloid and anaplastic melanocytic cells containing brown granular pigment, with 12 mitotic figures observed in 2.37 mm². For immunohistochemical evaluation, melanin bleaching was performed, and a melanoma cocktail (HMB-45, Melan-A/MART-1, and Tyrosinase) along with the proliferation marker Ki-67 was used. The neoplastic cells showed diffuse cytoplasmic immunoreactivity for all melanocytic markers and nuclear labeling for Ki-67, with a nuclear proliferation index of 38%, confirming the diagnosis of melanocytic iris melanoma. Excision of the eyeball using the transpalpebral technique was effective in the treatment of this neoplasm, providing a favorable prognosis. This case highlights the importance of considering iris melanoma as a differential diagnosis for ocular lesions in rabbits.