Mammary Gland Malignant Myoepithelioma in a Domestic Rabbit (Oryctolagus cuniculus)

Background: Malignant myoepithelioma is a rare neoplasm in humans and rats, poorly reported in dogs and not previously described in rabbits. This study aimed to report a case of malignant mammary myoepithelioma in a domestic rabbit (Oryctolagus cuniculus). Case: A domestic, hybrid, female, entire, nulliparous, seven-year-old rabbit was attended in the Veterinary Hospital Prof. Ricardo Alexandre Hippler at Universidade Vila Velha (UVV) with a history of apathy, lethargy and anorexia. In the clinical examination a 5 cm tumour was observed in the right abdominal mammary gland (M2), along with floating smaller lesions in M1 and M3. There were no palpable abnormalities in the axillary and inguinal lymph nodes. Complete blood count and serum biochemistry were within the normal range. Fine needle aspiration was performed for cytopathological evaluation of the tumour in M2 and it was suggestive of a malignant glandular neoplasm, with probable epithelial origin. Complete staging included chest and abdominal radiographs, but they were unremarkable. The patient was submitted to a right unilateral mastectomy, involving resection of the inguinal lymph node. The sample was sent to the Laboratory of Animal Pathology of UVV for routine histological evaluation. Histopathological evaluation in M2 was compatible with malignant myoepithelioma based on the literature, while nodules in M1 and M3 were classified as mammary cysts. The inguinal lymph node showed no significant abnormalities. An immunohistochemical panel was performed in the Comparative Pathology Laboratory of the Institute of Biological Sciences of the Universidade Federal de Minas Gerais (UFMG), using immunomarkers (pan-cytokeratin, p63 and vimentin) to characterize the tumour and confirm its diagnosis. Immunohistochemistry revealed expression of p63, and was negative for pan-cytokeratin and vimentin, which confirmed the myoepithelial origin, without involvement of the epithelial compartment. Clinical follow-up was recommended every 2-3 months, including abdominal ultrasonography, chest X-rays, complete blood count and serum biochemistry. Ultrasonography showed thinning of the uterine wall, dilation of 3.7 cm in diameter and presence of endometrial cysts of varied sizes, around 0.7 cm. Neutering was recommended but it was not elected by the pet´s responsible. These findings remained stable throughout the follow-up, which was greater than 405 days.Discussion: This is the first report of a malignant myoepithelioma