Mammary neuroendocrine tumor in a mare.

A 10-y-old American Paint Horse mare was presented to the Equine Field Services at Virginia-Maryland College of Veterinary Medicine (Blacksburg, VA, USA) for evaluation of mastitis and a possible abscess in the right mamma. Histologic evaluation of a mammary biopsy revealed a densely cellular, infiltrative neoplasm with cells arranged in packets supported by thin stroma. In a neuroendocrine immunohistochemistry panel, the neoplastic cells did not immunolabel for neuron-specific enolase or synaptophysin, positively immunolabeled for chromogranin A, and had moderate scattered immunolabeling for S100. These findings confirmed a diagnosis of mammary neuroendocrine tumor. Although the cells also did not immunolabel for cytokeratin, we cannot rule out a neuroendocrine carcinoma. Neuroendocrine mammary neoplasms are very rare and, to our knowledge, they have not been reported previously in horses. This neoplasm should be considered as a differential diagnosis for mammary tumors in mares.