A non-lactating 17-year-old grey barren Mangalarga Marchador mare was referred to the Large Animal Veterinary Hospital of the Federal Rural University of Rio de Janeiro (UFRRJ), presenting enlargement of the mammary gland. The owner reported that the animal had a lesion in the mammary gland for at least two years, diagnosed and treated as chronic mastitis. Initially only the right gland was involved, presenting ulceration and exudation on the cutaneous surface. After 20 months, the left gland also became affected. The diagnosis of tubulo-papillary adenocarcinoma was based on the typical microscopic lesions. This work shows the importance of the histopathological examination in the differential diagnosis between the neoplasms and the chronic inflammation in the mammary gland of mares, as well as, to show that the cytological examination cannot detect the tumor, in case the puncture is made in areas of secondary infection.
equine diseases; mammary neoplasms; mastitis
