Mammary simple adenoma of the right rear udder in a 15-month-old Holstein heifer

A 15-month-old Holstein heifer developed progressive swelling of the right rear udder over 2 months. The heifer was euthanized to investigate the cause. Postmortem examination revealed a solid mass measuring approximately 10 × 10 × 10 cm in the lactiferous sinus of the right rear udder. Histopathologically, the mass consisted of glandular and ductal proliferation of epithelial-like cells with slight atypia. Immunohistochemically, the cells were strongly positive for pan-cytokeratin (CK) and CK7, and approximately 25% of the cells were strongly positive for estrogen receptor. The Ki-67 proliferation index of tumor cells was 4.53%. No neoplastic proliferation of myoepithelial cells or fibrous connective tissue was observed. The mass was diagnosed as a simple adenoma. To our knowledge, this is the first report of mammary simple adenoma in a Holstein heifer, and in such a young heifer without a history of artificial insemination or pregnancy.