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821*Correspondence to: Iwaide S: fq3733@go.tuat.ac.jp, Research Unit of Innovative One Medicine, Advanced Research Center for One Welfare, Tokyo University of Agriculture and Technology, 3-5-8 Saiwai-cho, Fuchu, Tokyo 183-8509, Japan©2026 The Japanese Society of Veterinary ScienceThis is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives (by-nc-nd) License. (CC-BY-NC-ND 4.0: https://creativecommons.org/licenses/by-nc-nd/4.0/)NOTEPathologyMammary simple adenoma of the right rear udder in a 15-month-old Holstein heiferMikuya IWANAGA 1) , Momoka SUZUKI 1) , Hayate NISHIURA 2) , Susumu IWAIDE 2,3) *, Tomoyuki SHIBAHARA 2,4)1) Fukushima Prefectural Chuo Livestock Hygiene Service Center, Fukushima, Japan2) National Institute of Animal Health, National Agriculture and Food Research Organization (NARO), Ibaraki, Japan3) Research Unit of Innovative One Medicine, Advanced Research Center for One Welfare, Tokyo University of Agriculture and Technology, Tokyo, Japan4) Department of Veterinary Immunology, Graduate School of Veterinary Medical Sciences, Osaka Metropolitan University, Osaka, JapanABSTRACT. A 15-month-old Holstein heifer developed progressive swelling of the right rear udder 3 month. 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 fi brous connective tissue was observed. The mass was diagnosed as a simple adenoma. To our knowledge, this is the fi rst report of mammary simple adenoma in a Holstein heifer, and in such a young heifer without a history of artif i cial insemination or pregnancy.KEYWORDS: heifer, Holstein, lactiferous sinus, mammary gland, simple adenomaMammary tumors are common in dogs and cats, but extremely rare in ruminants [3, 5, 14]. Previous reports include fi broadenoma [7], papillary cystadenoma [10], and adenocarcinoma [9] in Holstein cattle; fi broadenoma in a lamb [6]; and carcinoma arising from an adenoma in a ewe [8]. Older literature sources have varying degrees of value because histologically def i nitive diagnoses had not been established [3]. The present study reports a rare case of a mammary simple adenoma that formed in the right rear udder of a 15-month-old Holstein heifer.In July 2025, a 15-month-old heifer in Fukushima Prefecture, Japan, presented with marked swelling of the right rear udder that had been present for 3 months. Palpation revealed a lump in the udder, with yellow-brown serosanguineous discharge expressed from the right rear udder. The heifer was treated with penicillin for suspected mastitis. However, the reduction in tumor size was not observed. No bacteria were isolated from culture of the expressed discharge. Subsequently, within 2 months, the swelling had expanded to encompass the entire right rear udder. To investigate the cause, the heifer was moved to the Fukushima Prefectural Chuo Livestock Hygiene Service Center and euthanized later that month.Clinical examination before necropsy revealed a fl uctuation in the cranial portion of the right rear udder (at the border with the right fore udder), whereas the caudal portion was fi rm and indurated (Fig. 1a). Ultrasonography revealed solid tissue consistent with a mass, papillary structures, and mammary ducts dilated in a mesh-like pattern within the mammary glands (Fig. 1b). No fever was observed, and the mammary glands exhibited no increased temperature. At necropsy, the mammary glands weighed 5.8 kg. The right mammary gland was swollen to approximately 20 × 30 × 30 cm, compared with approximately 10 × 15 × 20 cm for the left gland. The posterior right mammary section showed a well-def i ned, pedunculated, caulif l ower-like papillary tumor measuring approximately 10 × 10 × 10 cm within the lactiferous sinus (Fig. 1c). The tumor sections were yellow-orange, solid and water-rich. The tumor was connected to the mammary gland tissue at a distal point within the lactiferous sinus. Macroscopically, edema and hemorrhage were observed at the periphery of the mass sections. The surrounding mammary gland tissue was spongy, dilated, and contained the same yellow-brown, serosanguineous discharge observed during clinical examination. A corpus luteum 15 mm in diameter was observed in the right ovary. No signif i cant lesions were observed in other organs.No bacteria, including Mycoplasma species, were isolated from the mammary gland tissue and serosanguineous discharge. Received: 19 February 2026Accepted: 27 March 2026Advanced Epub: 8 April 2026 J Vet Med Sci 88(6): 821–825, 2026doi: 10.1292/jvms.26-0096