A 25-y-old Percheron mare was admitted to the teaching hospital because of lethargy and intractable dyspnea. Thoracoabdominal ultrasound examination identified severe peritoneal effusion, mild bilateral pleural effusion, and a diffuse pulmonary nodular pattern. Cytology of peritoneal fluid revealed a hypercellular sample with clusters of neoplastic polygonal cells and admixed macrophages. Euthanasia was followed by postmortem examination; marked bi-cavitary effusion was present, and innumerable up to 4-cm diameter, round-to-floriform nodules were diffusely evident throughout serosal surfaces as well as the pulmonary and hepatic parenchyma. Disseminated adenocarcinoma, predominantly affecting lung and liver with widespread serosal implantation, was confirmed on light microscopy. Neoplastic cells had strong immunolabeling for pancytokeratin and lacked immunoreactivity to vimentin, napsin A, and Pax8. Cytokeratin 7 and thyroid transcription factor-1 were non-contributory given absent and inconsistent internal control reactivity, respectively. Such results, combined with the lack of a major mass that would indicate a primary site, were supportive of carcinoma of unknown primary site, which remains a conundrum in human oncology, and is poorly explored in veterinary medicine, mainly as a result of clinical and diagnostic limitations.

一匹 25 岁的 Percheron 母马因嗜睡和顽固的呼吸困难住进了教学医院。胸腹超声检查发现严重的腹腔积液、轻度双侧胸腔积液和弥漫性肺结节模式。腹腔液的细胞学检查显示细胞过多的样本具有肿瘤多边形细胞簇和混合的巨噬细胞。安乐死后进行尸检；存在明显的双腔积液，无数直径达 4 厘米的圆形至花状结节在整个浆膜表面以及肺和肝实质中弥漫性明显。在光学显微镜下证实了播散性腺癌，主要影响肺和肝，伴有广泛的浆膜植入。肿瘤细胞对泛细胞角蛋白有很强的免疫标记，对波形蛋白、napsin A 和 Pax8 缺乏免疫反应性。细胞角蛋白 7 和甲状腺转录因子 1 分别因缺乏和不一致的内部控制反应性而无贡献。这些结果，再加上缺乏表明原发部位的主要肿块，支持了原发部位不明的癌，这在人类肿瘤学中仍然是一个难题，在兽医学中探索很少，主要是由于临床和诊断限制。