一头 23 个月大的荷斯坦-弗里斯兰小母牛出现不活动和腹泻。体格检查未见浅表淋巴结肿大。血液学检查发现淋巴细胞增多。牛白血病病毒（BLV）原病毒载量为2,122拷贝/10 ng DNA，根据全基因组系统发育分析，BLV被归类为C组。分别通过B细胞的克隆性测试和反向PCR检测B细胞的单克隆增殖和BLV原病毒在牛基因组中的单克隆整合。尽管尸检时淋巴结未肿胀，但组织病理学检查显示淋巴结中有肿瘤性淋巴细胞增殖，CD5和CD20免疫阳性，CD3免疫阴性。该小母牛被诊断患有由 BLV 引起的 EBL（C 组）。

A 23-month-old Holstein-Friesian heifer presented with inactivity and diarrhea. On physical examination, no enlargement of superficial lymph nodes was observed. Hematological examination revealed lymphocytosis. The bovine leukemia virus (BLV) proviral load was 2,122 copies/10 ng DNA, and BLV was classified as Group C based on whole genome phylogenetic analysis. Monoclonal proliferation of B-cells and monoclonal integration of the BLV provirus in the bovine genome were detected by a clonality test of B-cells and inverse PCR, respectively. Although lymph nodes were not swollen at necropsy, histopathological examination revealed neoplastic lymphocyte proliferation in lymph nodes, which were immune positive for CD5 and CD20, and negative for CD3. The heifer was diagnosed with EBL caused by BLV classified as Group C.