Peripheral B-lymphocyte clonality of 274 bovine leukemia virus -infected cattle with lymphocytosis was analyzed using clonality PCR based on sequences of the variable region of the bovine immunoglobulin H chain. None of the cattle showed monoclonal proliferation, while 10, 31, and 233 showed minor-clonal, oligoclonal, and polyclonal proliferation, respectively. A total of 163 cattle were analyzable the following year, and lymphocytosis was maintained in 157, indicating persistent lymphocytosis (PL). Blymphocyte clonality of the 157 PL cattle was minor-clonal in 6 (3.8 %), oligoclonal in 8(5.1 %), and polyclonal in 143 (91.1 %). A higher rate of enzootic bovine leukosis (EBL) onset within a year was observed in PL cattle with minor-clonal (50.0 % (3/6)) and oligoclonal (25.0 % (2/8)) proliferation compared to those with polyclonal (5.6% (8/143)) proliferation.Minor-clonal and oligoclonal proliferation in PL cattle may be a prognosis factor for developing EBL.

A 38-month-old Japanese Black bull presenting with anorexia was given supportive treatment without improvement. Findings including bovine leukemia virus positivity and monoclonal B-cell proliferation strongly suggested the onset of enzootic bovine leukosis (EBL). Pathological findings confirmed the diagnosis of EBL. B-cell clonality were analyzed over time using pre-onset preserved genomic DNA at ages 6 months, 16 months, and 30 months. In the B-cell clonality analysis, two minor peaks at 140 and 220 bp were observed before onset, but another large peak at 175 bp appeared at the time of EBL diagnosis. Although the reason for the proliferation of an independent clone is unknown, detection of clonality abnormalities may lead to the detection of cattle at high risk of developing EBL.