A hematologic key for bovine leukemia virus screening in Japanese black cattle.

The European Community's leukosis key (EC key) is a well-known hematologic method for detecting bovine leukemia virus (BLV) infection in dairy cattle. The key identifies infected cattle with persistent lymphocytosis via a combination of lymphocyte count (LC) and age. Using the EC key to identify BLV-infected Japanese black (JB) cattle is problematic, however, given the inherently lower LCs of JB cattle compared to dairy cattle. We analyzed the LC in BLV-positive and -negative JB cattle and estimated LC cutoff values by age using receiver operating characteristic curve analysis. Among the 716 JB blood samples collected, 452 (63%) JB cattle were confirmed as BLV-positive by an antibody ELISA for ≥1-y-old cattle and by real-time PCR for <1-y-old cattle. The cutoff values for the LC in each age group were calculated as 6.3 × 109/L for <1 y, 5.9 × 109/L for 1 to <2 y, 5.5 × 109/L for 2 to <3 y, 4.5 × 109/L for 3 to <6 y, 4.3 × 109/L for 6 to ≤10 y, and 3.7 × 109/L for >10 y. The sensitivity and specificity of the estimated cutoff values were 0.49 (95% confidence interval: 0.44-0.53) and 0.81 (0.75-0.85), whereas those of the EC key were 0.20 (0.16-0.24) and 0.99 (0.97-1.00). Our LC cutoff values for screening JB cattle for BLV infection appear to be preferable to those of the EC key.