Monitoring of proviral load in bovine leukemia virus-positive dairy cattle over 3 years: Implications for repeated testing in control programs.

Bovine leukemia virus (BLV) causes enzootic bovine leukosis, the most common neoplastic disease of dairy cattle worldwide. Infection is lifelong, with most cattle remaining asymptomatic, approximately one-third developing persistent lymphocytosis, and a small proportion progressing to lymphoma. The proviral load (PVL), measured as the number of provirus copies per estimated white blood cell, is an indicator of transmission risk and disease progression and can be used to identify animals that contribute most to virus spread in control programs. However, longitudinal data describing PVL dynamics at the individual cow level are limited. The objectives of this study were to quantify temporal changes in PVL in naturally infected dairy cows over 3 yr and evaluate the implications for repeated PVL testing in BLV control programs. A total of 716 BLV-positive cows from 25 commercial dairy herds in the Canadian Maritime provinces (Prince Edward Island, Nova Scotia, and New Brunswick) were included. Cows underwent annual PVL testing using quantitative PCR and were categorized as high, moderate, low, or undetectable PVL. Temporal changes in PVL were evaluated using repeated measurements, and conversion between PVL categories was assessed over 1- and 2-yr intervals. The BLV proviral load showed an overall temporal trend toward higher values, with corresponding shifts in PVL categories observed across initial classification groups and within individual animals. Of cows with an initial low PVL, 12% progressed to high PVL over the study period. Among cows initially classified as moderate PVL, 53% transitioned to high PVL. A subset of cows maintained undetectable PVL despite confirmed BLV seropositivity. These findings indicate that the likelihood of PVL progression is influenced by the initial PVL category, with moderate-PVL cows representing the group at greatest risk of becoming highly infectious. The results have practical implications for BLV control programs in dairy herds. Repeated PVL testing of cows with moderate PVL may help identify animals at increased risk of viral transmission and guide targeted culling or segregation decisions. In contrast, cows with low PVL may require less frequent testing, and cows with persistently undetectable PVL may not require repeated PVL monitoring. Tailoring PVL testing frequency according to initial PVL status may improve the efficiency of BLV control programs by prioritizing management of highly infectious animals and reducing herd-level virus transmission.