EFFECT OF BOVINE LEUKEMIA VIRUS INFECTION AND PROVIRAL LOAD ON THE SYSTEMIC PROFILE OF DAIRY HEIFERS DURING THE TRANSITION PERIOD

This study aimed to evaluate the effect of bovine leukemia virus (BLV) on the systemic profile of naturally infected dairy heifers during the transition period. Pregnant Holstein and Jersey heifers (n=24) were distributed in pairs into two experimental groups: (BLV+) and (BLV–). Animals from the BLV+ group were divided into two subgroups based on the median BLV proviral load: high and low proviral load. The animals were assessed at weeks -3, -2, -1, calving time (0), +1, +2, and +3. Blood samples were harvested for hematological and biochemical analyses in addition to haptoglobin measurements. A farm BLV screening revealed a herd BLV prevalence of 57.25% and a heifer BLV prevalence of 38.7%. Hematological variables were not affected by BLV groups, and mean corpuscular hemoglobin concentration was the only hematological variable for which group interaction was observed, with BLV+ cattle having higher values (33.29±3.39%) than BLV– cattle (31.08±2.31%). Additionally, the effect of aspartate aminotransferase on the group activity was detected, with higher values observed in the BLV+ heifers. Regarding acute-phase proteins, the BLV+ group had a higher prevalence of animals with haptoglobin and were 4.96 times more likely to be inflamed (haptoglobin ≥ 2.0 mg/dL) during the transition period than the BLV– group. Fibrinogen concentrations were higher at weeks 0 and +1 in BLV+ heifers than in BLV– heifers. A high proviral load affected total leukocyte count and the absolute number of lymphocytes; however, this profile was not observed in the low proviral load and paired BLV– heifers. To our knowledge, this is the first study to report the impact of BLV infection on the health of dairy heifers during the transition period, demonstrating the dependence of proviral load on WBC changes and the early high risk of inflammation in infected animals.