The infection hazard of carriers of proviral bovine leukemia virus and its evaluation with regard to leukocytosis

The spread of the bovine leukemia virus (BLV) brings significant economic damage to dairy and beef cattle still due to the problems to develop the optimal methods for its prevention. The situation is compounded by the fact that the clinical manifestation of lymphocytic leukemia has been observed in animals during 5 to 10 years after infection in approximately 5-7 % of animals. Subdivision of BLV infected B cells on the producers of mature viral particles and precursors of the formation of lymphomas necessitates of separately consideration of the risk of infecting animals and forecast for development of lymphocytic leukemia. Testing antibodies to viral antigens, or proviruse DNA insertion into the host genomes does not allow to reveal the most dangerous animals in view of their role in the infection spread, which is most essential for its control. In this regard, the particular urgency is the method development to assess the infection hazard from carriers of provirus DNA of bovine leukemia virus, associated with a large number of B lymphocytes, producing mature viral particles. In order to evaluate the possibility of using in these purposes not only testing individual animals on the quantity of viral RNA in blood samples, but the proliferative activity of leucocytes, these two characteristics were compared in the present work. The presence or absence of genome integrated provirus DNA in Black-and-White Holstein cows ( n = 57, commercial herd) was evaluated and the group of infected animals was revealed when using primers designed by us for BLV gag and pol genes (G.Yu. Kosovoskii et al., 2013). The relative quantity expression of provirus in individual infected cows using RT-PCR analysis and primers to the fragment of the BLV pol gene were assessed. The counting of leukocytes in blood samples of all cows, included in the analysis, was performed. The number of leucocytes in cows free from infection, with the exception of one cow, was less than 12х109 cells/l. This indicator in infected animals divided the investigated cows into two subgroups - in the first subgroup the leucocyte values did not exceed 17х109 cells/l, in the second subgroup the number of leukocytes was significantly higher, reaching 28х109 cells/l in some animals. It was important to note, that the amount (above 20х109 cells/l) of BLV RNA, sufficient to detect by RT-PCR, was revealed only on the second subgroup. The coincidence of the increased leucocyte amount and the BLV RNA in blood of infected animals indicates that particular these cows represent the greatest infection hazard and suggests that the combination of estimates of the number of leukocytes and RNA BLV can be quite a reliable approach for the herd recovery by priority of their liquidation.