Phylogenetic analysis of the sequenced segments of the env gene locus of the BLV provirus and the PCR RFLP genotyping strategy proposed in our works that is consistent with phylogenetic classification of this pathogenic agent allowed the taxonomic identification of BLV isolates identified in cattle in Tatarstan (Russian Federation) as representatives of BLV genotypes 4, 7, and 8. Of the 100 isolates identified, 64 were attributed to BLV genotype 4; 28 BLV representatives belong to the cluster of genotype 7; and the remaining 8 samples of the provirus belong to new genotype 8 of the pathogenic agent. The strategy of BLV PCR RFLP genotyping proposed in our work on the basis of five restriction endonucleases (PvuII, SspI, HphI, HaeIII, and BstYI) allowed for correct genotyping identification of the viral pathogen.

The article presents the results of studying the dynamics of the formation of antibodies and immune complexes, reveals the prospects for improving the early diagnosis of cattle leukaemia. Studies were conducted for 6 months on an experimental group of animals consisting of 12 cows. The titers of free and bound antibodies in blood serum and milk were determined by enzyme-linked immunosorbent assay. The results of studies showing that changes in titers of anti-leukaemia antibodies in the blood serum of cows naturally infected with BLV (bovine leukaemia virus) are significantly different from experimental infection data are adduced. In cows infected with BLV, there is no definite relationship between antibody titers in milk and in blood serum.  With sufficiently high titers of serum antibodies, antibody titers in milk can be minimal for the same cows; conversely, with low titers of serum antibodies, there can be high antibody titers in milk.  In the titers of antibodies free and bound in the immune complexes in blood serum with the development of the disease, a certain dependence is traced. With a decrease in titers of free antibodies, in most cases, an increase in titers of “bound” antibodies is observed, i.e., the degree of formation of circulating immune complexes (CECs) increases. There is no clearly defined dependence in the dynamics of changes in titers free and bound in immune complexes of antibodies in milk samples. They can remain at the same level for a long time, both at low and rather high levels.

The research presents the findings of DNA testing of allelic polymorphism by the AC-PCR method of the LEP gene. The research was conducted among 172 Holstein cows at Integrated Agricultural Production Centre “Stud farm named after Lenin” in Atninsky district of the Republic of Tatarstan in 2017–2018. All genotypes of the LEP gene were identified through the research. Associations of the leptin gene polymorphism with dynamics of milk production during three lactations of Holstein cows were established. The best indicators for all three lactations were found in a group of animals with the TT genotype of the LEP gene. These individuals are characterized by increased milk yield, a high yield index, and tend to increase the average daily milk yield during three lactations. These individuals are characterized by increased milk yield, a high milk yield index, and tend to increase the average daily milk yield during three lactations.

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Studies of the state of the antioxidant system and cellular-humoral parameters in dry cows were carried out. They showed a tendency to develop anemia, a decrease in the respiratory function of the blood and an increase in free radical oxidation, which was expressed in a decrease in the level of ceruloplasmin, an increase in catalase activity, the content of total lipids and malondialdehyde.