The analysis of the dynamics of changes in serological and hematological blood parameters of cattle over the period of comprehensive healthrelated measures against Bovine Leukemia Virus (BLV) infection is presented to assess their effectiveness. For analysis of the epizootological situation, veterinary reporting data and the results of serological and hematological diagnostics for cattle leukemia were used. The work was performed in three agricultural enterprises of the Siberian Federal District: Irkutsk Region Altai Territory and Novosibirsk Region. According to the principle of an individual approach, a programme of comprehensive diagnostic and health anti-leukemia measures for each enterprise was developed. The main purpose of these measures was to break the epizootic chain in chronic BLV infection and ultimately change the epizootic situation by reducing the percentage of infection in the livestock population. The positive dynamics of the reduction of infected and leukemiasick cattle was shown as part of a complex of antiepizootic measures, which was made possible by a deviation from the generally accepted regulations for serological diagnostics in agar gel (AGID). During the quarterly diagnosis, the infection rate decreased over 2 years from 72.41 to 1.94% in cows and from 66.6% to negative results in heifers of the breeding age. Using a more sensitive method of enzymelinked immunosorbent assay in the recovery of young and repair livestock, it was possible to reduce infection from 20.9 to 4.0% and from 43.0 to 2.7%, respectively. In this case the epizootic situation for BLV infection may be more stable due to the identifi cation of hidden carriers of the infection.

In recent years, using a genome-wide association study (GWAS), a number of single nucleotide polymorphisms (SNPs) have been suggested to be associated with susceptibility to leukemia in cattle. However, all studies have been done with purebred Holstein cows and their hybrids. In this regard, it is important to confirm the functional role of polymorphisms previously identified in a GWAS study in Russian cattle breeds. The aim of this study was to verify the association between rs110861313 in the intergenic region of bovine chromosome 23 and leukemia in the Russian Black Pied cattle. Based on the levels of bovine leukemia virus (BLV)-specific antibodies detected in serum using serodiagnostic techniques, animals were divided into three groups: healthy animals (n = 115), asymptomatic virus carriers (n = 145) and animals with leukemia (n = 107). Genotyping of rs110861313 was carried out using polymerase chain reaction followed by analysis of restriction fragment length polymorphisms. A significant decrease in the frequency of the A/A genotype (11.2 %) was revealed in animals with persistent lymphocytosis compared to virus carriers (27.6 %) (p < 0.002). At the same time, the frequency of animals with the C/C genotype in animals with persistent lymphocytosis (41.1 %) was significantly higher than that of virus carriers (21.4 %) (p < 0.001). In this case, asymptomatic virus carriers can be considered a more suitable control than healthy animals that have not been in contact with the virus. According to bioinformatics analysis, resistance to BLV can be due to the presence of the transcription factor FOXM1 binding site in the region of rs110861313. FOXM1 is expressed in immune cells and can potentially affect the expression of the neighboring genes (LY6G5B, GPANK1, ABHD16A, LY6G6F, LY6G6E, CSNK2B, ApoM). Thus, we found that SNP rs110861313 in the intergenic region of bovine chromosome 23 is associated with the development of leukemia following BLV infection in the Russian Black Pied cattle.

The analysis of the epizootic situation of cattle leukemia in the Siberian Federal District as a whole and in individual regions of the district is given. The study was carried out in the Republics of Altai, Tuva and Khakassia, in the Altai and Krasnoyarsk Territories and in five regions: Irkutsk, Kemerovo, Novosibirsk, Omsk and Tomsk. The materials of diagnostic studies obtained by veterinary specialists of the Siberian Federal District for 2017-2019 are presented. This information has been analyzed and summarized both for the Siberian Federal District as a whole and for its individual constituent entities. Studies show the spread of bovine leukemia virus infection in the whole district. A tense epizootic situation with cattle leukemia was noted, since the Siberian Federal District ranks second in Russia in terms of the number of adverse locations (322). However, compared to 2017, the number of adverse locations in 2019 decreased by 57, sick animals - by 977 heads, infected animals - by 70836 heads. The number of adverse locations has decreased slightly, while the number of infected animals in some regions has increased. As of January 1, 2020, no hematologically sick animals were found in the Republics of Altai, Khakassia, Buryatia and Tuva, but the number of animals infected with BLV increased. The experience of individual regions of the country that are free from BLV infection (Sverdlovsk, Vologda, Leningrad, Murmansk, Arkhangelsk, Kostroma, Kirov regions, Khanty-Mansi Autonomous District, etc.) shows that bovine leukemia is an infection that can be controlled. It is possible to eliminate the disease in herds of cattle with any level of BLV infection as a result of carrying out complex health-improving measures, as well as organizational and veterinary-sanitary measures with the obligatory removal of sick and BLV infected animals from the herd.

The analysis was carried out on the severity of the epizootic situation on leukemia and the presence of risk factors for the prevalence of this infection in 67 farms of Novosibirsk region, unfavorable for leukemia. Ten risk factors which can be attributed to the exogenous category of factors (technological, iatrogenic) were chosen as the subject of this study. These risk factors can have the greatest impact on the severity of the epizootic process in leukemia infection. The qualitative and quantitative representations of risk factors in the farms were analyzed, the most frequently registered of them were determined. The correlation between individual risk factors and such indicators as the level of leukemia incidence in cows and the infection rate of cows and heifers was assessed. It was established that there is a correlation between the leukemia incidence in cows and the following factors: the use of calves from cows with hematological diseases (p < 0.001) for reproduction of the herd; the absence of regular monitoring of the serological status of cows for BLV-infection (p < 0.001); the lack of regular monitoring of the serological status of cows for leukemia (p < 0.001); the use of natural insemination (p < 0.001). It was also identifi ed that there is a correlation between infection of heifers and joint maintenance and grazing of cattle with different status for BLV-infection (p < 0.001).