Phenotypic Selection of Dairy Cattle Infected with Bovine Leukemia Virus Demonstrates Immunogenetic Resilience through NGS-Based Genotyping of BoLA MHC Class II Genes

Characterization of the bovine leukocyte antigen (BoLA) DRB3 gene has shown that specific alleles associate with susceptibility or resilience to the progression of bovine leukemia virus (BLV), measured by proviral load (PVL). Through surveillance of multi-farm BLV eradication field trials, we observed differential phenotypes within seropositive cows that persist from months to years. We sought to develop a multiplex next-generation sequencing workflow (NGS-SBT) capable of genotyping 384 samples per run to assess the relationship between BLV phenotype and two BoLA genes. We utilized longitudinal results from milk ELISA screening and subsequent blood collections on seropositive cows for PVL determination using a novel BLV proviral load multiplex qPCR assay to phenotype the cows. Repeated diagnostic observations defined two distinct phenotypes in our study population, ELISA-positive cows that do not harbor detectable levels of provirus and those who do have persistent proviral loads. In total, 565 cows from nine Midwest dairy farms were selected for NGS-SBT, with 558 cows: 168 BLV susceptible (ELISA-positive/PVL-positive) and 390 BLV resilient (ELISA-positive/PVL-negative) successfully genotyped. Three BoLA-DRB3 alleles, including one novel allele, were shown to associate with disease resilience, *009:02, *044:01, and *048:02 were found at rates of 97.5%, 86.5%, and 90.3%, respectively, within the phenotypically resilient population. Alternatively, DRB3*015:01 and *027:03, both known to associate with disease progression, were found at rates of 81.1% and 92.3%, respectively, within the susceptible population. This study helps solidify the immunogenetic relationship between BoLA-DRB3 alleles and BLV infection status of these two phenotypic groupings of US dairy cattle.

Avoidance of Natural Suckling from Dams with Bovine Leukemia Virus Is a Low Priority Countermeasure against Postnatal Transmission

Although natural suckling from dams with bovine leukemia virus (BLV) has not been recommended in Japan, the frequency of BLV transmission through natural suckling under natural conditions is still unclear. The purpose of this study was to elucidate the risk of BLV transmission through natural suckling. Dams with BLV were classified into three groups (high, middle, low) based on the proviral loads (PVLs). PCR positivity of their colostrum and the correlations between the ratios of calves with BLV and types of feeding milk were analyzed. In dams with low PVLs, no colostrum or calves were confirmed to have BLV. In dams with middle and high PVLs, 17 out of 25 (68.0%) colostrum were PCR positive, and 10 out of 23 (43.4%) and 13 out of 29 (44.8%) calves with natural suckling and artificial rearing were infected with BLV, respectively. No difference was confirmed between the infection rates of natural-suckled and artificially reared calves. Thus, we concluded that the avoidance of natural suckling from dams with BLV and the introduction of artificial rearing were low priority countermeasures against BLV transmission.

Kinetic Study of BLV Infectivity in BLV Susceptible and Resistant Cattle in Japan from 2017 to 2019

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leukosis. Polymorphism in bovine lymphocyte antigen (BoLA)-DRB3 alleles is related to susceptibility to BLV proviral load (PVL), which is a useful index for estimating disease progression and transmission risk. However, whether differential BoLA-DRB3 affects BLV infectivity remains unknown. In a three-year follow-up investigation using a luminescence syncytium induction assay for evaluating BLV infectivity, we visualized and evaluated the kinetics of BLV infectivity in cattle with susceptible, resistant and neutral BoLA-DRB3 alleles which were selected from 179 cattle. Susceptible cattle showed stronger BLV infectivity than both resistant and neutral cattle. The order of intensity of BLV infectivity was as follows: susceptible cattle > neutral cattle > resistant cattle. BLV infectivity showed strong positive correlation with PVL at each testing point. BLV-infected susceptible cattle were found to be at higher risk of horizontal transmission, as they had strong infectivity and high PVL, whereas BLV-infected resistant cattle were low risk of BLV transmission owing to weak BLV infection and low PVL. Thus, this is the first study to demonstrate that the BoLA-DRB3 polymorphism is associated with BLV infection.

Analysis of the episootic situation of cattle leukemia in the Siberian Federal District

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.