Association between bovine-leukosis virus seroprevalence and herd-level productivity on US dairy farms

Estimation of genetic parameters and genome-wide association study for enzootic bovine leukosis resistance in Canadian Holstein cattle

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leukosis (hereafter referred to as leukosis), frequently observed in North American dairy herds. Infection with BLV can lead to persistent lymphocytosis and tumors and is associated with decreased production, immunity. and fertility. With no available treatment or vaccine, reducing the prevalence of leukosis through management and culling has not yet been successful. Genetic selection could contribute to permanent improvement in dairy cattle resistance to leukosis. This study aimed to examine the prevalence and impact of leukosis in Canada and to assess the potential for including leukosis resistance in Canadian national genetic evaluations by characterizing the genetic architecture of leukosis resistance using pedigree and genomic information. A total of 117,349 milk ELISA test records on 96,779 Holstein cows from 950 Canadian herds taken between 2007 and 2021 were provided by Lactanet Canada (Guelph, ON, Canada). Each cow was classified as test-positive for leukosis or test-negative for leukosis. Leukosis was present in ∼77% of herds tested; within those herds, an average of 39% of cows tested were test-positive for leukosis. Heritabilities of 0.10 (SE = 0.001) and 0.07 (SE <0.001) were estimated for leukosis resistance using a linear animal model and BLUP or single-step GBLUP methodology, respectively. Breeding value correlations were estimated between leukosis resistance and economically important and phenotypically relevant traits. Most correlations between leukosis resistance and traits already included in Canadian genetic evaluations were favorable, with the exception of SCS. The candidate genes for leukosis resistance identified using a genome-wide association study were on chromosome 23, with some being part of the major histocompatibility complex. This study showed that genetic evaluation for leukosis resistance is possible, and could be considered for inclusion in Canadian national selection indices.

Inter-laboratory comparison of eleven quantitative or digital PCR assays for detection of proviral bovine leukemia virus in blood samples

Bovine leukemia virus (BLV) is the etiological agent of enzootic bovine leukosis and causes a persistent infection that can leave cattle with no symptoms. Many countries have been able to successfully eradicate BLV through improved detection and management methods. However, with the increasing novel molecular detection methods there have been few efforts to standardize these results at global scale. This study aimed to determine the interlaboratory accuracy and agreement of 11 molecular tests in detecting BLV. Each qPCR/ddPCR method varied by target gene, primer design, DNA input and chemistries. DNA samples were extracted from blood of BLV-seropositive cattle and lyophilized to grant a better preservation during shipping to all participants around the globe. Twenty nine out of 44 samples were correctly identified by the 11 labs and all methods exhibited a diagnostic sensitivity between 74 and 100%. Agreement amongst different assays was linked to BLV copy numbers present in samples and the characteristics of each assay (i.e., BLV target sequence). Finally, the mean correlation value for all assays was within the range of strong correlation. This study highlights the importance of continuous need for standardization and harmonization amongst assays and the different participants. The results underscore the need of an international calibrator to estimate the efficiency (standard curve) of the different assays and improve quantitation accuracy. Additionally, this will inform future participants about the variability associated with emerging chemistries, methods, and technologies used to study BLV. Altogether, by improving tests performance worldwide it will positively aid in the eradication efforts.

Development of Dry and Liquid Duplex Reagent Mix-Based Polymerase Chain Reaction Assays as Novel Tools for the Rapid and Easy Quantification of Bovine Leukemia Virus (BLV) Proviral Loads

Bovine leukemia virus (BLV) is prevalent worldwide, causing serious problems in the cattle industry. The BLV proviral load (PVL) is a useful index for estimating disease progression and transmission risk. We previously developed a quantitative real-time PCR (qPCR) assay to measure the PVL using the coordination of common motif (CoCoMo) degenerate primers. Here, we constructed a novel duplex BLV-CoCoMo qPCR assay that can amplify two genes simultaneously using a FAM-labeled MGB probe for the BLV LTR gene and a VIC-labeled MGB probe for the BoLA-DRA gene. This liquid duplex assay maintained its original sensitivity and reproducibility in field samples. Furthermore, we developed a dry duplex assay composed of PCR reagents necessary for the optimized liquid duplex assay. We observed a strong positive correlation between the PVLs measured using the dry and liquid duplex assays. Validation analyses showed that the sensitivity of the dry duplex assay was slightly lower than that of the other methods for the detection of a BLV molecular clone, but it showed similar sensitivity to the singleplex assay and slightly higher sensitivity than the liquid duplex assay for the PVL quantification of 82 field samples. Thus, our liquid and dry duplex assays are useful for measuring the BLV PVL in field samples, similar to the original singleplex assay.

Herd-level prevalence of bovine leukemia virus, Salmonella Dublin and Neospora caninum in Alberta, Canada, dairy herds using ELISA on bulk tank milk samples

Endemic infectious diseases remain a major challenge for dairy producers worldwide. For effective disease control programs, up-to-date prevalence estimates are of utmost importance. The objective of this study was to estimate the herd-level prevalence of bovine leukemia virus (BLV), Salmonella Dublin, and Neospora caninum in dairy herds in Alberta, Canada using a serial cross-sectional study design. Bulk tank milk samples from all Alberta dairy farms were collected 4 times, in December 2021 (n = 489), April 2022 (n = 487), July 2022 (n = 487), and October 2022 (n = 480), and tested for antibodies against BLV, S. Dublin, and N. caninum using ELISAs. Herd-level apparent prevalence was calculated as positive samples divided by total tested samples at each time point. A mixed effect modified Poisson regression model was employed to assess the association of prevalence with region, herd size, herd type, and type of milking system. Apparent prevalence of BLV was 89.4, 88.7, 86.9 and 86.9% in December, April, July, and October, respectively, whereas for S. Dublin apparent prevalence was 11.2, 6.6, 8.6, and 8.5%, and for N. caninum apparent prevalence was 18.2, 7.4, 7.8, and 15.0%. For BLV, S. Dublin and N. caninum, a total of 91.7, 15.6, and 28.1% of herds, respectively, were positive at least once, whereas 82.5, 3.6, and 3.0% of herds were ELISA-positive at all 4 times. Compared with the north region, central Alberta had a high prevalence (prevalence ratio (PR) = 1.13) of BLV-antibody positive herds, whereas south Alberta had a high prevalence (PR = 2.56) of herds positive for S. Dublin antibodies. Furthermore, central (PR = 0.52) and south regions (PR = 0.46) had low prevalence of N. caninum-positive herds compared with the north. Hutterite colony herds were more frequently BLV-positive (PR = 1.13) but less frequently N. caninum-positive (PR = 0.47). Large herds (>7,200 L/day milk delivered ∼ > 250 cows) were 1.1 times more often BLV-positive, whereas small herds (≤3,600 L/day milk delivered ∼ ≤ 125 cows) were 3.2 times more often N. caninum-positive. For S. Dublin, Hutterite-colony herds were less frequently (PR = 0.07) positive than non-colony herds only in medium and large stratum but not in small stratum. Moreover, larger herds were more frequently (PR = 2.20) S. Dublin-positive than smaller herds only in non-colony stratum but not in colony stratum. Moreover, N. caninum prevalence was 1.6 times higher on farms with conventional milking systems compared with farms with an automated milking system. These results provide up-to-date information of the prevalence of these infections that will inform investigations of within-herd prevalence of these infections and help in devising evidence-based disease control strategies.

Development of a novel monoclonal antibody-based competitive ELISA for antibody detection against bovine leukemia virus

Infection with bovine leukemia virus (BLV) leads to enzootic bovine leukosis, the most prevalent neoplastic disease in cattle. Due to the lack of commercially available vaccines, reliable eradication of the disease can be achieved through the testing and elimination of BLV antibody-positive animals. In this study, we developed a novel competitive ELISA (cELISA) to detect antibodies against BLV capsid protein p24. Recombinant p24 protein expressed by Escherichia coli, in combination with the monoclonal antibody 2G11 exhibiting exceptional performance, was used for the establishment of the cELISA. Receiver-operating characteristic curve analysis showed that the sensitivity and specificity of the assay were 98.85 % and 98.13 %, respectively. Furthermore, the established cELISA was specific for detecting BLV-specific antibodies, without cross-reactivity to antisera for six other bovine viruses. Significantly, experimental infection of cattle and sheep with BLV revealed that the cELISA accurately monitors seroconversion. In a performance evaluation, the established cELISA displayed a high agreement with Western blotting and the commercial BLV gp51 cELISA kit in the detection of 242 clinical samples, respectively. In conclusion, the novel p24 cELISA exhibited the potential to be a reliable and efficient diagnostic tool for BLV serological detection with a broad application prospect.

Genetic Variability of Bovine Leukemia Virus: Evidence of Dual Infection, Recombination and Quasi-Species

We have characterized the intrahost genetic variation in the bovine leukemia virus (BLV) by examining 16 BLV isolates originating from the Western Siberia-Tyumen and South Ural-Chelyabinsk regions of Russia. Our research focused on determining the genetic composition of an 804 bp fragment of the BLV env gene, encoding for the entire gp51 protein. The results provide the first indication of the quasi-species genetic nature of BLV infection and its relevance for genome-level variation. Furthermore, this is the first phylogenetic evidence for the existence of a dual infection with BLV strains belonging to different genotypes within the same host: G4 and G7. We identified eight cases of recombination between these two BLV genotypes. The detection of quasi-species with cases of dual infection and recombination indicated a higher potential of BLV for genetic variability at the intra-host level than was previously considered.

The Global Epidemiology of Bovine Leukemia Virus: Current Trends and Future Implications

Bovine leukemia virus (BLV) is a retrovirus that causes enzootic bovine leucosis (EBL), which is the most significant neoplastic disease in cattle. Although EBL has been successfully eradicated in most European countries, infections continue to rise in Argentina, Brazil, Canada, Japan, and the United States. BLV imposes a substantial economic burden on the cattle industry, particularly in dairy farming, as it leads to a decline in animal production performance and increases the risk of disease. Moreover, trade restrictions on diseased animals and products between countries and regions further exacerbate the problem. Recent studies have also identified fragments of BLV nucleic acid in human breast cancer tissues, raising concerns for public health. Due to the absence of an effective vaccine, controlling the disease is challenging. Therefore, it is crucial to accurately detect and diagnose BLV at an early stage to control its spread and minimize economic losses. This review provides a comprehensive examination of BLV, encompassing its genomic structure, epidemiology, modes of transmission, clinical symptoms, detection methods, hazards, and control strategies. The aim is to provide strategic information for future BLV research.

Effects of bovine leukemia virus seropositivity and proviral load on milk, fat, and protein production of dairy cows

The objective was to evaluate the effects of bovine leukemia virus (BLV) infection, as determined by BLV seropositivity and proviral load, on 305-d milk, fat, and protein production of dairy cows. A cross-sectional study was conducted among 1,712 cows from 9 dairy herds in Alberta, Canada. The BLV status was assessed using an antibody ELISA, whereas BLV proviral load in BLV-seropositive cattle was determined with quantitative PCR. Dairy Herd Improvement 305-d milk, fat, and protein production data were obtained for all enrolled cattle. Differences in these milk end points were assessed in 2 ways: first, by categorizing cows based on BLV serostatus (i.e., BLV positive or negative), and second, by categorizing based on BLV proviral load (i.e., BLV negative, low proviral load [LPL] BLV positive, and high proviral load [HPL] BLV positive). A mixed-effect multivariable linear regression model was used to assess differences in milk parameters. We found that BLV positivity, adjusted for parity and natural log-transformed somatic cell count (SCC), was not associated with reduction in 305-d milk, fat, or protein production. However, significant reductions in 305-d milk, fat, and protein yield occurred in HPL cows, but not in LPL cows, compared with BLV-negative cows, when adjusted for parity number and natural log-transformed SCC. In summary, BLV proviral load may predict effects of BLV infection on milk, fat, and protein production.

BoLA-DRB3 Polymorphism Associated with Bovine Leukemia Virus Infection and Proviral Load in Holstein Cattle in Egypt

Bovine leukemia virus (BLV) is the etiological agent of enzootic bovine leukosis, the most prevalent neoplastic disease of cattle worldwide. The immune response to BLV and disease susceptibility and resistance in cattle are strongly correlated with the bovine leukocyte antigen (BoLA)-DRB3 allelic polymorphism. BLV infection continues to spread in Egypt, in part because the relationships between BLV infection, proviral load in Egypt, and BoLA-DRB3 polymorphism are unknown. Here, we identified 18 previously reported alleles in 121 Holstein cows using a polymerase chain reaction sequence-based typing method. Furthermore, BoLA-DRB3 gene polymorphisms in these animals were investigated for their influence on viral infection. BoLA-DRB3*015:01 and BoLA-DRB3*010:01 were identified as susceptible and resistant alleles, respectively, for BLV infection in the tested Holsteins. In addition, BoLA-DRB3*012:01 was associated with low PVL in previous reports but high PVL in Holstein cattle in Egypt. This study is the first to demonstrate that the BoLA-DRB3 polymorphism confers resistance and susceptibility to PVL and infections of BLV in Holstein cattle in Egypt. Our results can be useful for the disease control and eradication of BLV through genetic selection.

Effectiveness of integrated bovine leukemia virus eradication strategies utilizing cattle carrying resistant and susceptible major histocompatibility complex class II DRB3 alleles

Bovine leukemia virus (BLV) has spread worldwide and causes serious problems in the cattle industry owing to the lack of effective treatments and vaccines. Bovine leukemia virus is transmitted via horizontal and vertical infection, and cattle with high BLV proviral load (PVL), which is a useful index for estimating disease progression and transmission risk, are considered major infectious sources within herds. The PVL strongly correlates with highly polymorphic bovine lymphocyte antigen (BoLA)-DRB3 alleles. The BoLA-DRB3*015:01 and *012:01 alleles are known susceptibility-associated markers related to high PVL, and cattle with susceptible alleles may be at a high risk of BLV transmission via direct contact with healthy cows. In contrast, the BoLA-DRB3*009:02 and *014:01:01 alleles comprise resistant markers associated with the development of low PVL, and cattle with resistant alleles may be low-risk spreaders for BLV transmission and disrupt the BLV transmission chain. However, whether polymorphisms in BoLA-DRB3 are useful for BLV eradication in farms remains unknown. Here, we conducted a validation trial of the integrated BLV eradication strategy to prevent new infection by resistant cattle and actively eliminate susceptible cattle in addition to conventional BLV eradication strategies to maximally reduce the BLV prevalence and PVL using a total of 342 cattle at 4 stall-barn farms in Japan from 2017 to 2019. First, we placed the resistant milking cattle between the BLV-positive and BLV-negative milking cattle in a stall barn for 3 yr. Interestingly, the resistant cattle proved to be an effective biological barrier to successfully block the new BLV infections in the stall-barn system among all 4 farms. Concomitantly, we actively eliminated cattle with high PVL, especially susceptible cattle. Indeed, 39 of the 60 susceptible cattle (65%), 76 of the 140 neutral cattle (54%), and 20 of the 41 resistant cattle (48.8%) were culled on 4 farms for 3 years. Consequently, BLV prevalence and mean PVL decreased in all 4 farms. In particular, one farm achieved BLV-free status in May 2020. By decreasing the number of BLV-positive animals, the revenue-enhancing effect was estimated to be ¥5,839,262 ($39,292.39) for the 4 farms over 3 yr. Our results suggest that an integrated BLV eradication program utilization of resistant cattle as a biological barrier and the preferential elimination of susceptible cattle are useful for BLV infection control.

Development of a novel fluorogenic assay method for screening inhibitors of bovine leukemia virus protease and identification of mitorubrinic acid as an anti-BLV compound

Bovine leukemia virus (BLV) causes enzootic bovine leukosis, a fatal cattle disease that leads to significant economic losses in the livestock industry. Currently, no effective BLV countermeasures exist, except testing and culling. In this study, we developed a high-throughput fluorogenic assay to evaluate the inhibitory activity of various compounds on BLV protease, an essential enzyme for viral replication. The developed assay method was used to screen a chemical library, and mitorubrinic acid was identified as a BLV protease inhibitor that exhibited stronger inhibitory activity than amprenavir. Additionally, the anti-BLV activity of both compounds was evaluated using a cell-based assay, and mitorubrinic acid was found to exhibit inhibitory activity without cytotoxicity. This study presents the first report of a natural inhibitor of BLV protease-mitorubrinic acid-a potential candidate for the development of anti-BLV drugs. The developed method can be used for high-throughput screening of large-scale chemical libraries.

Identifying Pathogen and Allele Type Simultaneously in a Single Well Using Droplet Digital PCR

In the transmission control of chronic and untreatable livestock diseases such as bovine leukemia virus (BLV) infection, the removal of viral superspreaders is a fundamental approach. On the other hand, selective breeding of cattle with BLV-resistant capacity is also critical for reducing the viral damage to productivity by keeping infected cattle. To provide a way of measuring BLV proviral load (PVL) and identifying susceptible/resistant cattle simply and rapidly, we developed a fourplex droplet digital PCR method targeting the BLV pol gene, BLV-susceptible bovine major histocompatibility complex (BoLA)-DRB3*016:01 allele, resistant DRB3*009:02 allele, and housekeeping RPP30 gene (IPATS-BLV). IPATS-BLV successfully measured the percentage of BLV-infected cells and determined allele types precisely. Furthermore, it discriminated homozygous from heterozygous carriers. Using this method to determine the impact of carrying these alleles on the BLV PVL, we found DRB3*009:02-carrying cattle could suppress the PVL to a low or undetectable level, even with the presence of a susceptible heterozygous allele. Although the population of DRB3*016:01-carrying cattle showed significantly higher PVLs compared with cattle carrying other alleles, their individual PVLs were highly variable. Because of the simplicity and speed of this single-well assay, our method has the potential of being a suitable platform for the combined diagnosis of pathogen level and host biomarkers in other infectious diseases satisfying the two following characteristics of disease outcomes: (i) pathogen level acts as a critical maker of disease progression; and (ii) impactful disease-related host genetic biomarkers are already identified. IMPORTANCE While pathogen-level quantification is an important diagnostic of disease severity and transmissibility, disease-related host biomarkers are also useful in predicting outcomes in infectious diseases. In this study, we demonstrate that combined proviral load (PVL) and host biomarker diagnostics can be used to detect bovine leukemia virus (BLV) infection, which has a negative economic impact on the cattle industry. We developed a fourplex droplet digital PCR assay for PVL of BLV and susceptible and resistant host genes named IPATS-BLV. IPATS-BLV has inherent merits in measuring PVL and identifying susceptible and resistant cattle with superior simplicity and speed because of a single-well assay. Our new laboratory technique contributes to strengthening risk-based herd management used to control within-herd BLV transmission. Furthermore, this assay design potentially improves the diagnostics of other infectious diseases by combining the pathogen level and disease-related host genetic biomarker to predict disease outcomes.

Clone Dynamics and Its Application for the Diagnosis of Enzootic Bovine Leukosis

Bovine leukemia virus (BLV) infection results in polyclonal expansion of infected B lymphocytes, and ~5% of infected cattle develop enzootic bovine leukosis (EBL). Since BLV is a retrovirus, each individual clone can be identified by using viral integration sites. To investigate the distribution of tumor cells in EBL cattle, we performed viral integration site analysis by using a viral DNA capture-sequencing method. We found that the same tumor clones existed in peripheral blood, with a dominance similar to that in lymphoma tissue. Additionally, we observed that multiple tumor tissues from different sites harbored the identical clones, indicating that tumor cells can circulate and distribute systematically in EBL cattle. To investigate clonal expansion of BLV-infected cells during a long latent period, we collected peripheral blood samples from asymptomatic cattle every 2 years, among which several cattle developed EBL. We found that no detectable EBL clone existed before the diagnosis of EBL in some cases; in the other cases, clones that were later detected as malignant clones at the EBL stage were present several months or even years before the disease onset. To establish a feasible clonality-based method for the diagnosis of EBL, we simplified a quick and cost-effective method, namely, rapid amplification of integration sites for BLV infection (BLV-RAIS). We found that the clonality values (Cvs) were well correlated between the BLV-RAIS and viral DNA capture-sequencing methods. Furthermore, receiver operating characteristic (ROC) curve analysis identified an optimal Cv cutoff value of 0.4 for EBL diagnosis, with excellent diagnostic sensitivity (94%) and specificity (100%). These results indicated that the RAIS method efficiently and reliably detected expanded clones not only in lymphoma tissue but also in peripheral blood. Overall, our findings elucidated the clonal dynamics of BLV- infected cells during EBL development. In addition, Cvs of BLV-infected cells in blood can be used to establish a valid and noninvasive diagnostic test for potential EBL onset. IMPORTANCE Although BLV has been eradicated in some European countries, BLV is still endemic in other countries, including Japan and the United States. EBL causes huge economic damage to the cattle industry. However, there are no effective drugs or vaccines to control BLV infection and related diseases. The strategy of eradication of infected cattle is not practical due to the high endemicity of BLV. Furthermore, how BLV-infected B cell clones proliferate during oncogenesis and their distribution in EBL cattle have yet to be elucidated. Here, we provided evidence that tumor cells are circulating in the blood of diseased cattle. Thus, the Cv of virus-infected cells in blood is useful information for the evaluation of the disease status. The BLV-RAIS method provides quantitative and accurate clonality information and therefore is a promising method for the diagnosis of EBL.

Diagnosis and Early Prediction of Lymphoma Using High-Throughput Clonality Analysis of Bovine Leukemia Virus-Infected Cells

Bovine leukemia virus (BLV), a retrovirus, infects B cells of ruminants and is integrated into the host genome as a provirus for lifelong infection. After a long latent period, 1% to 5% of BLV-infected cattle develop aggressive lymphoma, enzootic bovine leukosis (EBL). Since the clonal expansion of BLV-infected cells is essential for the development of EBL, the clonality of proviral integration sites could be a molecular marker for diagnosis and early prediction of EBL. Recently, we developed Rapid Amplification of the Integration Site without Interference by Genomic DNA Contamination (RAISING) and an analysis software of clonality value (CLOVA) to analyze the clonality of transgene-integrated cells. RAISING-CLOVA is capable of assessing the risk of adult T-cell leukemia/lymphoma development in human T-cell leukemia virus-I-infected individuals through the clonality analysis of proviral integration sites. Thus, we herein examined the performance of RAISING-CLOVA for the clonality analysis of BLV-infected cells and conducted a comprehensive clonality analysis by RAISING-CLOVA in EBL and non-EBL cattle. RAISING-CLOVA targeting BLV was a highly accurate and reproducible method for measuring the clonality value. The comprehensive clonality analysis successfully distinguished EBL from non-EBL specimens with high sensitivity and specificity. A longitudinal clonality analysis in BLV-infected sheep, an experimental model of lymphoma, also confirmed the effectiveness of RAISING-CLOVA for early detection of EBL development. Therefore, our study emphasizes the usefulness of RAISING-CLOVA as a routine clinical test for monitoring virus-related cancers. IMPORTANCE Bovine leukemia virus (BLV) infection causes aggressive B-cell lymphoma in cattle and sheep. The virus has spread to farms around the world, causing significant economic damage to the livestock industry. Thus, the identification of high-risk asymptomatic cattle before they develop lymphoma can be effective in reducing the economic damage. Clonal expansion of BLV-infected cells is a promising marker for the development of lymphoma. Recently, we have developed a high-throughput method to amplify random integration sites of transgenes in host genomes and analyze their clonality, named as RAISING-CLOVA. As a new application of our technology, in this study, we demonstrate the value of the RAISING-CLOVA method for the diagnosis and early prediction of lymphoma development by BLV infection in cattle. RAISING-CLOVA is a reliable technology for monitoring the clonality of BLV-infected cells and would contribute to reduce the economic losses by EBL development.

Immune epitopes identification and designing of a multi-epitope vaccine against bovine leukemia virus: a molecular dynamics and immune simulation approaches

Background

Bovine leukemia virus (BLV) is an oncogenic delta-retrovirus causing bovine leucosis. Studies on BLV have shown the association with human breast cancer. However, the exact molecular mechanism is neither known nor their appropriate preventative measure to halt the disease initiation and progression. In this study, we designed a multi-epitope vaccine against BLV using a computational analyses.

Methods

Following a rigorous assessment, the vaccine was constructed using the T-cell epitopes from each BLV-derived protein with suitable adjuvant and linkers. Both physicochemistry and immunogenic potency as well as the safeness of the vaccine candidate were assessed. Population coverage was done to evaluate the vaccine probable efficiency in eliciting the immune response worldwide. After homology modeling, the three-dimensional structure was refined and validated to determine the quality of the designed vaccine. The vaccine protein was then subjected to molecular docking with Toll-like receptor 3 (TLR3) to evaluate the binding efficiency followed by dynamic simulation for stable interaction.

Results

Our vaccine construct has the potential immune response and good physicochemical properties. The vaccine is antigenic and immunogenic, and has no allergenic or toxic effect on the human body. This novel vaccine contains a significant interactions and binding affinity with the TLR3 receptor.

Conclusions

The proposed vaccine candidate would be structurally stable and capable of generating an effective immune response to combat BLV infections. However, experimental evaluations are essential to validate the exact safety and immunogenic profiling of this vaccine.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00262-022-03181-w.

Kinetics of Bovine leukemia virus aspartic protease reveals its dimerization and conformational change

The retropepsin (PR) of the Bovine leukemia virus (BLV) plays, as in other retroviruses, a crucial role in the transition from the non-infective viral particle to the infective virion by processing the polyprotein Gag. PR is expressed as an immature precursor associated with Gag, after an occasional −1 ribosomal frameshifting event. Self-hydrolysis of PR at specific N- and C-terminal sites releases the monomer that dimerizes giving rise to the active protease. We designed a strategy to express BLV PR in E. coli as a fusion protein with maltose binding protein, with a six-histidine tag at its N-terminal end, and bearing a tobacco etch virus protease hydrolysis site. This allowed us to obtain soluble and mature recombinant PR in relatively good yields, with exactly the same amino acid composition as the native protein. As PR presents relative promiscuity for the hydrolysis sites we designed four fluorogenic peptide substrates based on Förster resonance energy transfer (FRET) in order to characterize the activity of the recombinant enzyme. These substrates opened the way to perform kinetic studies, allowing us to characterize the dimer-monomer equilibrium. Furthermore, we obtained kinetic evidence for the existence of a conformational change that enables the interaction with the substrate. These results constitute a starting point for the elucidation of the kinetic properties of BLV-PR, and may be relevant not only to improve the chemical warfare against this virus but also to better understand other viral PRs.

Assessment of Natural Transmission of Bovine Leukemia Virus in Dairies from Southern Chile

Bovine leukemia virus (BLV) is a retrovirus that affects cattle worldwide. A longitudinal study was conducted with the aim to (a) estimate the incidence rate of the BLV infection of dairy farms in the regions of Los Ríos and Los Lagos (Chile), and (b) describe the frequency and epidemiological association of risk management practices related to new cases in cattle on dairy farms in Southern Chile. Infection status was based on commercial blocking ELISA results, on serum and milk. Individual information on animals and management practices was extracted from farm records, and then the most likely date of infection for new cases was estimated. The number of new infections was used to calculate the within-herd incidence rate. Adult animals had an incidence rate of 1.16 (95% CI 0.96; 1.20) cases per 100 cow-months at risk, while for young animals it was 0.64 (95% CI 0.44; 1.00) cases per 100 animal-months at risk. Rectal palpation, artificial insemination, and injections were the most common practices related to infection. Further studies are needed to determine if these are the only practices that facilitate spreading or if there are other practices that can be handled better in order to reduce the spread of BLV.

Phylogenomics and Spatiotemporal Dynamics of Bovine Leukemia Virus Focusing on Asian Native Cattle: Insights Into the Early Origin and Global Dissemination

Bovine leukemia virus (BLV), the causative agent of enzootic bovine leukosis, is currently one of the most important pathogens affecting the cattle industry worldwide. Determining where and in which host it originated, and how it dispersed across continents will provide valuable insights into its historical emergence as the cattle pathogen. Various species in the Bos genus were domesticated in Asia, where they also diversified. As native cattle (taurine cattle, zebu cattle, yak, and water buffalo) are indigenous and adapted to local environments, we hypothesized that Asian native cattle could have harbored BLV and, therefore, that they were important for virus emergence, maintenance, and spread. In this study, phylogeographic and ancestral trait analyses—including sequences obtained from Asian native cattle—were used to reconstruct the evolutionary history of BLV. It was shown that, since its probable emergence in Asia, the virus spread to South America and Europe via international trade of live cattle. It was inferred that zebu cattle were the hosts for the early origin of BLV, while taurine cattle played the significant role in the transmission worldwide. In addition, the results of positive selection analysis indicate that yak had a substantially minor role in the transmission of this virus. In this study, endogenous deltaretrovirus sequences in bats, collected in Asian countries, were also analyzed on whether these sequences were present in the bat genome. Endogenous deltaretrovirus sequences were detected from bat species endemic to specific regions and geographically isolated for a long time. Endogenous deltaretrovirus sequences from these geographically isolated species represent ancient exogenous deltaretroviruses distributions. The phylogenetic analysis revealed that these newly obtained endogenous deltaretrovirus sequences were closely related to those of BLV from Asian native cattle, indicating that BLV-related ancient deltaretroviruses circulated in Asia long before the emergence of BLV. Together, our analyses provide evidence for origin and spatiotemporal dynamics of BLV.

Comparative Evaluation of Three Commercial Quantitative Real-Time PCRs Used in Japan for Bovine Leukemia Virus

Bovine leukemia virus (BLV) is an oncogenic virus belonging to the genus Deltaretrovirus and is the causative agent of enzootic bovine leukosis. Proviral load (PVL) determined by real-time quantitative PCR (qPCR) is now widely used as an indicator of not only BLV infection, but also BLV disease progression. To interpret PVLs determined by different qPCRs used in Japan, we compared a chimeric cycling probe-based qPCR, CY415, targeting the BLV tax region; a TaqMan probe-based qPCR, RC202, targeting the BLV pol region; and a TaqMan probe-based qPCR, CoCoMo, targeting the BLV long terminal repeat (LTR) region. Whole-blood samples collected from 317 naturally BLV-infected cattle (165 Holstein–Friesian and 152 Japanese Black) and tumor tissue samples collected from 32 cattle at a meat inspection center were used. The PVLs determined by each qPCR were strongly correlated. However, the PVL and the proportion of BLV-infected cells determined by RC202 or CoCoMo were significantly higher than those determined by CY415. Genetic analysis of three tumor tissue samples revealed that LTR region mutations or a deletion affected the PVL determined by CoCoMo. These results suggest that the TaqMan-based RC202 or CoCoMo qPCR is better than CY415 for BLV PVL analysis. However, qPCR target region mutations were not rare in tumors and could hamper PVL analysis by using qPCR.

Design and Synthesis of Bovine Leukemia Virus-Associated Peptide-Based Qβ Conjugate Eliciting Long-Lasting Neutralizing Antibodies in Mice

Bovine leukemia virus (BLV) is a C-type retrovirus of cattle that causes huge economic losses with high infection rates in the majority of countries worldwide. To develop an anti-BLV vaccine, we constructed a peptide conjugate using the envelope glycoprotein gp51-peptide epitope, a putative receptor-binding site. This highly antigenic peptide was covalently linked to a mutant bacteriophage carrier (mQβ) using two different linker strategies, isothiocyanate (NCS) and dinitrophenyl adipate. Both constructs elicited higher anti-BLV peptide IgG titers than the corresponding conjugate with keyhole limpet hemocyanin protein carrier (gold standard) in mice with the NCS linker strategy requiring less sample processing. The mQβ-gp51-peptide construct is the first BLV peptide-based vaccine candidate to generate durable immunity (>539 days), which recognized both native gp51 protein and BLV particles and significantly decreased fusion of a susceptible cell line exposed to infectious BLV. These results support the high translational and animal health potential of the vaccine construct.

Molecular Characterization of Bovine Leukemia Virus with the Evidence of a New Genotype Circulating in Cattle from Kazakhstan

Bovine leukemia virus (BLV) is a retrovirus that causes enzootic bovine leukosis (EBL) and has worldwide distribution. Infections with BLV have been reported in cattle from Kazakhstan but the virus has not yet been thoroughly characterized. In this study, we detect and estimate the level of BLV proviral DNA by qPCR in DNA samples from 119 cattle naturally infected with BLV, from 18 farms located in four different geographical regions of Kazakhstan. Furthermore, we conducted the phylogenetic and molecular analysis of 41 BLV env-gp51 gene sequences from BLV infected cattle. Phylogenetic analysis showed the affiliation of sequences to two already known genotypes G4 and G7 and also to a new genotype, classified as genotype G12. In addition, a multivariate method was employed for analysis of the association between proviral load and different variables such as the geographical location of the herd, cattle breeds, age of animals, and the presence of particular BLV genotypes. In summary, the results of this study provide the first evidence on molecular characterization of BLV circulating in cattle from Kazakhstan.

A pooled testing system to rapidly identify cattle carrying the elite controller BoLA-DRB3*009:02 haplotype against bovine leukemia virus infection

As genetically resistant individuals, the “elite controllers” (ECs) of human immunodeficiency virus infection have been focused on as the keys to developing further functional treatments in medicine. In the livestock production field, identifying the ECs of bovine leukemia virus (BLV) infection in cattle is desired to stop BLV transmission chains on farms. Cattle carrying the bovine leukocyte antigen (BoLA)‐DRB3*009:02 allele (DRB3*009:02) have a strong possibility of being BLV ECs. Most of cattle carrying this allele maintain undetectable BLV proviral loads and do not shed virus even when infected. BLV ECs can act as transmission barriers when placed between uninfected and infected cattle in a barn. To identify cattle carrying DRB3*009:02 in large populations more easily, we developed a pooled testing system. It employs a highly sensitive, specific real‐time PCR assay and TaqMan MGB probes (DRB3*009:02‐TaqMan assay). Using this system, we determined the percentage of DRB3*009:02‐carrying cattle on Kyushu Island, Japan. Our pooled testing system detected cattle carrying the DRB3*009:02 allele from a DNA pool containing one DRB3*009:02‐positive animal and 29 cattle with other alleles. Its capacity is sufficient for herd‐level screening for DRB3*009:02‐carrying cattle. The DRB3*009:02‐TaqMan assay showed high‐discriminative sensitivity and specificity toward DRB3*009:02, making it suitable for identifying DRB3*009:02‐carrying cattle in post‐screening tests on individuals. We determined that the percentage of DRB3*009:02‐carrying cattle in Kyushu Island was 10.56%. With its ease of use and reliable detection, this new method strengthens the laboratory typing for DRB3*009:02‐carrying cattle. Thus, our findings support the use of BLV ECs in the field.

Bovine Leukaemia Virus: Current Epidemiological Circumstance and Future Prospective

Bovine leukaemia virus (BLV) is a deltaretrovirus that is closely related to human T-cell leukaemia virus types 1 and 2 (HTLV-1 and -2). It causes enzootic bovine leukosis (EBL), which is the most important neoplastic disease in cattle. Most BLV-infected cattle are asymptomatic, which potentiates extremely high shedding rates of the virus in many cattle populations. Approximately 30% of them show persistent lymphocytosis that has various clinical outcomes; only a small proportion of animals (less than 5%) exhibit signs of EBL. BLV causes major economic losses in the cattle industry, especially in dairy farms. Direct costs are due to a decrease in animal productivity and in cow longevity; indirect costs are caused by restrictions that are placed on the import of animals and animal products from infected areas. Most European regions have implemented an efficient eradication programme, yet BLV prevalence remains high worldwide. Control of the disease is not feasible because there is no effective vaccine against it. Therefore, detection and early diagnosis of the disease are essential in order to diminish its spreading and the economic losses it causes. This review comprises an overview of bovine leukosis, which highlights the epidemiology of the disease, diagnostic tests that are used and effective control strategies.

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.

Cytokine TNF-α and its receptors TNFRI and TNFRII play a key role in the in vitro proliferative response of BLV infected animals

Bovine leukemia virus (BLV) main host cells are B lymphocytes. Infected animals can be classified into high or low proviral load (HPL or LPL respectively), regarding the number of proviral copies infected lymphocytes they carry. After infection, there is an overexpression of several cytokines, particularly TNF-α, which has a delicate regulation mediated by receptors TNFRI and TNFRII; the first one involved with apoptosis, while the other stimulates cell proliferation. The study aimed to quantify TNF-α and its receptors mRNA expression, and in which extent in vitro proliferation was affected, in peripheral blood mononuclear cells (PBMC) from BLV-infected animals with different proviral loads, after the addition or not of synthetic TNF-α (rTNF-α) for 48 h. PBMC from BLV-infected animals showed spontaneous proliferation after 48 h in culture but did not show changes in proliferation rates after 48 h incubation in the presence of the rTNF-α. TNF-α mRNA expression after 48 h culture without exogenous stimulation was significantly lower, regardless of the proviral load of the donor, compared to non-infected animals. In the LPL animals, the expression of TNF-α mRNA was significantly lower with respect to the control group while the expression of TNFRI mRNA was significantly increased. The HPL animals showed a significant decrease in the expression of TNF-α and TNFRII mRNA respect to the control group. After 48 h incubation with rTNF-α, PBMC from infected animals had different responses: TNF-α and TNFRI mRNA expression was reduced in PBMC from the LPL group compared to the BLV negative group, but no differences were observed in PBMC from the HPL group. TNFRII mRNA expression showed no differences between HPL, LPL, and BLV negative groups, though HPL animals expressed 10.35 times more TNFRI mRNA than LPL. These results support the hypothesis that LPL animals, when faced with viral reactivation, present a pro-apoptotic and anti-proliferative state. However, complementary studies are needed to explain the influence of TNFRII on the development of the HLP profile. On the other hand, exogenous stimulation studies reinforce the hypothesis that BLV infection compromises the immune response of the animals.

Molecular Characterization of the env Gene of Bovine Leukemia Virus in Cattle from Pakistan with NGS-Based Evidence of Virus Heterogeneity

Characterization of the global genetic diversity of the bovine leukemia virus (BLV) is an ongoing international research effort. Up to now BLV sequences have been classified into eleven distinct genotypes. Although BLV genotyping and molecular analysis of field isolates were reported in many countries, there is no report describing BLV genotypes present in cattle from Pakistan. In this study we examined 27 env gene sequences from BLV-infected cattle coming from four farms located in Khyber Pakhtunkwa, Gilgit Baltisan and Punjab provinces. Phylogenetic analyses revealed the classification of Pakistani sequences into genotypes G1 and G6. The alignment with the FLK-BLV sequence revealed the presence of 45 mutations, namely, seven in genotype G1 and 33 in genotype G6. Five mutations were found in both, G1 and G6 genotypes. Twelve amino acid substitutions were found in the analyzed sequences, of which only one P264S was specific for sequences from Pakistan. Furthermore, a certain degree of nucleotide heterogeneity was identified by NGS. These results highlight the need for further study on the importance of genetic variability of BLV, especially in the context of its pathogenicity and potential effect on serological detection.

Specific antiviral effect of violaceoid E on bovine leukemia virus

Bovine leukemia virus (BLV) infection has spread worldwide causing significant economic losses in the livestock industry. In countries with a high prevalence of BLV, minimizing economic losses is challenging; thus, research into various countermeasures is important for improving BLV control. Because anti-BLV drugs have not been developed, the present study explored a promising chemical compound with anti-BLV activity. Initially, screening of a chemical compound library revealed that violaceoid E (vioE), which is isolated from fungus, showed antiviral activity. Further analysis demonstrated that the antiviral effect of vioE inhibited transcriptional activation of BLV. Cellular thermal shift assay and pulldown assays provided evidence for a direct interaction between vioE and the viral transactivator protein, Tax. These data indicate that interference with Tax-dependent transcription could be a novel target for development of anti-BLV drugs. Therefore, it is suggested that vioE is a novel antiviral compound against BLV.

Overview of Mitigation Programs for Non-EU-Regulated Cattle Diseases in Austria

Background: The non-regulation of animal diseases due to missing regulation at the European Union (EU) level enables member states to implement mitigation programs based on their own country-specific conditions such as priority settings of the governments, availability of financial resources, and epidemiological situation. This can result in a heterogeneous distribution of mitigation activities and prevalence levels within and/or between countries, which can cause difficulties for intracommunity trade. This article aims to describe the past, current, and future mitigation activities and associated prevalence levels for four non-regulated animal diseases, i.e., enzootic bovine leukosis (EBL), infectious bovine rhinotracheitis/infectious pustular vulvovaginitis (IBR/IPV), bovine viral diarrhea (BVD), and bluetongue disease (BT) for Austria. Over a period of 40 years (1978-2020), regulations concerning EBL, IBR/IPV, BVD, and BT were retraced to analyze the changes of legislation, focusing on sampling, testing, and mitigation activities in Austria, and were linked to the collected diagnostic testing results. The study results clearly demonstrate the adoption of the legislation by the Austrian governments in dependency of the epidemiological situations. Furthermore, our study shows that, related to the forthcoming Animal Health Law on April 21, 2021, Austria has a good initial situation to achieve disease-free status and/or free from infection status based on the current available epidemiological situation and previously implemented mitigation activities. The study results presented here are intended to contribute to a better comparison of the eradication status across the European countries for non-EU-regulated cattle diseases by providing information about the mitigation activities and data of testing results over a period of 40 years.

Bayesian Estimation of the True Seroprevalence and Risk Factor Analysis of Bovine Leukemia Virus Infection in Pakistan

Simple Summary

The enzootic bovine leucosis is the most common and economically important viral disease of cattle, caused by bovine leukemia virus (BLV). In cattle, infection with BLV leads to decreased milk production and premature culling and also impairs the immune system predisposing animals to other infections and increasing severity of disease. The aim of this study was to estimate the true seropositivity to BLV at the farm and within-farm levels in Pakistan, using a latent class analysis. In addition, some factors influencing BLV seropositivity were analyzed. We tested 1380 dairy cattle from 451 herds and 92 water buffalo. Analysis at the within-herd and herd levels showed 3.8% of cattle and 1.4% of herds were truly seropositive. All 92 serum samples from water buffalo were negative. The study demonstrated strong association between BLV seroprevalence and herd size but not with common housing of cattle representing indigenous breeds with exotic breed or their crossbred and also common housing of cattle and water buffalo.

Abstract

The objective of this study was to determine the true seroprevalence of bovine leukemia virus (BLV) infection in dairy cattle from Pakistan at the animal and herd-level. We tested 1380 dairy cattle from 451 herds and 92 water buffalo. The sera were tested by ELISA and the results were analyzed using Bayesian inference. The median posterior estimate of the herd level true BLV prevalence was 1.4%, with a 95% credible interval (CI) 0.7–3.1, whereas the median posterior estimate of the within-farm true seroprevalence was 3.8% with a 95% CI 2.8–4.8. All 92 sera collected from water buffalo were negative. Several risk factors potentially associated with seropositivity to BLV infections in Pakistan were analyzed using logistic regression model based on calculation of an odds ratio (OR). The study showed an association between seropositivity and medium herd (≥50) size (OR = 23.57, 95% CI: 3.01–103.48). Common housing of indigenous cattle with exotic-breed cattle (OR = 0.67, 95% CI: 06–2.35) or housing indigenous or their crossbred cattle with exotic-breed cattle (OR = 0.95, 95% CI: 0.14–3.01) had no effect on the BLV seroprevalence. Similarly, common housing of cattle and water buffalo was not risk factor for increased BLV seropositivity (OR = 27.10, 95% CI: 0.63–119.34).

Risk Assessment of Bovine Major Histocompatibility Complex Class II DRB3 Alleles for Perinatal Transmission of Bovine Leukemia Virus

Perinatal transmission plays a critical role in the spread of bovine leukemia virus (BLV) infection in cattle herds. In the Holstein breed, we previously identified BLV resistant and susceptible bovine leukocyte antigen (BoLA)-DRB3 alleles, including BoLA-DRB3*009:02 and *014:01:01 with a low BLV proviral load (PVL), and *015:01 and *012:01 with a high PVL. Here, we evaluated the perinatal BLV transmission risk in dams with different BoLA-DRB3 alleles. BoLA-DRB3 alleles of 120 dam-calf pairs from five dairy farms in Japan were identified; their PVL was quantified using the BLV-Coordination of Common Motifs (CoCoMo)-qPCR-2 assay. Ninety-six dams were BLV-positive, and 29 gave birth to BLV-infected calves. Perinatal transmission frequency was 19% in dams with resistant alleles suppressed to a low PVL level, and 38% and 25% in dams with susceptible and neutral alleles that maintained high PVL levels, respectively. Notably, all calves with resistant alleles were BLV free, whereas 30% of calves with susceptible genes were infected. Thus, vertical transmission risk was extremely lower for dams and calves with resistant alleles compared to those with susceptible alleles. Our results can inform the development of effective BLV eradication programs under field conditions by providing necessary data to allow for optimal selection of dams for breeding.

Effectiveness of on-farm continuous flow high-temperature short-time pasteurization for inactivation of bovine leukemia virus in milk

The effectiveness of on-farm continuous flow high-temperature short-time (HTST) pasteurization (i.e., 72°C for 15 s) for the inactivation of bovine leukemia virus (BLV) in milk was investigated with a sheep bioassay. Four sheep that had been inoculated with completely pasteurized milk containing approximately 3.4 × 10⁷ BLV-infected peripheral blood mononuclear cells (PBMC) and treated by either HTST pasteurization or laboratory-scale low-temperature long-time (LTLT) pasteurization (i.e., 60°C for 30 min), remained negative for BLV for at least 17 weeks after inoculation. In contrast, all sheep inoculated with unpasteurized or inadequately pasteurized milk containing the same number of BLV-infected PBMC were tested positive for BLV and anti-BLV antibodies within 3 weeks after inoculation. These results suggest that on-farm continuous flow HTST pasteurization was equivalent value with inactivated BLV on the LTLT procedure and can effectively inactivate BLV in the milk. Therefore, on-farm HTST pasteurization of the pooled colostrum or milk used in automated feeding systems is likely to protect group-housed preweaned calves from BLV infection, thereby improving animal health on dairy farms.

Seroprevalence, Risk Factors and Molecular Identification of Bovine Leukemia Virus in Egyptian Cattle

Simple Summary

Bovine leukemia virus (BLV) is distributed worldwide and affects dairy cattle causing significant economic losses. This study’s objective was to assess the risk factors associated with BLV infection and identify the Egyptian BLV strain’s genetic diversity. The overall seroprevalence of BLV infection in Egyptian dairy cattle was 18.2%, and the grazing cattle in the losing house system had a higher probability of getting BLV infection. The sequencing and phylogenetic analysis for one Egyptian BLV strain was performed, and the obtained results confirmed the clustering of Egyptian BLV strain into genotype-1.

Abstract

Bovine leukemia virus (BLV) is distributed worldwide and affects dairy cattle causing severe economic losses. The BLV has been serologically reported in Egypt, but few studies have evaluated its associated risk factors and genetic classification. Therefore, this study assessed risk factors associated with BLV infection and identified the genetic diversity of the Egyptian strain. The study was conducted on 500 dairy cattle distributed in four Governorates located in Northern Egypt. Overall, the seroprevalence of BLV infection among Egyptian dairy cattle was 18.2%. The grazing cattle in the losing house system had higher odds for BLV seropositivity, and bad practice such as the use of a single needle or one plastic glove for more than one animal was considered a significant risk factor for BLV infection. Besides, the sequencing and phylogenetic analysis for one Egyptian BLV strain was performed, and the obtained results confirmed the clustering of Egyptian BLV strain into genotype-1. The assessment of associated risk factors for BLV infection and determination of its genetic classification are essential to implement an effective control program.

Current Developments in the Epidemiology and Control of Enzootic Bovine Leukosis as Caused by Bovine Leukemia Virus

Enzootic Bovine Leukosis (EBL) caused by the bovine leukemia virus (BLV) has been eradicated in over 20 countries. In contrast, the U.S. and many other nations are experiencing increasing prevalence in the absence of efforts to control transmission. Recent studies have shown that BLV infection in dairy cattle has a greater impact beyond the long-recognized lymphoma development that occurs in <5% of infected cattle. Like other retroviruses, BLV appears to cause multiple immune system disruptions, affecting both cellular and humoral immunity, which are likely responsible for increasingly documented associations with decreased dairy production and decreased productive lifespan. Realization of these economic losses has increased interest in controlling BLV using technology that was unavailable decades ago, when many nations eradicated BLV via traditional antibody testing and slaughter methods. This traditional control is not economically feasible for many nations where the average herd antibody prevalence is rapidly approaching 50%. The ELISA screening of cattle with follow-up testing via qPCR for proviral load helps prioritize the most infectious cattle for segregation or culling. The efficacy of this approach has been demonstrated in at least four herds. Breeding cattle for resistance to BLV disease progression also appears to hold promise, and several laboratories are working on BLV vaccines. There are many research priorities for a wide variety of disciplines, especially including the need to investigate the reports linking BLV and human breast cancer.

Quantitative Risk Assessment for the Introduction of Bovine Leukemia Virus-Infected Cattle Using a Cattle Movement Network Analysis

The cattle industry is suffering economic losses caused by bovine leukemia virus (BLV) and enzootic bovine leukosis (EBL), the clinical condition associated with BLV infection. This pathogen spreads easily without detection by farmers and veterinarians due to the lack of obvious clinical signs. Cattle movement strongly contributes to the inter-farm transmission of BLV. This study quantified the farm-level risk of BLV introduction using a cattle movement analysis. A generalized linear mixed model predicting the proportion of BLV-infected cattle was constructed based on weighted in-degree centrality. Our results suggest a positive association between weighted in-degree centrality and the estimated number of introduced BLV-infected cattle. Remarkably, the introduction of approximately six cattle allowed at least one BLV-infected animal to be added to the farm in the worst-case scenario. These data suggest a high risk of BLV infection on farms with a high number of cattle being introduced. Our findings indicate the need to strengthen BLV control strategies, especially along the chain of cattle movement.

Sero-Prevalence of Bovine Leukemia Virus Infection in Kosovo Cattle

A cross-sectional survey was conducted in Kosovo to determine the presence and prevalence of bovine leukemia virus antibodies in cattle. A total of 5,051 serum samples from 315 villages were collected during 2016. Samples were tested using commercial indirect enzyme-linked immunosorbent assay. At least one sero-positive animal was found in 55 (17.5%) of the villages sampled, of which 23 had more than one sero-positive detected. Overall individual sero-prevalence, corrected for geographic distribution of samples, was found to be 2.26% (95% c.i. 1.62% to 3.04%). There were no statistically significant associations between serological status and herd size or age of animal, although sero-prevalence in 5-year-old cattle was higher than in the other ages. There was a statistical significant heterogeneity in sero-prevalence between different geographic zones of Kosovo (chi-square value = 20.68 (4 d.f.); (p=0.0004). Pairwise comparisons showed that sero-prevalence in the south was significantly higher than in the east and in the north and sero-prevalence in the west was significantly higher than in the north. The 3.11% aggregated sero-prevalence for the two highest sero-prevalence zones, south and west, was significantly higher than the 1.57% aggregated sero-prevalence for the remaining zones, centre, east and north (Fisher exact p-value (2-tail) = 0.0004). The reason for higher prevalence in the south and west of Kosovo is uncertain. These results may serve to enrich the information of bovine leukemia virus distribution in the region, as well as a starting point for the future control and eradication strategy in Kosovo.

North American Big Brown Bats (Eptesicus fuscus) Harbor an Exogenous Deltaretrovirus

Bats are the reservoir for a large number of zoonotic viruses, including members of Coronaviridae (severe acute respiratory syndrome coronavirus [SARS-CoV] and SARS-CoV-2), Paramyxoviridae (Hendra and Nipah viruses), Rhabdoviridae (rabies virus), and Filoviridae (Ebola virus) as exemplars. Many retroviruses, such as human immunodeficiency virus, are similarly zoonotic; however, only infectious exogenous gammaretroviruses have recently been identified in bats. Here, viral metagenomic sequencing of samples from bats submitted for rabies virus testing, largely due to human exposure, identified a novel, highly divergent exogenous Deltaretrovirus from a big brown bat (Eptesicus fuscus) in South Dakota. The virus sequence, corresponding to Eptesicus fuscus deltaretrovirus (EfDRV), comprised a nearly complete coding region comprised of canonical 5'-gag-pro-pol-env-3' genes with 37% to 51% identity to human T-lymphotropic virus (HTLV), an infectious retrovirus that causes T-cell lymphoma. A putative tax gene with 27% identity to HTLV was located downstream of the pol gene along with a gene harbored in an alternative reading frame which possessed a conserved domain for an Epstein-Barr virus nuclear antigen involved in gene transactivation, suggesting a regulatory function similar to that of the deltaretrovirus rex gene. A TaqMan reverse transcriptase PCR (RT-PCR) targeting the pol gene identified 4/60 (6.7%) bats as positive for EfDRV, which, combined with a search of the E. fuscus genome that failed to identify sequences with homology to EfDRV, suggests that EfDRV is an infectious exogenous virus. As all known members of Deltaretrovirus can cause malignancies and E. fuscus is widely distributed in the Americas, often with a colonial roosting behavior in human dwellings, further studies are needed to investigate potential zoonosis.IMPORTANCE Bats host a large numbers of viruses, many of which are zoonotic. In the United States, the big brown bat (Eptesicus fuscus) is widely distributed and lives in small colonies that roost in cavities, often in human dwellings, leading to frequent human interaction. Viral metagenomic sequencing of samples from an E. fuscus bat submitted for rabies testing identified the first exogenous bat Deltaretrovirus The E. fuscus deltaretrovirus (EfDRV) genome consists of the typical deltaretrovial 5'-gag-pro-pol-env-3' genes along with genes encoding two putative transcriptional transactivator proteins distantly related to the Tax protein of human T-cell lymphotrophic virus and nuclear antigen 3B of Epstein-Barr virus. Searches of the E. fuscus genome sequence failed to identify endogenous EfDRV. RT-PCR targeting the EfDRV pol gene identified 4/60 (6.7%) bats with positive results. Together, these results suggest that EfDRV is exogenous. As all members of Deltaretrovirus are associated with T- and B-cell malignancies or neurologic disease, further studies on possible zoonosis are warranted.

Changes in bovine leukemia virus serological status and lymphocyte count between dry-off and early lactation in Michigan dairy cows

This study addresses how the serological status of bovine leukemia virus (BLV) and lymphocyte count fluctuate from dry-off to early lactation in dairy cattle. Very few studies have investigated how BLV antibody status and lymphocyte count of cows changes longitudinally during the lactation cycle. Blood samples were collected from dairy cattle (n = 149) on 5 commercial dairy herds in Michigan at dry-off, close-up, and 7 to 10 d after calving. Plasma was analyzed for anti-BLV antibodies using a BLV-ELISA and whole blood was analyzed for lymphocyte counts. We found that BLV seroprevalence increased from dry-off (38.9%) to close-up (43.6%), then slightly decreased from close-up to 7 to 10 d after calving (43.0%). However, the change in seroprevalence was only significant from dry-off to close-up. Cows of third or higher parity were more likely to seroconvert than cows of lower parity and had the highest ELISA-negative prevalence of BLV. Lymphocyte counts were significantly higher in ELISA-positive animals, but only among second and third or greater parity animals. These results indicate that the use of lymphocyte counts as a disease severity monitoring tool for BLV should differ by parity group. Future studies should investigate if changes in seroprevalence are due to new infections or natural changes in antibody concentrations as the cow prepares for colostrum production. More accurate lymphocyte guidelines to be used for monitoring the progression of BLV should be created that consider parity and lactation stage.

Development of multipurpose recombinant reporter bovine leukemia virus

Bovine leukemia virus (BLV) is a global problem that results in significant economic losses to the livestock industry. We developed three virus strains by inserting the HiBiT reporter tag from NanoLuc luciferase (NLuc) into limited sites within BLV molecular clones. Initial analysis for site selection of the tag insertion revealed a permissible site immediately downstream of the viral envelope gene. Therefore, NLuc activity could be used to measure virus copy numbers in the supernatant and the levels of cell infection. Productivity and growth kinetics of the reporter virus were similar to those of the wild-type strain; therefore, the reporter virus can be used to characterize the replication of chimeric viruses as well as responses to the antiviral drug, amprenavir. Collectively, our results suggest that the BLV reporter virus with a HiBiT tag insertion is a highly versatile system for various purposes such as evaluating virus replication and antiviral drugs.

Risk factor analysis of bovine leukemia virus infection in dairy cattle in Egypt

Identification of the risk factors associated with Enzootic bovine leukosis (EBL) is essential for the adoption of potentially prevention strategies. Accordingly, our objectives were to determine the geographic distribution of Bovine Leukemia Virus (BLV) infection and identify the risk factors associated with cow-level BLV infection in the Egyptian dairy cattle. A cross-sectional study was conducted on 1299 mixed breed cows distributed over four provinces in the Nile Delta of Egypt in 2018. The randomly selected cows on each farm were serologically tested for BLV, and the cow's information was obtained from the farm records. Four variables (geographic location, herd size, number of parities, and age) were used for risk analysis. A total of 230 serum samples (17.7 %) were serologically positive for BLV. The highest prevalence of BLV infection was associated with parity (OR = 3.4, 95 %CI 2.4-4.9) with 80 % probability of being BLV-positive at parity ≥5, followed by herd size (OR = 1.8, 95 %CI 1.4-2.2). However, geographic location seems to have no impact on the prevalence of BLV infection in Egypt. Our findings strongly indicate that the intensive surveillance and effective prevention strategies against BLV infection in Egypt should be provided to multiparous cows with ≥5 parities and live in large farm with more than 200 cows.

Breeding bulls as a potential source of bovine leukemia virus transmission in beef herds

OBJECTIVE

To determine the prevalence of bovine leukemia virus (BLV) in beef bulls; evaluate the presence of BLV provirus DNA in blood, smegma, and semen samples; and analyze whether blood BLV proviral load was associated with differential blood cell counts.

DESIGN

Observational cross-sectional study.

ANIMALS

121 beef bulls ≥ 2 years old from 39 Michigan herds.

PROCEDURES

Blood, smegma, and semen samples were collected from each bull during a routine breeding soundness examination. An ELISA was used to detect serum anti-BLV antibodies. A coordination of common motifs-quantitative PCR assay was used to detect BLV provirus DNA in blood, smegma, and semen samples. Bulls with positive results on both the BLV serum ELISA and coordination of common motifs-quantitative PCR assay were considered infected with BLV.

RESULTS

19 of 39 (48.7%) herds and 54 of 121 (44.6%) bulls were infected with BLV. Provirus DNA was detected in the blood of all 54 and in smegma of 4 BLV-infected bulls but was not detected in any semen sample. Lymphocyte count was significantly greater in BLV-infected bulls than in uninfected bulls. The proportion of BLV-infected bulls with lymphocytosis (16/54 [29.6%]) was greater than the proportion of uninfected bulls with lymphocytosis (6/67 [9%]). Lymphocyte count was positively associated with BLV proviral load in BLV-infected bulls.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that almost half of beef bulls and herds were infected with BLV, and BLV provirus DNA was detected in the smegma of some BLV-infected bulls. Bulls may have an important role in BLV transmission in beef herds.

PRMT5 Is Required for Bovine Leukemia Virus Infection In Vivo and Regulates BLV Gene Expression, Syncytium Formation, and Glycosylation In Vitro

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leukosis, which is the most common neoplastic disease of cattle and is closely related to human T-cell leukemia viruses. We investigated the role of a new host protein, PRMT5, in BLV infection. We found that PRMT5 is overexpressed only in BLV-infected cattle with a high proviral load, but not in those with a low proviral load. Furthermore, this upregulation continued to the lymphoma stage. PRMT5 expression was upregulated in response to experimental BLV infection; moreover, PRMT5 upregulation began in an early stage of BLV infection rather than after a long period of proviral latency. Second, siRNA-mediated PRMT5 knockdown enhanced BLV gene expression at the transcript and protein levels. Additionally, a selective small-molecule inhibitor of PRMT5 (CMP5) enhanced BLV gene expression. Interestingly, CMP5 treatment, but not siRNA knockdown, altered the gp51 glycosylation pattern and increased the molecular weight of gp51, thereby decreasing BLV-induced syncytium formation. This was supported by the observation that CMP5 treatment enhanced the formation of the complex type of N-glycan more than the high mannose type. In conclusion, PRMT5 overexpression is related to the development of BLV infection with a high proviral load and lymphoma stage and PRMT5 inhibition enhances BLV gene expression. This is the first study to investigate the role of PRMT5 in BLV infection in vivo and in vitro and to reveal a novel function for a small-molecule compound in BLV-gp51 glycosylation processing.

Impact of bovine leukemia virus infection on beef cow longevity

Bovine leukosis is a chronic lymphoproliferative disorder caused by bovine leukemia virus (BLV). Previous studies estimate that 38 % of cow-calf beef herds and 10.3 % of individual beef cows in the US are BLV seropositive. About 70 % of BLV infected animals are asymptomatic carriers of the virus, while less than 5% develop lymphosarcoma, the leading reason for carcass condemnation at the US slaughterhouses. Studies provide evidence that BLV infection leads to decreased immune function making animals more vulnerable to other diseases, which could shorten their productive lifespan and increase economic losses in the cattle industry. BLV seropositive dairy cows are reportedly more likely to be culled sooner compared with their uninfected herd mates. Beyond simple prevalence studies, little is known about the impact of BLV infection in beef cattle production or specifically on beef cow longevity. Our objective was to determine the association between BLV infection and cow longevity in beef cow-calf operations. Twenty-seven cow-calf herds from the Upper Midwest volunteered to participate in this study. Female beef cattle (n = 3146) were tested for serum BLV antibodies by ELISA. A subsample of 648 cows were also tested for BLV proviral load (PVL). Culling data was collected for the subsequent 24 months. Twenty-one herds (77.7 %) had at least one BLV-infected animal, and 29.2 % (930/3146) of tested animals were BLV seropositive. Of the BLV-positive cows, 33.7 % (318/943) were culled compared with 32.1 % (541/1682) of the seronegative cows. BLV status did not affect cows' longevity within herds (P = 0.062). However, cows with high BLV PVL had decreased survival within the herd compared with ELISA- negative cows (P = 0.01). Overall, infection with BLV did not impact beef cow longevity unless the disease had progressed to a point of high BLV PVL.

Serological evidence of enzootic bovine leukosis in the periurban dairy cattle production system of Al Ain, United Arab Emirates

The serostatus of enzootic bovine leukosis (EBL) was determined at three dairy farms and the Al Ain Livestock Market (AALM), within the Al Ain region of Abu Dhabi, UAE. Of the 957 bovine sera tested by ELISA, 657 were from Holstein-Friesians from three dairy farms, and 300 from Bos indicus cattle at AALM. The chi-square homogeneity test (CSHT) and the Marascuilo multiple comparison procedure (MMCP) assessed the level of significance between the proportions of EBL-seropositive cattle (ESPC) across the study farms and AALM, and between the age groups at farms 1 and 3. Overall, the proportion of ESPC was 25.7% at dairy farms and AALM, 37.0% for farms and 1.0% for AALM. Furthermore, the proportions of ESPC at farms 1, 2 and 3 were 54.7%, 0.0% and 26.3% respectively, and statistically significant differences were seen across the farm/farm and farm/AALM comparisons, and between two age groups at farms 1 and 3. The 37-72-month-old age group showed the highest proportion of ESPC. This is the first serological evidence of EBL in the UAE. As previously reported, the ESPC are comparatively higher in dairy than Bos indicus cattle. Molecular and more extensive serological studies are needed to further corroborate the present data. Meanwhile, the UAE veterinary authorities will need to formulate national EBL control policies.

Early events following bovine leukaemia virus infection in calves with different alleles of the major histocompatibility complex DRB3 gene

Cattle maintaining a low proviral load (LPL) status after bovine leukaemia virus (BLV) infection have been recognized as BLV controllers and non-transmitters to uninfected cattle in experimental and natural conditions. LPL has been associated with host genetics, mainly with the BoLA class II DRB3 gene. The aim of this work was to study the kinetics of BLV and the host response in Holstein calves carrying different BoLA-DRB3 alleles. Twenty BLV-free calves were inoculated with infected lymphocytes. Two calves were maintained uninfected as controls. Proviral load, total leukocyte and lymphocyte counts, anti-BLVgp51 titres and BLVp24 expression levels were determined in blood samples at various times post-inoculation. The viral load peaked at 30 days post-inoculation (dpi) in all animals. The viral load decreased steadily from seroconversion (38 dpi) to the end of the study (178 dpi) in calves carrying a resistance-associated allele (*0902), while it was maintained at elevated levels in calves with *1501 or neutral alleles after seroconversion. Leukocyte and lymphocyte counts and BLVp24 expression did not significantly differ between genetic groups. Animals with < 20 proviral copies/30 ng of DNA at 178 dpi or < 200 proviral copies at 88 dpi were classified as LPL, while calves with levels above these limits were considered to have high proviral load (HPL) profiles. All six calves with the *1501 allele progressed to HPL, while LPL was attained by 6/7 (86%) and 2/6 (33%) of the calves with the *0902 and neutral alleles, respectively. One calf with both *0902 and *1501 developed LPL. This is the first report of experimental induction of the LPL profile in cattle.

Visualizing bovine leukemia virus (BLV)-infected cells and measuring BLV proviral loads in the milk of BLV seropositive dams

Bovine leukemia virus (BLV) infects cattle and causes serious problems for the cattle industry, worldwide. Vertical transmission of BLV occurs via in utero infection and ingestion of infected milk and colostrum. The aim of this study was to clarify whether milk is a risk factor in BLV transmission by quantifying proviral loads in milk and visualizing the infectivity of milk. We collected blood and milk from 48 dams (46 BLV seropositive dams and 2 seronegative dams) from seven farms in Japan and detected the BLV provirus in 43 blood samples (89.6%) but only 22 milk samples (45.8%) using BLV-CoCoMo-qPCR-2. Although the proviral loads in the milk tended to be lower, a positive correlation was firstly found between the proviral loads with blood and milk. Furthermore, the infectivity of milk cells with BLV was visualized ex vivo using a luminescence syncytium induction assay (LuSIA) based on CC81-GREMG cells, which form syncytia expressing enhanced green fluorescent protein (EGFP) in response to BLV Tax and Env expressions when co-cultured with BLV-infected cells. Interestingly, in addition to one BLV-infected dam with lymphoma, syncytia with EGFP fluorescence were observed in milk cells from six BLV-infected, but healthy, dams by an improved LuSIA, which was optimized for milk cells. This is the first report demonstrating the infectious capacity of cells in milk from BLV-infected dams by visualization of BLV infection ex vivo. Thus, our results suggest that milk is a potential risk factor for BLV vertical spread through cell to cell transmission.

Controlling bovine leukemia virus in dairy herds by identifying and removing cows with the highest proviral load and lymphocyte counts

The objective of this field trial was to reduce bovine leukemia virus (BLV) transmission and prevalence in commercial dairy herds using proviral load (PVL) and lymphocyte count (LC) measurements as indicators of the most infectious animals for culling or segregation. Bovine leukemia virus causes lymphoma in <5% of infected cattle, and increased lymphocyte counts (lymphocytosis) in about one-third. Recent research has shown that dairy cows infected with BLV have altered immune function associated with decreases in milk production and lifespan. Recent findings show that a minority of infected cattle have PVL concentrations in blood and other body fluids of over 1,000 times that of other infected cattle. In combination with a high LC, these animals are thought to be responsible for most transmission of BLV in a herd. Milk or blood samples from adult cows in our 3 Midwestern dairy farm field trials were tested semiannually with ELISA for BLV antibodies, and ELISA-positive cattle were then retested using a blood LC and a quantitative PCR test for PVL to identify the animals presumed to be most infectious. Herd managers were encouraged to consider PVL and LC status when making cull decisions, and to segregate cows with the highest PVL and LC from their BLV ELISA-negative herd mates where possible. After 2 to 2.5 yr of this intervention, the incidence risk of new infections decreased in all 3 herds combined, from 13.8 to 2.2, and the overall herd prevalence decreased in all 3 herds combined from 62.0 to 20.7%, suggesting that this approach can efficiently reduce BLV transmission as well as prevalence. This is encouraging, because a very low prevalence of BLV infection would make it economically feasible to cull the remaining ELISA-positive cattle, as was achieved in national eradication programs in other countries decades ago.

Control of Bovine Leukemia Virus in Three US Dairy Herds by Culling ELISA-Positive Cows

The objective of this trial was to evaluate a test-and-cull approach to controlling bovine leukemia virus (BLV) in US dairy herds with a low BLV prevalence. Despite worldwide distribution of the virus, 21 nations have eradicated BLV from their dairy cattle and are currently considered 'BLV-free.' In contrast, the US has attempted no industry-wide BLV control programs and has experienced an increase in BLV prevalence among dairy cows to about 40%. This raises concerns about production efficiency, herd health, and sustainability. In a pilot field trial with three Midwestern-US dairy herds, a test-and-cull approach using ELISA screening of milk samples was successful in reducing BLV prevalence in two herds. In the third herd, BLV prevalence increased following the introduction of infected heifers that were raised at an out-of-state calf raising facility. This trial demonstrated that a test-and-cull approach to BLV control can be successful in US dairy herds with low BLV prevalence, but ongoing surveillance is necessary to prevent reintroduction of the virus.