The recent prevalence of bovine leukemia virus (BLV) infection among Japanese cattle

Screening for Japanese Black cattle herds at risk of bovine leukemia virus transmission based on the presence of persistent lymphocytosis

This study aimed to develop a screening method to identify Japanese Black (JB) cattle farms at high risk of bovine leukemia virus (BLV) transmission. We introduced a recently established lymphocyte count (LC) cut-off to detect cattle with persistent lymphocytosis (PL). To identify high-risk farms, we examined the relationship between the proportion of cattle with PL and the mean blood proviral loads (PVL) per farm. The results showed a strong correlation between the proportion of cattle with high LC, higher than the LC cut-off values for JB cattle, and the mean blood PVL per farm. This indicates that the mean blood PVL on a herd basis of JB cattle can be estimated from the proportion of cattle with high LC. Specifically, the mean blood PVL on farms with >30 % cattle having high LC was estimated to be greater than 100 copies/10 ng DNA. The higher the proportion of cattle with high LC per farm, the higher the proportion of cattle at risk as sources of BLV infection. The study demonstrated that the proportion of cattle with high LC, based on the new LC cut-off, serves as a practical index for screening high-risk farms with PL cattle that have high PVL. This finding is meaningful for prioritizing farms with a high proportion of cattle with high LC, necessitating BLV infection prevention measures, such as voluntary culling and segregation, in order to develop a regional, stepwise BLV eradication strategy.

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.

Cytokine secretion in stem cells of cattle infected with bovine leukaemia virus

Introduction

Bovine leukaemia virus (BLV) is a Deltaretrovirus responsible for enzootic bovine leukosis, the most common neoplastic disease of cattle. It deregulates the immune system, favouring secondary infections and changes in the blood and lymphatic tissues. Blood homeostasis depends on functional haematopoietic stem cells (HSCs). Bone marrow is populated by these cells, which express CD34+ and CD35+ surface antigens and produce and release cytokines involved in the maintenance of haematopoiesis. The aim of the study was determination of the profile of cytokine production by CD34+ stem cells of cattle naturally infected with BLV.

Material and Methods

The HSCs were generated from the blood and lymphoid organs of cows infected with BLV and healthy control cows with immunomagnetic separation and anti-CD34+ monoclonal antibodies. Isolated CD34+ cells were cultivated for two weeks with interleukin (IL)-4 and granulocyte-macrophage colony-stimulating factor. The levels of IL-6, IL-10, IL-12p40, IL-12p70, interferon gamma (IFN-γ) and tumour necrosis factor alpha (TNF-α) were determined in culture fluid by flow cytometry.

Results

The expression of IL-6, IL-12p70 and TNF-α in blood HSCs was higher in BLV+ cows than in the control animals. In bone marrow HSCs of infected cows, IL-12, TNF-α and IFN-γ were more concentrated, but in these cows' spleen HSCs only expression of IL-10 was elevated. In HSCs isolated from the lymph nodes of leukaemic cows, only TNF-α secretion was lower than in control cows, the other cytokines being more potently secreted.

Conclusion

Infection with BLV caused statistically significant differences in cytokine expression by HSC CD34+ cells.

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.

Seroprevalence and risk factors associated with bovine Leukemia virus infection in argentine beef cattle

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leukosis, an endemic disease in dairy cattle of Argentina. However, little is known about the seroprevalence of BLV in beef cattle. In this study, we conducted a cross-sectional study including farms from thirteen provinces of Argentina. A total of 5827 bovine serum samples were collected from 76 farms and analyzed using an in-house developed enzyme-linked immunosorbent assay. Information about herd management was collected through a questionnaire, and univariate and multivariate analyses were performed to detect risk factors associated with BLV infection. Herd-level seroprevalence was 71.05%, while the mean animal-level seroprevalence was 7.23% (median = 2.69%; min = 0, max = 75). Only two provinces had no positive BLV samples. The other eleven provinces showed more than 50% of their farms infected with BLV. The multivariate model revealed that BLV prevalence was significantly associated with the use of animals raised in the same farm for cattle replacement (P = 0.005), breeding cows by natural mating with a bull (P < 0.001), and weaning calves after 6 months of age (P = 0.011). This extensive study revealed that BLV seroprevalence in Argentine beef farms has increased during the last years and allowed identifying some management practices associated with BLV prevalence. These data deserve special attention because BLV infection in beef cattle seems to lead to a dissemination pattern similar to that observed during the last decades in dairy cattle, especially considering that Argentina is the sixth beef producer in the world, with about 5% of global beef production.

Detection of bovine leukemia virus in beef cattle kept in the Central Coast Regions of Vietnam

Bovine leukemia virus (BLV) is the etiologic agent of enzootic bovine leucosis. Our previous study showed the BLV existence in cattle kept in the Red River Delta Region of Vietnam. However, no positive samples were identified in beef cattle. Besides, information related to the BLV circulation in the remained parts of Vietnam is limited. Therefore, we tested the existence of BLV in 48 beef cattle kept in the Central Coast Regions. Nested PCR targeting the BLV-env-gp51 confirmed the prevalence of 14.6% in investigated regions. Phylogenetic analysis suggested the co-existence of genotypes 1 and 10. The close relationship between strains found in Vietnam, Thailand, Myanmar, and China was revealed suggesting the possibility of BLV transmission through the movement of live cattle.

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.

No evidence of bovine leukemia virus proviral DNA and antibodies in human specimens from Japan

Background

The potential risk and association of bovine leukemia virus (BLV) with human remains controversial as it has been reported to be both positive and negative in human breast cancer and blood samples. Therefore, establishing the presence of BLV in comprehensive human clinical samples in different geographical locations is essential.

Result

In this study, we examined the presence of BLV proviral DNA in human blood and breast cancer tissue specimens from Japan. PCR analysis of BLV provirus in 97 Japanese human blood samples and 23 breast cancer tissues showed negative result for all samples tested using long-fragment PCR and highly-sensitive short-fragment PCR amplification. No IgG and IgM antibodies were detected in any of the 97 human serum samples using BLV gp51 and p24 indirect ELISA test. Western blot analysis also showed negative result for IgG and IgM antibodies in all tested human serum samples.

Conclusion

Our results indicate that Japanese human specimens including 97 human blood, 23 breast cancer tissues, and 97 serum samples were negative for BLV.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12977-022-00592-6.

Molecular investigation of possible relationships concerning bovine leukemia virus and breast cancer

Worldwide, breast cancer has an eminent morbidity and mortality rate, as it is a neoplastic disease among females. The query of the prospective danger of bovine leukemia virus (BLV) to humans is an old but exceedingly topical focus of scientific debate. The objective of the current study was to determine the possible relationship between BLV and breast cancer. A total of 2710 formalin-fixed paraffin-embedded (FFPE) breast cancer samples were selected regardless of the age, ethnicity, or municipality origin of the subjects. The presence of BLV in human breast cancer was determined through nested PCR by amplifying tax and gag genes followed by partial sequencing. Homology was confirmed by using the online BLAST Tool. BLV genes were found to be positive in 26.8% (728/2710) of the samples from breast cancer patients and 10% (10/80) of the samples without cancer (negative control). The results indicated a correlation between the presence of the BLV gene and breast cancer (odds ratio = 0.3889; confidence interval = 1,18; p = 0.0029). The current findings suggest a possible link between BLV and human breast carcinoma. Therefore, screening cattle herds and milk products is suggested to reduce the viral transmission risk to humans.

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.

Potential Risk Factors Associated with Infection with Bovine Leukaemia Virus in Dairy and Beef Cattle in Taiwan

Bovine leukaemia virus (BLV), which is classified as a Deltaretrovirus, is the aetiologic agent of enzootic bovine leukosis (EBL), a chronic lymphoproliferative disorder with a worldwide distribution. EBL is widespread in dairy herds and causes a direct economic impact due to reduced milk production and the early culling of BLV-infected cattle. The BLV infection status in Taiwan remains largely unknown; a high prevalence of BLV in dairy cows was recently revealed. The present study further investigated BLV infections in beef cattle. Surprisingly, the prevalence of BLV proviral DNA was as low as 11.8% (23/195), which is significantly lower than that noted in dairy cows, which was 42.5% (102/240) (p < 0.001). Factors associated with BLV infections were subsequently investigated. Due to the differences in herd management, an analysis of risk factors for a BLV infection was independently conducted in these two sectors. Several factors associated with a BLV infection were identified. Age was significantly associated with BLV infection status in dairy cows (p < 0.001) but not in beef cattle. A high prevalence of BLV was observed in cattle >15.5 months old (57.8%) compared with those ≤15.5 months old (11.4%). Moreover, after stratification analysis, based on the critical age of 15.5 months, as determined by the receiver operating characteristic (ROC) curve, a significantly higher BLV prevalence was demonstrated in lactating dairy cows, cattle undergoing bull breeding, heifers at older ages, and those undergoing routine rectal palpation. Due to the high prevalence of BLV in Taiwan, the development of an effective control program, based on the identified risk factors, is important for interrupting the routes of BLV transmission within herds.

Abnormal clonalities of B-lymphocytes in bovine leukemia virus-infected cattle with persistent lymphocytosis

Peripheral B-lymphocyte clonality of 274 bovine leukemia virus-infected cattle with lymphocytosis was analyzed using clonality PCR based on sequences of the variable region of the bovine immunoglobulin H chain. None of the cattle showed monoclonal proliferation, while 10, 31, and 233 showed minor-clonal, oligoclonal, and polyclonal proliferation, respectively. A total of 163 cattle were analyzable the following year, and lymphocytosis was maintained in 157, indicating persistent lymphocytosis (PL). B-lymphocyte clonality of the 157 PL cattle was minor-clonal in 6 (3.8%), oligoclonal in 8 (5.1%), and polyclonal in 143 (91.1%). A higher rate of enzootic bovine leukosis (EBL) onset within a year was observed in PL cattle with minor-clonal (50.0% (3/6)) and oligoclonal (25.0% (2/8)) proliferation compared to those with polyclonal (5.6% (8/143)) proliferation. Minor-clonal and oligoclonal proliferation in PL cattle may be a prognosis factor for developing EBL.

Estimation of economic loss by carcass weight reduction of Japanese dairy cows due to infection with bovine leukemia virus

Bovine leukemia virus (BLV) infection is endemic in Japanese dairy farms. To promote the participation of farmers in BLV infection control in Japan, it is important to provide estimates of the economic losses caused by this infection. We hypothesized that decreased immune function due to BLV infection would increase visceral abnormalities, in turn reducing carcass weight. We employed mediation analysis to estimate the annual economic loss due to carcass weight reduction caused by BLV infection. Culled Holstein cows from 12 commercial dairy farms in the Nemuro and Kushiro regions of Hokkaido, Japan, were traced. Information on age and the last delivery day were collected. A non-infected culled cow was defined as a cow from which BLV provirus was not detected. A high-proviral-load (H-PVL) cow was defined as a cow whose PVL titer was above 2465 copies/50 ng DNA or 56,765 copies/10⁵ cells. A BLV-infected cow with PVL titer lower than the thresholds was categorized as low-proviral load (L-PVL). Post-mortem examination results for culled cows were collected from a meat inspection center. The hypothesis was tested by three models, using data from 222 culled dairy cows. Model 1, a generalized linear mixed-effects model, selected carcass weight as an outcome variable, BLV status and the potential confounders (lactation stage and age) as explanatory variables, and herd as a random effect. Model 2 additionally included the number of abnormal findings in the post-mortem examination (AFPE) as an explanatory variable. Model 3 applied a Bayesian generalized linear mixed model, which employed a mediator separately modeled for AFPE, to estimate the amount of direct, indirect, and total carcass weight loss with adjustment for known confounding factors. Compared to the mean carcass weight for the non-infected culled cows, the carcass weight for H-PVL culled cows was significantly decreased by 30.4 kg on average. For each increase of one in the number of AFPE, the mean carcass weight was decreased by 8.6 kg. Only the indirect effect of BLV H-PVL status on carcass weight loss through AFPE was significant, accounting for 21.6 % of the total effect on carcass weight reduction. In 2017, 73,650 culled dairy cows were slaughtered in Hokkaido, and the economic loss due to carcass weight loss caused by BLV infection that year was estimated to be US $1,391,649. In summary, unlike L-PVL cows, H-PVL status was associated with carcass weight reduction, which was partially mediated by an increase in the number of visceral abnormalities.

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.

Seropositivity and risk factors associated with the presentation of bovine leukosis virus in Sotaquirá, Colombia

Background and Aim

Enzootic bovine leukosis is a disease economically important to the dairy farming industry worldwide. The virus is of the Deltaretrovirus genus and is primarily transmitted iatrogenically. Most bovines infected with the virus remain asymptomatic with only 5-10% of cattle having lymphomas. This study aimed to determine the seroprevalence of bovine leukosis virus (BLV) in Sotaquirá, Boyacá, Colombia.

Materials and Methods

We conducted a descriptive, observational epidemiological cross-sectional study using the simple random sampling method with a sample size of 1000. Blood samples from random bovine were processed using the SERELISA^® BLV Ab Mono Blocking indirect enzyme-linked immunosorbent assay kit (Zoetis, USA). The assay had a sensitivity of 97% and a specificity of 98%. The collected data were processed using Epi Info^® (Centers for Disease Control and Prevention; Atlanta, Georgia). From the study, we could determine a high seroprevalence of BLV in Sotaquirá.

Results

We established a high seroprevalence on BLV in the municipality, with 31.1% apparent seroprevalence and 30.6% real seroprevalence rate. We found that male cattle more than 4 years old (39.4%) and the Ayrshire breed (45.5%) had the highest prevalence rates of the virus. In this study, we could establish statistically significant associations according to breed, age, and gender of the cattle under study. Moreover, we identified the risk factors for BLV infection. We found that in cattle aged <1 year and those older than 4 years of age and those of the Holstein breed, the presentation of infectious bovine rhinotracheitis, mucosal secretions, mastitis, fetal death, the presence of a corral, and the implementation of artificial insemination practices were risk factors for BLV infection.

Conclusion

Determining the prevalence of BLV within the herd and identifying the associated risk factors for the disease are fundamental in developing efficient programs for the control and eradication of BLV within herds.

Genetic variation in Japanese Holstein cattle for EBL development

BACKGROUND

Infection with bovine leukemia virus (BLV), the causative agent for enzootic bovine leukosis (EBL), is increasing in dairy farms of Japan. The tendency of tumor development following BLV infection in certain cow families and bull lines has previously been described. We therefore hypothesized the existence of a genetic component which differentiates cattle susceptibility to the disease.

RESULTS

We analyzed routinely collected large-scale data including postmortem inspection data, which were combined with pedigree information and epidemiological data of BLV infection. A total of 6,022 postmortem inspection records of Holstein cattle, raised on 226 farms served by a regional abattoir over 10 years from 2004 to 2015, were analyzed for associations between sire information and EBL development. We then identified statistically the relative susceptibility to EBL development for the progeny of specific sires and paternal grandsires (PGSs). The heritability of EBL development was calculated as 0.19. Similarly, proviral loads (PVLs) of progeny from identified sires and PGSs were analyzed, but no significant differences were found.

CONCLUSIONS

These observations suggest that because EBL development in our Holstein population is, at least in part, influenced by genetic factors independent of PVL levels, genetic improvement for lower incidence of EBL development in cattle notwithstanding BLV infection is possible.

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.

Effects of Naturally Occurring Mutations in Bovine Leukemia Virus 5'-LTR and Tax Gene on Viral Transcriptional Activity

Bovine leukemia virus (BLV) is a deltaretrovirus infecting bovine B cells and causing enzootic bovine leucosis (EBL). The long terminal repeat (LTR) plays an indispensable role in viral gene expression. The BLV Tax protein acts as the main transactivator of LTR-driven transcription of BLV viral genes. The aim of this study was to analyze mutations in the BLV LTR region and tax gene to determine their association with transcriptional activity. LTRs were obtained from one hundred and six BLV isolates and analyzed for their genetic variability. Fifteen variants were selected and characterized based on mutations in LTR regulatory elements, and further used for in vitro transcription assays. Reporter vectors containing the luciferase gene under the control of each variant BLV promoter sequence, in addition to variant Tax expression vectors, were constructed. Both types of plasmids were used for cotransfection of HeLa cells and the level of luciferase activity was measured as a proxy of transcriptional activity. Marked differences in LTR promoter activity and Tax transactivation activity were observed amongst BLV variants. These results demonstrate that mutations in both the BLV LTR and tax gene can affect the promoter activity, which may have important consequences on proviral load, viral fitness, and transmissibility in BLV-infected cattle.

Phylogenetic Analysis of South African Bovine Leukaemia Virus (BLV) Isolates

Bovine leukaemia virus (BLV) causes chronic lymphoproliferative disorder and fatal lymphosarcoma in cattle, leading to significant economic losses in the beef and dairy industries. BLV is endemic globally and eleven genotypes have been identified. To date, only Zambian isolates have been genotyped from Africa. Although high BLV prevalence has been reported in South Africa, there has been no molecular characterisation of South African BLV isolates. To characterise BLV isolates in South Africa for the first time, we investigated the phylogenetic relationships and compared the genetic variability of eight South African BLV isolates with BLV isolates representing the eleven known genotypes from different geographical regions worldwide. Phylogenetic analyses based on full-length and partial env sequences as well as full-length gag sequences revealed that at least two genotypes, genotypes 1 (G1) and 4 (G4), are present in cattle in South Africa, which is consistent with studies from Zambia. However, our analysis revealed that the G1 South African isolate is more similar to other G1 isolates than the G1 Zambian isolates whereas, the G4 South African isolates are more divergent from other G4 isolates but closely related to the G4 Zambian isolate. Lastly, amino acid sequence alignment identified genotype-specific as well as novel amino acid substitutions in the South African isolates. The detection of two genotypes (G1 and G4) in southern Africa highlights the urgent need for disease management and the development of an efficacious vaccine against local strains.

Detection and genotyping of bovine leukemia virus (BLV) in Vietnamese cattle

Bovine leukemia virus (BLV) belongs to the genus, Deltaretrovirus of the family, Retroviridae and it is the causative agent of enzootic bovine leukosis. The prevalence of BLV in three provinces in the Red River Delta Region in the North of Vietnam, Hanoi, Vinhphuc and Bacninh was studied from April 2017 to June 2018. A total of 275 blood samples collected from cattle were used for serum isolation and DNA extraction. Of these samples, 266 sera were subjected to ELISA test for detecting antibody against BLV gp51 protein and 152 DNA samples were used to detect the 444 bp fragment corresponding to a part of the gp51 region of the env by nested PCR. The results showed that 16.5% (n=44) and 21.1% (n=32) of samples were positive for BLV gp51 antibody and BLV proviral DNA, respectively. Phylogenetic analysis of the partial (423 bp) and complete (913 bp) BLV env-gp51 gene indicated that Vietnamese strains were clustered into genotypes 1, 6 and 10 (G1, G6 and G10). Of those genotypes, G1 genotype was dominant; G6 strains were designated as G6e and G6f subgenotypes; the existence of genotype 10 was confirmed for the first time in Vietnam. The present study provides important information regarding the prevalence of BLV infection and genetic characteristics of BLV strains identified in Vietnam, contributing to promote the establishment of disease control and eradication strategies in Vietnam.

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.

Epizootic situation with bovine leukemia in the Central Federal District of the Russian Federation in 2012–2016

Leukemia is a chronic infectious viral disease of a tumor nature. The disease is widespread in different countries of the world and is relevant for most regions of the Russian Federation. The purpose of the work is to study the epizootic situation of leukemia in cattle in livestock farms of the Central Federal District of the Russian Federation. A complex epizootological method and evidence-based epizootology methods were used in the work. Objects of research are dysfunctional points, sick cattle. The paper presents a comparative analysis of the epizootic situation for the period 2012–2016. In the analyzed period, a total of 470 dysfunctional points for leukemia were identified, which amounted to 29.59 % of the total number of dysfunctional points in the Russian Federation and 62908 animals with leukemia were registered, which amounted to 36.90 % of the total number of diseased animals in Russia. Despite the decrease in the number of cattle infected with leukemia in five subjects, and the absence of animals that died from leukemia in all 18 subjects of the Central Federal District of the Russian Federation during 2012–2016, the epizootic situation of leukemia in the whole region remains quite tense, which is explained by the failure to comply with the basic requirements of the state veterinary service regarding sanitary anti-leukemia measures, according to the “Rules for the Prevention and Control of Leukemia of Large R gatogo cattle “ (1999.).

Molecular Epidemiological and Serological Studies of Bovine Leukemia Virus in Taiwan Dairy Cattle

Bovine leukemia virus (BLV) infection results in a decrease in milk yield and quality, a compromise in immunity, and shortening in the longevity of cows. The current status of BLV infection of dairy cattle in Taiwan remains unclear. To evaluate BLV infection, anti-BLV gp51 antibody and proviral DNA were detected. Surprisingly, the seroprevalence of BLV at the animal and herd level was as high as 81.8% (540/660 cattle) and 99.1% (109/110 herds), respectively. Among 152 blood samples analyzed, 132 (86.8%) were detected as positive for BLV-proviral DNA. When the complete blood count (CBC) was taken into account, the white blood cell (WBC) number appears to be the factor with the highest predicted potential for BLV infection. Moreover, based on receiver operating characteristic (ROC) curve analysis, the sensitivity and specificity are 72.0 and 75.0%, respectively, when the cut-off value of the WBC was set at 10.215 K/μL. Despite the co-circulation of genotype 1 and 3 in Taiwan, genotype 1 was much more prevalent (29/30). Taken together, due to the high prevalence of BLV, the identification of risk factors for interrupting the routes of transmission of BLV are critical for the control and prevention of further BLV infection.

Association between bovine leukemia virus proviral load and severity of clinical mastitis

The purpose of this study was to clarify the effect of Bovine leukemia virus (BLV) infection on natural immunity in the bovine mammary gland and on the severity of clinical mastitis. We classified milk samples from clinical mastitic cows into BLV-positive (n=76) and BLV-negative (n=12). BLV-positive cows were further divided into cows with High BLV proviral load (H-PVL) (n=23) and Low BLV proviral load (L-PVL) (n=53). Severity of clinical mastitis was classified as MILD, MODERATE, or SEVERE. Multiple logistic regression analysis was performed on the host factors and environmental factors with severity of clinical mastitis as the objective variable. BLV proviral load (PVL) and season at onset of mastitis showed significant correlation with the severity of clinical mastitis. Binary logistic regression analysis was performed on natural immunity factors lactoferrin and lingual antimicrobial peptide (LAP) concentration in milk, with PVL as the objective variable. Of these natural immunity factors, LAP concentration in milk showed significant correlation with PVL. The results of the present study suggested that PVL and season are associated with severity of clinical mastitis, and that the immune function in the mammary gland is decreased in cows with H-PVL compared to that in cows with L-PVL.

A point mutation to the long terminal repeat of bovine leukemia virus related to viral productivity and transmissibility

It is important to establish the molecular basis of the high transmissibility of bovine leukemia virus (BLV) to develop new methods of preventing viral transmission. Hence, the aim of this study was to determine whether some strains had transmission advantages. First, we determined the whole BLV genome sequences of all 34 BLV-infected cows from one farm. Phylogenetic analysis divided strains into 26 major and 8 minor strains. The major strains dominantly spread independent of host factor, bovine leucocyte antigen. Further analysis, with molecular clones, associated transmissibility with viral productivity in vitro. In addition, the two groups could be classified by group-specific mutations. The reverse genetic approach demonstrated that a spontaneous mutation at nucleotide 175 of the BLV genome, which is located in the viral promoter region, could alter viral productivity by changing viral transactivation, suggesting that BLV transmissibility is affected by a spontaneous mutation associated with viral productivity.

Detection and molecular characterization of bovine leukemia virus in various regions of Iran

Purpose. Bovine leukemia virus (BLV) infects cattle worldwide, imposing an economic impact on the dairy cattle industry. The purpose of this study was to evaluate the molecular epidemiology of BLV in Iran.Methodology. Blood samples taken from 280 cows aged over 2 years old from 13 provinces of Iran were used for leukocyte count and blocking ELISA. Genomic DNA was extracted from the peripheral blood leukocytes of BLV-infected samples and fetal lamb kidney cells to perform PCR of partial env, rex and tax genes and long-terminal-repeat region. The PCR products were sequenced, the phylogenetic tree of each gene was constructed, and nucleotide and amino acid sequence pair distances were calculated.Results. The frequency of BLV infection was 32.8 % among animals and was 80 % among provinces. In BLV seropositive animals, the rate of persistent lymphocytosis was 36.9 %. The constructed phylogenetic trees showed the presence of two BLV genotypes (1 and 4) in Iranian strains. As previous studies, our results showed that the env gene was more variable than previously thought, the Rex protein could withstand more amino acid changes compared to the Tax protein, and no significant differences were observed in average changes of the nucleotide of these genes between clinical stages.Conclusions. Our data indicates an increase in the frequency of this infection in Iran. This is the first study report of the presence of BLV genotype 4 in Iranian farms. These findings may have an important role in the control and prevention of BLV infection in Iran and other countries.

Increasing Bovine leukemia virus (BLV) proviral load is a risk factor for progression of Enzootic bovine leucosis: A prospective study in Japan

Bovine leukemia virus (BLV) belongs to the genus Deltaretrovirus in the family Retroviridae, and is etiologically associated with Enzootic Bovine Leukosis (EBL). The majority of BLV-infected cattle remain asymptomatic throughout their productive lives, whereas approximately 5%-10% of infected cattle develop EBL. Data accumulated recently indicate that whole blood proviral load (PVL) levels of BLV-infected cattle could be an indicator of disease progression in the field. However, a few cross-sectional studies have been reported. Here, we prospectively evaluated 866 cattle enrolled between August 2015 and December 2015, and followed until November 2018, identifying 407 asymptomatic BLV-infected cattle. There were no significant differences in the median PVL level among the category of herd seroprevalence (p = 0.57), herd size (p = 0.19), nor among the category of past EBL history in the herd (p = 0.31). During the study period, 12 cattle developed EBL. The PVL levels of EBL cattle at the time of enrollment were significantly higher than that of cattle that did not progress to EBL (median, 90,695 vs 39,139 copies/105 cells, p = 0.0005). Moreover, the adjusted hazard ratio for the increase in PVL was 2.61 (95% CI, 1.51-4.53) as estimated by the Cox proportional hazards frailty model. These results indicate that a high PVL level is a significant risk factor for progression to EBL, and could potentially be used as an indicator for the identification of cattle to be culled from the herd long before the progression of EBL. This knowledge might be useful to design a strategy for decreasing economic loss from EBL or even eradicating it from herds.

Development of a fluorescent loop-mediated isothermal amplification assay for rapid and simple diagnosis of bovine leukemia virus infection

Bovine leukemia virus (BLV) causes enzootic bovine leukosis (EBL), a condition that threatens the sustainability of the livestock industry. A fluorescent loop-mediated isothermal amplification (fLAMP) assay targeting BLV env sequences was developed and used to evaluate 100 bovine blood samples. Compared with a conventional real-time PCR (rPCR) assay, the fLAMP assay achieved 87.3% (62/71) sensitivity and 100% (29/29) specificity. The rPCR assay took 65 min, while the fLAMP assay took 8 min to 30 min from the beginning of DNA amplification to final judgement with a comparable limit of detection. The fLAMP is a potential tool for the rapid and simple diagnosis of BLV infection to supplement ELISA testing and can be used by local laboratories and slaughterhouses without special equipment.

Prevalence of Bovine Leukemia Virus Antibodies in US Dairy Cattle

OBJECTIVE

To estimate current US herd-level and animal-level prevalence of bovine leukemia virus (BLV) in dairy cows and characterize epidemiologic features.

DESIGN

Cross-sectional observational study design and survey.

ANIMALS

4120 dairy cows from 103 commercial dairy herds in 11 states across the US.

PROCEDURES

Milk samples were collected from dairy cows through routine commercial sampling and tested for anti-BLV antibodies by antibody capture ELISA. Based on the ELISA results of a sample of an average of 40 cows per herd, within-herd apparent prevalence (AP) was estimated by a directly standardized method and by a lactation-weighted method for each herd. Within-herd AP estimates were summarized to give estimates of US herd-level and animal-level AP. Differences in AP by lactation, region, state, breed, and herd size were examined to characterize basic epidemiologic features of BLV infection.

RESULTS

94.2% of herds had at least one BLV antibody positive cow detected. The average within-herd standardized AP was 46.5%. Lactation-specific AP increased with increasing lactation number, from 29.7% in first lactation cows to 58.9% in 4th and greater lactation cows. Significant differences were not observed based on region, state, breed, or herd size.

CONCLUSIONS AND CLINICAL RELEVANCE

These results are consistent with a historical trend of increasing prevalence of BLV among US dairy cattle. Given the findings of other studies on the negative impacts of BLV infection on milk production and cow longevity, these findings are clinically relevant for veterinarians counseling dairy clients on the risks of BLV to their herds.

Development of a new recombinant p24 ELISA system for diagnosis of bovine leukemia virus in serum and milk

Bovine leukemia virus (BLV) is a retrovirus that causes enzootic bovine leucosis. Here, we designed a p24 enzyme-linked immunosorbent assay (ELISA) to detect antibodies specific for BLV capsid protein p24 (encoded by the gag gene) in bovine serum samples. The p24 gene was inserted into an Escherichia coli expression system, and recombinant proteins (GST-p24, p24, and His-p24) were purified. His-p24 was the most suitable antigen for using in the ELISA. The cut-off point was calculated from a receiver operating characteristic curve derived from a set of 582 field samples that previously tested positive or negative by BLV-CoCoMo-qPCR-2, which detects BLV provirus. The new p24 ELISA showed almost the same specificity and sensitivity as a commercial gp51 ELISA kit when used to test field serum samples, and allowed monitoring of p24 antibodies in raw milk and whey. Comparing the results for the p24 ELISA and gp51 ELISA revealed that p24 antibodies were detected earlier than gp51 antibodies in three out of eight calves experimentally infected with BLV, indicating improved detection without diminishing BLV serodiagnosis. Thus, the p24 ELISA is a robust and reliable assay for detecting BLV antibodies in serum or milk, making it is a useful tool for large-scale BLV screening.

Cooperation of PD-1 and LAG-3 in the exhaustion of CD4+ and CD8+ T cells during bovine leukemia virus infection

Bovine leukemia virus (BLV) is a retrovirus that infects B cells in cattle and causes bovine leukosis after a long latent period. Progressive exhaustion of T cell functions is considered to facilitate disease progression of BLV infection. Programmed death-1 (PD-1) and lymphocyte activation gene-3 (LAG-3) are immunoinhibitory receptors that contribute to T-cell exhaustion caused by BLV infection in cattle. However, it is unclear whether the cooperation of PD-1 and LAG-3 accelerates disease progression of BLV infection. In this study, multi-color flow cytometric analyses of PD-1- and LAG-3-expressing T cells were performed in BLV-infected cattle at different stages of the disease. The frequencies of PD-1+LAG-3+ heavily exhausted T cells among CD4+ and CD8+ T cells was higher in the blood of cattle with B-cell lymphoma over that of BLV-uninfected and BLV-infected cattle without lymphoma. In addition, blockade assays of peripheral blood mononuclear cells were performed to examine whether inhibition of the interactions between PD-1 and LAG-3 and their ligands by blocking antibodies could restore T-cell function during BLV infection. Single or dual blockade of the PD-1 and LAG-3 pathways reactivated the production of Th1 cytokines, interferon-γ and tumor necrosis factor-α, from BLV-specific T cells of the infected cattle. Taken together, these results indicate that PD-1 and LAG-3 cooperatively mediate the functional exhaustion of CD4+ and CD8+ T cells and are associated with the development of B-cell lymphoma in BLV-infected cattle.

New hematological key for bovine leukemia virus-infected Japanese Black cattle

The European Community’s (EC) Key, which is also called Bendixen’s Key, is a well-established bovine leukemia virus (BLV) diagnostic method that classifies cattle according to the absolute lymphocyte count and age. The EC Key was originally designed for dairy cattle and is not necessarily suitable for Japanese Black (JB) beef cattle. This study revealed the lymphocyte counts in the BLV-free and -infected JB cattle were significantly lower than those in the Holstein cattle. Therefore, applying the EC Key to JB cattle could result in a large number of undetected BLV-infected cattle. Our proposed hematological key, which was designed for JB cattle, improves the detection of BLV-infected cattle by approximately 20%. We believe that this study could help promote BLV control.

Intrauterine infection with bovine leukemia virus in pregnant dam with high viral load

Enzootic bovine leukemia is caused by the bovine leukemia virus (BLV). BLV is transmitted vertically or horizontally through the transfer of infected cells via direct contact, through milk, insect bites and contaminated iatrogenic procedures. However, we lacked direct evidence of intrauterine infection. The purpose of this study was to confirm intrauterine BLV infection in two pregnant dams with high viral load by cesarean delivery. BLV was detected in cord and placental blood, and the BLV in the newborns showed 100% nucleotide identity with the BLV-env sequence from the dams. Notably, a newborn was seropositive for BLV but had no colostral antibodies. In this study, we presented a direct evidence of intrauterine BLV transmission in pregnant dam with a high proviral load. These results could aid the development of BLV control measures targeting viral load.

Epidemiology and genetic diversity of bovine leukemia virus

Bovine leukemia virus (BLV), an oncogenic member of the Deltaretrovirus genus, is closely related to human T-cell leukemia virus (HTLV-I and II). BLV infects cattle worldwide and causes important economic losses. In this review, we provide a summary of available information about commonly used diagnostic approaches for the detection of BLV infection, including both serological and viral genome-based methods. We also outline genotyping methods used for the phylogenetic analysis of BLV, including PCR restriction length polymorphism and modern DNA sequencing-based methods. In addition, detailed epidemiological information on the prevalence of BLV in cattle worldwide is presented. Finally, we summarize the various BLV genotypes identified by the phylogenetic analyses of the whole genome and env gp51 sequences of BLV strains in different countries and discuss the distribution of BLV genotypes worldwide.

Development and evaluation of an immunochromatographic assay using a gp51 monoclonal antibody for the detection of antibodies against the bovine leukemia virus

Infection of cattle with bovine leukemia virus (BLV) has been observed and reported worldwide, including in Korea. The onsite identification of infected cattle would help decreasing and eradicating BLV infections on farms. Here, we present a new immunochromatographic assay that employs monoclonal antibodies (MAbs) for the detection of antibodies against BLV in the field. BLV envelope glycoprotein (gp)51 was expressed in E. coli, and MAbs against recombinant BLV gp51 were generated for the development of an immunochromatographic assay to detect BLV antibodies in cattle. The sensitivity and specificity of the assay were determined by comparing these results with those obtained from a standard enzyme linked immunosorbent assay (ELISA). A total of 160 bovine sera were used to evaluate the new immunochromatographic assay. Using ELISA as a reference standard, the relative specificity and sensitivity of this assay were determined to be 94.7% and 98%, respectively. Because of its high sensitivity and specificity, this BLV antibody detection assay would be suitable for the onsite identification of BLV infection in the field.

Phylogenetic analysis of env gene of bovine leukemia virus strains spread in Miyazaki prefecture, Japan

To understand how the latest dominant bovine leukemia virus (BLV) strains were introduced and spread in the Miyazaki prefecture, we collected blood samples from 3 geographic areas (north, central and south) and carried out sequence analysis of the BLV env gene. Two genotypes, genotype I, and III, were identified and the majority of the strains belonged to genotype I (71/74). To clarify a route of BLV introduction, we divided the strains into 20 subgenotypes based on their nucleotide sequences and performed phylogenetic analysis. Our study indicated that common BLV strains were comparatively evenly distributed even in the area, where the farmers have not introduced cattle from other areas and the cattle have limited exposure to BLV infection in grazing fields.

Co-infection with Mycobacterium bovis does not alter the response to bovine leukemia virus in BoLA DRB3*0902, genetically resistant cattle

High proviral load (HPL) profile in bovine leukemia virus infected animals poses increased risk of transmission, and development of HPL or low proviral load (LPL) profile may be attributed to host genetics. Genetic resistance and susceptibility has been mapped to the Major Histocompatibility Complex class II DRB3 gene (BoLA DRB3). The aim of this work was to determine the effect of Mycobacterium bovis infection on certain virological and host immunological parameters of BLV experimental infection. Twenty-six Argentinian Holstein calves carrying the resistance-associated marker allele BoLA DRB3*0902, susceptibility-associated marker allele BoLA DRB3*1501, or neutral BoLA DRB3 alleles, exposed to M. bovis were used. Twenty calves were inoculated with BLV, three were naturally infected and other three were BLV-negative. Seven from twenty six (27%) of the animals resulted positive to the PPD test. The proviral load, absolute leukocyte and lymphocyte counts, time to seroconversion, antibody titer against BLV, and viral antigen expression in vitro at various times post inoculation were determined and compared between PPD+ and PPD- animals. From a total of 23 BLV positive animals (naturally and experimentally infected), 13 (56.5%) developed HPL, and 10 (43.5%) developed LPL. None of the investigated parameters were affected by infection with M. bovis. We concluded that the ability of cattle carrying resistance-associated marker to control BLV and to progress towards a LPL phenotype was not altered by M. bovis co-infection.

First Report of Bovine Leukemia Virus Infection in Yaks (Bos mutus) in China

Enzootic bovine leukosis (EBL) is a chronic lymphosarcoma disease of cattle caused by bovine leukemia virus (BLV). No information is available concerning the epidemiology of BLV infection in yaks (Bos mutus). One thousand five hundred and eighty-four serum samples from 610 black yaks and 974 white yaks from Gansu province, northwest China, were collected between April 2013 and March 2014 and tested for BLV antibodies using a commercially available ELISA kit. The overall BLV seroprevalence in yaks was 21.09% (334/1584), with 24.26% (148/610) black yaks and 19.10% (186/974) white yaks yielding positive results. Risk factor analysis indicated that with the exception of breed (OR = 1.36, 95% CI = 1.06–1.73, P < 0.05), the age, region, gender, farm, and the numbers of pregnancies were not considered as risk factors for the presence of BLV in yaks included in this study. This is the first report of BLV infection in yaks in China, which provides information for controlling BLV infection in yaks.

Risk factors associated with increased bovine leukemia virus proviral load in infected cattle in Japan from 2012 to 2014

Bovine leukemia virus (BLV) is the causative agent of enzootic bovine leukosis, a malignant B cell lymphoma. BLV has spread worldwide and causes serious problems. After infection, the BLV genome is integrated into the host DNA and can be amplified during periods of latency. We previously designed degenerate primers using the Coordination of Common Motifs (CoCoMo) algorithm to establish a new quantitative real-time PCR method (BLV-CoCoMo-qPCR-2) of measuring the proviral load of both known and novel BLV variants. Here, we aimed to examine the correlation between proviral load and risk factors for BLV infection, such as breeding systems, parousity, and colostrum feeding. Blood and serum samples were collected from 83 BLV-positive farms in 22 prefectures of Japan, and the BLV proviral load and anti-BLV antibody levels were measured. BLV was detected in 73.3% (1039/1,417) of cattle by BLV-CoCoMo-qPCR-2 and the provirus was detected in 93 of 1039 antibody-negative samples. The results showed that the proviral load increased with progression of lymphocytosis. Next, the risk factors associated with increasing BLV infection rate were examined along with any association with proviral load. The proviral load was higher in cattle with lymphocytosis than in healthy cattle, and higher in multiparous cows than in nulliparous cows. Finally, proviral loads were higher in contact breeding systems than in non-contact breeding systems. Taken together, these findings may help to formulate a plan for eliminating BLV from contaminated farms. This is the first nationwide study to estimate BLV proviral load in Japanese cattle.

Horizontal transmission and phylogenetic analysis of bovine leukemia virus in two districts of Miyazaki, Japan

Horizontal transmission is recognized as a major infection route for bovine leukemia virus (BLV), and cattle with high viral loads are considered to be a major infectious source in a herd. However, a correlation between viral loads and the risk of infection has been insufficient to use as a foundation for BLV control strategies. In this report, we examined the epidemiology of BLV infection and the infectious source in a local area. In 2013–2014, BLV infection was investigated in 1,823 cattle from 117 farms in two adjacent districts, Miyazaki, Japan. Seropositive samples for BLV were detected with 88 cattle and in 14 farms. Phylogenetic analysis revealed that 94% of the isolates clustered into genotype I and the remaining isolate into genotype III. Among genotype I, genetically distinct strains were spread at each farm, and cattle infected with less than 3 copies/100 cells did not transmit BLV to other cattle for more than thirty months. This is the first report of concrete data of viral load in relation to viral horizontal transmission under the field condition. The data facilitate farmers and veterinarians understanding the status of BLV infected cattle. This research contributes to BLV infection control and the development of effective BLV eradication programs.

Sequencing and phylogenetic analysis of the gp51 gene from Korean bovine leukemia virus isolates

BACKGROUND

Bovine Leukemia virus (BLV) infection of cattle has been reported in Korea for more than three decades. However, to date, there have been few studies regarding Korean BLV since 1980s. Thus, the purpose of this study is to perform a diagnosis and molecular characterization of BLV strains circulating in Korea and to estimate genetic diversity of different genotypes of BLV.

METHOD

To investigate the distribution of BLV variants in the world and assess the evolutionary history of Korean BLV isolates, a comprehensive molecular analysis of the BLV env gp51 gene was conducted using recent worldwide BLV isolates. The isolates included 50 samples obtained from two cattle farms in southeastern Korea in 2014.

RESULTS

Sequence and phylogenetic analyses of partial 444-nt fragment sequences and complete gp51 sequences of BLV revealed eight distinct genotypes of BLV showing geographic distribution of the world. Most Korean BLV isolates were found to belong to genotype 1 which is a major genotype prevailed throughout the world, and only four isolates from one farm were classified as genotype 3 related to the US and Japan isolates. Analysis of amino acids of Korean BLV isolates showed several sequence substitutions in the leader peptide, conformational epitope, and neutralizing domain regions. The observations suggest the possibility of affecting on viral infectivity and formation.

CONCLUSION

Korean BLV isolates showed the close relationship to genotype 1 and 3. Further study to identify the diversity of BLV circulating in Korea is necessary with samples collected nationwide because this study is the first report of BLV genotype 3 being in circulation in Korea.

Evaluation of the natural perinatal transmission of bovine leukaemia virus

The perinatal transmission of bovine leukaemia virus (BLV) plays a critical role in the spread and persistence of BLV infection in cattle herds. The purpose of this study was to examine the frequency of perinatal infections in an area in Japan and investigate some risk factors associated with infection. Altogether, 129 calves born to BLV-infected cows in a herd in Japan were tested for infection immediately after birth and again at one month of age using nested PCR. Twenty-four calves (18.6 per cent) were infected with BLV, of which 14 (10.8 per cent) and 10 (7.7 per cent) calves were infected via the transplacental and the birth canal routes, respectively. Maternal viral loads, breed, the presence or absence of assistance during parturition and the number of births per dam were evaluated to investigate risk factors associated with infection. Maternal viral load was significantly correlated with the frequency of perinatal infection, and more than 40 per cent of newborn calves born to dams with high viral loads were infected with BLV. The results of this study could contribute towards developing effective eradication programmes by providing necessary data for replacement of breeding cow in the field.

Distribution and characteristics of bovine leukemia virus integration sites in the host genome at three different clinical stages of infection

Bovine leukemia virus (BLV) is an oncogenic retrovirus closely related to human T-cell lymphotropic virus. BLV-infected cattle are categorized as asymptomatic carriers or as having persistent lymphocytosis or enzootic bovine leukemia, depending on the clinical stage. We investigated the BLV integration site distribution at three BLV clinical stages and examined genome sequence features around the integration sites. In all, 264 BLV integration sites, at various locations on each chromosome, were identified in 28 cattle by inverse PCR and BLAST searches. Approximately one-third of BLV proviruses were independently integrated within transcriptional units, and approximately 10 % were integrated near transcription start sites. Moreover, less than 7 % of BLV integration sites were located near CpG islands. BLV did not preferentially integrate into transcriptionally active regions during any of the clinical stages. At the nucleotide level, regions around BLV integration points were significantly A/T rich with weak sequence consensus. BLV preferentially integrated within long interspersed nuclear repeat elements. Although BLV integration sites may not be associated with disease progression, integration is selective at the nucleotide level.

Animal models of bovine leukemia virus and human T-lymphotrophic virus type-1: insights in transmission and pathogenesis

Bovine leukemia virus (BLV) and human T-lymphotrophic virus type-1 (HTLV-1) are related retroviruses associated with persistent and lifelong infections and a low incidence of lymphomas within their hosts. Both viruses can be spread through contact with bodily fluids containing infected cells, most often from mother to offspring through breast milk. Each of these complex retroviruses contains typical gag, pol, and env genes but also unique, nonstructural proteins encoded from the pX region. These nonstructural genes encode the Tax and Rex regulatory proteins, as well as novel proteins essential for viral spread in vivo. Improvements in the molecular tools to test these viral determinants in cellular and animal models have provided new insights into the pathogenesis of each virus. Comparisons of BLV and HTLV-1 provide insights into mechanisms of spread and tumor formation, as well as potential approaches to therapeutic intervention against the infections.