Molecular epidemiology of bovine leukemia virus associated with enzootic bovine leukosis in Japan

The Prevalence and Molecular Characterization of Bovine Leukemia Virus among Dairy Cattle in Henan Province, China

Enzootic bovine leukosis, a neoplastic disease caused by the bovine leukemia virus (BLV), was the primary cancer affecting cattle in China before 1985. Although its prevalence decreased significantly between 1986 and 2000, enzootic bovine leukosis has been re-emerging since 2000. This re-emergence has been largely overlooked, possibly due to the latent nature of BLV infection or the perceived lack of sufficient evidence. This study investigated the molecular epidemiology of BLV infections in dairy cattle in Henan province, Central China. Blood samples from 668 dairy cattle across nine farms were tested using nested polymerase chain reaction assays targeting the partial envelope (env) gene (gp51 fragment). Twenty-three samples tested positive (animal-level prevalence of 3.4%; 95% confidence interval: 2.2, 5.1). The full-length env gene sequences from these positive samples were obtained and phylogenetically analyzed, along with previously reported sequences from the GenBank database. The sequences from positive samples were clustered into four genotypes (1, 4, 6, and 7). The geographical annotation of the maximum clade credibility trees suggested that the two genotype 1 strains in Henan might have originated from Japan, while the genotype 7 strain is likely to have originated from Moldova. Subsequent Bayesian stochastic search variable selection analysis further indicated a strong geographical association between the Henan strains and Japan, as well as Moldova. The estimated substitution rate for the env gene ranged from 4.39 × 10-4 to 2.38 × 10-3 substitutions per site per year. Additionally, codons 291, 326, 385, and 480 were identified as positively selected sites, potentially associated with membrane fusion, epitope peptide vaccine design, and transmembrane signal transduction. These findings contribute to the broader understanding of BLV epidemiology in Chinese dairy cattle and highlight the need for measures to mitigate further BLV transmission within and between cattle herds in China.

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.

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.

Detection and molecular characterization of bovine leukemia virus in beef cattle presented for slaughter in Egypt

Bovine leukemia virus (BLV) is the etiological agent of enzootic bovine leukosis, the most common neoplastic disease of cattle worldwide and a serious problem for the cattle industry. Previous studies have shown the molecular prevalence of BLV and the coexistence of BLV genotype-1 and -4 in Egyptian dairy cattle; however, the molecular characteristics of BLV in Egyptian beef cattle are unknown. Therefore, we collected blood samples of 168 beef cattle from slaughterhouses in three governorates in Egypt. Based on BLV-CoCoMo-qPCR-2 targeting long terminal repeats and nested PCR targeting the env-gp51 gene, the BLV provirus infection rates were found to be 47/168 (28.0%) and 42/168 (25.0%), respectively. Phylogenetic analysis based on 501 bp of the BLV env-gp51 gene from 42 BLV isolates revealed that at least six distinctive strains (b, e, f, g, x, and z) were prevalent in cattle across the examined regions. Furthermore, phylogenetic analysis of the 420 bp sequence of the BLV env-gp51 region of the six strains against 11 known genotypes showed that the strains b, e, f, and g were clustered into genotype-1, and strains x and z were clustered into genotype-4. Our results also indicated that strains b and x exist in both dairy and beef cattle in Egypt. The present study is the first to detect and genotype BLV among beef cattle in Egypt.

Lack of association between amino acid sequences of the bovine leukemia virus envelope and varying stages of infection in dairy cattle

We collected 724 blood samples from dairy cattle from six Mexican states, and tested them for the presence of antibodies against BLV using a commercial ELISA test. Our study groups consisted of 32 samples: 12 asymptomatic cows, 12 cows with lymphocytosis and 8 samples of tumor tissue of the abomasum and heart of cattle with lymphoma. We designed three pairs of primers to amplify the complete BLV env gene, and obtained a fragment of 1548 nucleotides in length with the sequenced products. According to the phylogenetic tree we constructed to identify the viral genotype, 96.87 % of the sequences grouped into genotype 1, while a single sample from a cow with lymphocytosis (3.13 %) was associated with genotype 3 sequences. The similarity between the Mexican BLV sequences ranged from 0.985-1.00. In addition, the proportion of non-synonymous and synonymous mutations indicated negative selection. We did not identify any conserved residues in the viral protein sequences that could be related to BLV infection stage in cattle. Proviral quantification was performed using quantitative polymerase chain reaction, and we used Mood´s median test as statistical analysis. We found no significant association between proviral load and phase of infection. The sequences showed high similarity without any association between BLV surface glycoprotein and the different infection stages, nor differences in the proviral load. BLV genotype 1 was identified as prevalent in the studied samples, and for the first time in Mexico, we identified BLV genotype 3 in cattle.

New evidence of bovine leukemia virus circulating in Myanmar cattle through epidemiological and molecular characterization

Bovine leukemia virus (BLV) is the etiological agent of enzootic bovine leukosis, which is the most common neoplastic disease of cattle. BLV infects cattle worldwide and causes serious problems for the cattle industry. In this study, we examined the prevalence of BLV infection and the distribution of BLV genotypes in cattle in the northern, central, and southern parts of Myanmar. The prevalence of BLV infection among Myanmar cattle (37.04%) in this study was markedly higher than the prevalence (9.1%) observed in our earlier study in which BLV was detected from the limited number of cattle only from a small area of Myanmar. Phylogenetic analysis of partial env-gp51 sequence of the isolated BLV strains revealed that there are at least three BLV genotypes (genotype-1, genotype-6, and genotype-10) in Myanmar, which have also been detected in the neighboring countries. We performed this study to estimate the BLV proviral load, which is a major diagnosis index for determining the virus transmission risk. The cattle of the three test regions with warm, wet, and humid climatic conditions (upper Sagaing, Yangon, and Kayin) exhibited a high mean proviral load, while cattle of three other regions with low annual rainfall and very high temperature (Mandalay, Magway, and upper Bago) exhibited a low mean proviral load. Further, the level of proviral load and the prevalence of BLV infection in Myanmar native cattle (N = 235) were lower than that in the hybrid cattle (Holstein Friesian × Myanmar native) (N = 62). We also observed that the cattle with high risk for BLV transmission, which have high proviral load, may enhance the BLV infection rate. Hence, to control BLV transmission, it is necessary to eliminate these cattle with high-risk for BLV transmission and to diagnose BLV provirus in cattle in the remaining regions/states of Myanmar sharing a boundary with neighboring countries.

Molecular detection and characterization of genotype 1 bovine leukemia virus from beef cattle in the traditional sector in Zambia

Whilst bovine leukemia virus (BLV) causes considerable economic losses to the dairy industry worldwide, information on its molecular epidemiology and economic impact in beef cattle is limited. Here, blood from 880 animals from Zambia's major cattle-rearing provinces was screened for BLV by nested PCR. Positive pools were sequenced and phylogenetically analyzed. The estimated pooled prevalence was 2.1%. All strains belonged to genotype 1 and formed a distinct phylogenetic cluster. The study suggests circulation of genotype 1 BLV in beef cattle in these regions. This is the first report on molecular detection and characterization of BLV from beef cattle in Africa.

Genotyping bovine leukemia virus in dairy cattle of Heilongjiang, northeastern China

Background

Bovine leukemia virus (BLV) causes enzootic bovine leukosis in cattle and leads to heavy economic losses in the husbandry industry. Heilongjiang Province, China, is rich in dairy cattle. However, its current BLV epidemiology and genotypes have still not been evaluated and confirmed. In this report, we investigated the BLV epidemiology in dairy cattle in the major regions of Heilongjiang Province via the nested PCR assay.

Results

A total of 730 blood samples were collected from nine different farms in six regions of Heilongjiang. The results showed that the infection rate of these regions ranged from null to 31%. With a clustering analysis of 60 published BLV env sequences, genotypes 1 and 6 were confirmed to be circulating in Heilongjiang. Importantly, a new genotype, 11, and a new subgenotype, 6E, were also identified in the Harbin and Daqing regions, respectively. An epitope analysis showed that a cluster of T-X-D-X-R-XXXX-A sequences in genotype 11 gp51 neutralizing domain 2 was unique among all currently known BLV isolates and was therefore a defining feature of this new genotype.

Conclusions

BLV epidemics and genotypes were initially investigated in dairy cattle of Heilongjiang. A relatively high infection rate was found in some regions of this province. A new genotype, G11, with a highly specific motif, was identified and thus added as a new member to the current BLV genotype family. This report provides an initial reference for future investigations and subsequent control of BLV transmission and spread in this region.

Electronic supplementary material

The online version of this article (10.1186/s12917-019-1863-3) contains supplementary material, which is available to authorized users.

Heterogeneity determination of bovine leukemia virus genome in Santa Catarina state, Brazil

ABSTRACT: Bovine leukemia virus (BLV) is a member of Retroviridae family, genus Deltaretrovirus, and the main viral agent responsible for economic loses in dairy herds. Some studies have been carried out about BLV genotypes, and at least seven genotypes were found out in samples of different regions of the world. The objective of this study was to identify BLV samples from seropositive dairy cattle in Santa Catarina state, Brazil, using molecular techniques. Blood samples were collected (454) from dairy cattle from 31 different farms, and serology using agar gel immunodiffusion test (AGID) was performed. After that, 191 seropositive samples were submitted to DNA extraction, and in 77 samples the polymerase chain reaction (PCR) for amplification of a 440 bp fragment of the env gene was performed. Nineteen DNA samples were subjected to restriction fragment length polymorphism (RFLP) analysis by digestion of the PCR fragment by five restriction endonucleases - BamHI, HaeIII, Tru9I, TaqI, and MwoI. It was found 42% seropositive animals (191/454) and 68% positives of the farms (21/31). The PCR showed 80.5% (62/77) of animals positive. The RFLP analysis identified five different genotypes dispersed by Santa Catarina state, with the highest prevalence for genotype X (47.4%). Overall, our results identified the viral genotypes present in dairy cattle and the prevalence of new variants in representative farms from Santa Catarina state.

Variations in the viral genome and biological properties of bovine leukemia virus wild-type strains

Bovine leukemia virus (BLV) is the etiological agent of enzootic bovine leukosis (EBL), which causes enormous economic losses in the livestock industry worldwide. To reduce the economic loss caused by BLV infection, it is important to clarify the characters associated with BLV transmissibility and pathogenesis in cattle. In this study, we focused on viral characters and examined spontaneous mutations in the virus and viral properties by analyses of whole genome sequences and BLV molecular clones derived from cows with and without EBL. Genomic analysis indicated that all 28 strains harbored limited genetic variations but no deletion mutations that allowed classification into three groups (A, B, and C), except for one strain. Some nucleotide/amino acid substitutions were specific to a particular group. On the other hand, these genetic variations were not associated with the host bovine leukocyte antigen-DRB3 allele, which is known to be related to BLV pathogenesis. The viral replication activity in vitro was high, moderate, and low in groups A, B, and C, respectively. In addition, the proviral load, which is related to BLV transmissibility and pathogenesis, was high in cows infected with group A strains and low in those infected with group B/C strains. Therefore, these results suggest that limited genetic variations could affect viral properties relating to BLV transmissibility and pathogenesis.

Computational analysis of envelope glycoproteins from diverse geographical isolates of bovine leukemia virus identifies highly conserved peptide motifs

BACKGROUND

Bovine leukemia virus (BLV) is a deltaretrovirus infecting bovine B cells and causing enzootic bovine leucosis. The SU or surface subunit, gp51, of its envelope glycoprotein is involved in receptor recognition and virion attachment. It contains the major neutralizing and CD4+ and CD8+ T cell epitopes found in naturally infected animals. In this study, we aimed to determine global variation and conservation within gp51 in the context of developing an effective global BLV vaccine.

RESULTS

A total of 256 sequences extracted from the NCBI database and collected in different parts of the world, were studied to identify conserved segments along the env gene sequences that encode the gp51 protein. Using the MEME server and the conserved DNA Region module for analysis within DnaSP, we identified six conserved segments, referred to as A-F, and five semi-conserved segments, referred to as G-K. The amino acid conservation ranged from 98.8 to 99.8% in conserved segments A to F, while segments G to K had 89.6-95.2% conserved amino acid sequence. Selection analysis of individual segments revealed that residues of conserved segments had undergone purifying selection, whereas, particular residues in the semi-conserved segments are currently undergoing positive selection, specifically at amino acid positions 48 in segment K, 74 in segment G, 82 in segment I, 133 and 142 in segment J, and residue 291 in segment H. Each of the codons for these six residues contain the most highly variable nucleotides within their respective semi-conserved segments.

CONCLUSIONS

The data described here show that the consensus amino acid sequence constitutes a strong candidate from which a global vaccine can be derived for use in countries where eradication by culling is not economically feasible. The most conserved segments overlap with amino acids in known immunodeterminants, specifically in epitopes D-D', E-E', CD8+ T-cell epitopes, neutralizing domain 1 and CD4+ T-cell epitopes. Two of the segments reported here represent unique segments that do not overlap with previously identified antigenic determinants. We propose that evidence of positive selection in some residues of the semi-conserved segments suggests that their variation is involved in viral strategy to escape immune surveillance of the host.

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.

The molecular epidemiological study of bovine leukemia virus infection in Myanmar cattle

Bovine leukemia virus (BLV) is the etiological agent of enzootic bovine leukosis, which is the most common neoplastic disease of cattle. BLV infects cattle worldwide and affects both health status and productivity. However, no studies have examined the distribution of BLV in Myanmar, and the genetic characteristics of Myanmar BLV strains are unknown. Therefore, the aim of this study was to detect BLV infection in Myanmar and examine genetic variability. Blood samples were obtained from 66 cattle from different farms in four townships of the Nay Pyi Taw Union Territory of central Myanmar. BLV provirus was detected by nested PCR and real-time PCR targeting BLV long terminal repeats. Results were confirmed by nested PCR targeting the BLV env-gp51 gene and real-time PCR targeting the BLV tax gene. Out of 66 samples, six (9.1 %) were positive for BLV provirus. A phylogenetic tree, constructed using five distinct partial and complete env-gp51 sequences from BLV strains isolated from three different townships, indicated that Myanmar strains were genotype-10. A phylogenetic tree constructed from whole genome sequences obtained by sequencing cloned, overlapping PCR products from two Myanmar strains confirmed the existence of genotype-10 in Myanmar. Comparative analysis of complete genome sequences identified genotype-10-specific amino acid substitutions in both structural and non-structural genes, thereby distinguishing genotype-10 strains from other known genotypes. This study provides information regarding BLV infection levels in Myanmar and confirms that genotype-10 is circulating in Myanmar.

A new genotype of bovine leukemia virus in South America identified by NGS-based whole genome sequencing and molecular evolutionary genetic analysis

BACKGROUND

Bovine leukemia virus (BLV) is a member of retroviridae family, together with human T cell leukemia virus types 1 and 2 (HTLV-1 and -2) belonging to the genes deltaretrovirus, and infects cattle worldwide. Previous studies have classified the env sequences of BLV provirus from different geographic locations into eight genetic groups. To investigate the genetic variability of BLV in South America, we performed phylogenetic analyses of whole genome and partial env gp51 sequences of BLV strains isolated from Peru, Paraguay and Bolivia, for which no the molecular characteristics of BLV have previously been published, and discovered a novel BLV genotype, genotype-9, in Bolivia.

RESULTS

In Peru and Paraguay, 42.3 % (139/328) and over 50 % (76/139) of samples, respectively, were BLV positive. In Bolivia, the BLV infection rate was up to 30 % (156/507) at the individual level. In Argentina, 325/420 samples were BLV positive, with a BLV prevalence of 77.4 % at the individual level and up to 90.9 % at herd level. By contrast, relatively few BLV positive samples were detected in Chile, with a maximum of 29.1 % BLV infection at the individual level. We performed phylogenetic analyses using two different approaches, maximum likelihood (ML) tree and Bayesian inference, using 35 distinct partial env gp51 sequences from BLV strains isolated from Peru, Paraguay, and Bolivia, and 74 known BLV strains, representing eight different BLV genotypes from various geographical locations worldwide. The results indicated that Peruvian and Paraguayan BLV strains were grouped into genotypes-1, -2, and -6, while those from Bolivia were clustered into genotypes-1, -2, and -6, and a new genotype, genotype-9. Interestingly, these results were confirmed using ML phylogenetic analysis of whole genome sequences obtained by next generation sequencing of 25 BLV strains, assigned to four different genotypes (genotypes-1, -2, -6, and -9) from Peru, Paraguay, and Bolivia. Comparative analyses of complete genome sequences clearly showed some specific substitutions, in both structural and non-structural BLV genes, distinguishing the novel genotype-9 from known genotypes.

CONCLUSIONS

Our results demonstrate widespread BLV infection in South American cattle and the existence of a new BLV genotype-9 in Bolivia. We conclude that at least seven BLV genotypes (genotypes-1, -2, -4, -5, -6, -7, and -9) are circulating in South America.

Detection of the BLV provirus from nasal secretion and saliva samples using BLV-CoCoMo-qPCR-2: Comparison with blood samples from the same cattle

Bovine leukemia virus (BLV) induces enzootic bovine leukosis, which is the most common neoplastic disease in cattle. Sero-epidemiological studies show that BLV infection occurs worldwide. Direct contact between infected and uninfected cattle is thought to be one of the risk factors for BLV transmission. Contact transmission occurs via a mixture of natural sources, blood, and exudates. To confirm that BLV provirus is detectable in these samples, matched blood, nasal secretion, and saliva samples were collected from 50 cattle, and genomic DNA was extracted. BLV-CoCoMo-qPCR-2, an assay developed for the highly sensitive detection of BLV, was then used to measure the proviral load in blood (n=50), nasal secretions (n=48), and saliva (n=47) samples. The results showed that 35 blood samples, 14 nasal secretion samples, and 6 saliva samples were positive for the BLV provirus. Matched blood samples from cattle that were positive for the BLV provirus (either in nasal secretion or saliva samples) were also positive in their blood. The proviral load in the positive blood samples was >14,000 (copies/1×10(5) cells). Thus, even though the proviral load in the nasal secretion and saliva samples was much lower (<380 copies/1×10(5) cells) than that in the peripheral blood, prolonged direct contact between infected and healthy cattle may be considered as a risk factor for BLV transmission.

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.

Detection and molecular characterization of bovine leukemia virus in Philippine cattle

Bovine leukemia virus (BLV) is the etiological agent of enzootic bovine leukosis, which is the most common neoplastic disease of cattle. BLV infects cattle worldwide, imposing a severe economic impact on the dairy cattle industry. However, there are no comprehensive studies on the distribution of BLV in the Philippines, and the genetic characteristics of Philippine BLV strains are unknown. Therefore, the aim of this study was to detect BLV infections in the Philippines and determined their genetic variability. Blood samples were obtained from 1116 cattle from different farms on five Philippine islands, and BLV provirus was detected by BLV-CoCoMo-qPCR-2 and nested PCR targeting BLV long terminal repeats. Out of 1116 samples, 108 (9.7 %) and 54 (4.8 %) were positive for BLV provirus, as determined by BLV-CoCoMo-qPCR-2 and nested PCR, respectively. Of the five islands, Luzon Island showed the highest prevalence of BLV infection (23.1 %). Partial env gp51 genes from 43 samples, which were positive for BLV provirus by both methods, were sequenced for phylogenetic analysis. Phylogenetic analysis based on a 423-bp fragment of the env gene revealed that Philippine BLV strains clustered into either genotype 1 or genotype 6. Substitutions were mainly found in antigenic determinants, such as the CD4(+) T-cell epitope, the CD8(+) T-cell epitope, the second neutralizing domain, B and E epitopes, and these substitutions varied according to genotype. This study provides comprehensive information regarding BLV infection levels in the Philippines and documents the presence of two BLV genotypes, genotypes 1 and 6, in this population.

L233P mutation of the Tax protein strongly correlated with leukemogenicity of bovine leukemia virus

The bovine leukemia virus (BLV) Tax protein is believed to play a crucial role in leukemogenesis by the virus. BLV usually causes asymptomatic infections in cattle, but only one-third develop persistent lymphocytosis that rarely progress after a long incubation period to lymphoid tumors, namely enzootic bovine leucosis (EBL). In the present study, we demonstrated that the BLV tax genes could be divided into two alleles and developed multiplex PCR detecting an L233P mutation of the Tax protein. Then, in order to define the relationship between the Tax protein and leukemogenicity, we examined 360 tumor samples randomly collected from dairy or breeding cattle in Japan, of which Tax proteins were categorized, for age at the time of diagnosis of EBL. The ages of 288 animals (80.0%) associated with L233-Tax and those of 70 animals (19.4%) with P233-Tax individually followed log-normal distributions. Only the two earliest cases (0.6%) with L233-Tax disobeyed the log-normal distribution. These findings suggest that the animals affected by EBL were infected with the virus at a particular point in life, probably less than a few months after birth. Median age of those with P233-Tax was 22 months older than that with L233-Tax and geometric means exhibited a significant difference (P<0.01). It is also quite unlikely that viruses carrying the particular Tax protein infect older cattle. Here, we conclude that BLV could be divided into two categories on the basis of amino acid at position 233 of the Tax protein, which strongly correlated with leukemogenicity.

Detection of bovine leukemia virus in brains of cattle with a neurological syndrome: pathological and molecular studies

Bovine leukemia virus (BLV) was investigated in the central nervous system (CNS) of cattle with neurological syndrome. A total of 269 CNS samples were submitted to nested-PCR (BLV env gene gp51), and the viral genotypes were identified. The nested-PCR was positive in 4.8% (13/269) CNS samples, with 2.7% (2/74) presenting at histological examination lesions of nonpurulent meningoencephalitis (NPME), whereas 5.6% (11/195) not presenting NPME (P > 0.05). No samples presented lymphosarcoma. The PCR products (437 bp) were sequenced and submitted to phylogenetic analysis by neighbor-joining and maximum composite likelihood methods, and genotypes 1, 5, and 6 were detected, corroborating other South American studies. The genotype 6 barely described in Brazil and Argentina was more frequently detected in this study. The identity matrices showed maximum similarity (100%) among some samples of this study and one from Argentina (FJ808582), recovered from GenBank. There was no association among the genotypes and NPME lesions.

The molecular characterization of bovine leukaemia virus isolates from Eastern Europe and Siberia and its impact on phylogeny

Recent studies have shown that bovine leukemia virus (BLV) sequences can be classified into seven distinct genotypes based on full gp51 sequence. This classification was based on available sequence data that mainly represented the BLV population that is circulating in cattle from the US and South America. In order to aid with a global perspective inclusion of data from Eastern Europe is required. In this study we examined 44 BLV isolates from different geographical regions of Poland, Belarus, Ukraine, and Russia. Phylogenetic analysis based on a 444bp fragment of env gene revealed that most of isolates belonged to genotypes 4 and 7. Furthermore, we confirmed the existence of a new genotype, genotype 8, which was highly supported by phylogenetic computations. A significant number of amino acid substitutions were found in the sequences of the studied Eastern European isolates, of which 71% have not been described previously. The substitutions encompassed mainly the C-part of the CD4+ epitope, zinc binding peptide region, CD8+ T cell epitope, and overlapping linear epitope E. These observations highlight the use of sequence data to both elucidate phylogenetic relationships and the potential effect on serological detection of geographically diverse isolates.

Detection and molecular characterization of bovine leukemia viruses from Jordan

Bovine leukemia virus (BLV) is distributed worldwide. BLV has many effects on the health status and productivity of infected animals and is a potential risk for humans. In this study, we aimed to investigate the presence of and genotype bovine leukemia viruses on Jordanian dairy farms. Nested PCR coupled with RFLP and direct sequencing of a partial fragment of the env gene were carried out. Two BLV genotypes were found, genotypes 1 and 6. These genotypes were identified by nested PCR-RFLP of 444 bp of the env gene by restriction digestion with HaeIII, Bcl I and Pvu II. However, BLV-Jordan-10 seems to represent an entirely new genotype in our phylogenetic analysis. The nucleotide sequence identity between these two Jordanian BLV genotypes (1 and 6) was 96.2 %. The nucleotide sequence identity between Jordanian BLV genotype 1 and other reference BLV genotype 1 strains ranged from 99 % to 99.5 %. The nucleotide sequence similarity of the Jordanian BLV genotype 6 to other BLV genotypes ranged from 90 % to 96.7 %. A neutralizing motif and CD8(+) T-cell epitope were found in the env protein of both Jordanian isolates. In this study, we documented the presence of two BLV genotypes (1 and 6) on Jordanian dairy farms.

Identification of a new genotype of bovine leukemia virus

To investigate the degree of genetic variability of bovine leukemia virus (BLV) strains circulating in Croatia, 29 isolates from the six largest dairy farms were examined by PCR for a segment of the gp51 env gene, followed by DNA sequencing and phylogenetic analysis. The nucleotide sequences were compared with other previously characterized BLV strains from different geographical areas, comprising all seven known BLV genotypes. The Croatian sequences showed six to eight nucleotide substitutions: six silent substitutions and two amino acid changes. Four of those substitutions were within epitopes. In comparison to the sequences of other BLV genotypes, our isolates showed the closest relationship to genotype 1 isolates PL-3252 (FJ808585) and AL-148 (FJ808573) from Argentina. The degree of variation between our sequences and those of genotype 1 was 0.2- 4.6 %. In phylogenetic trees based on 400-nt and 519-nt sequences, all of the Croatian sequences clustered separately from the other sequences, revealing a new genotype.

Preventive and therapeutic strategies for bovine leukemia virus: lessons for HTLV

Bovine leukemia virus (BLV) is a retrovirus closely related to the human T-lymphotropic virus type 1 (HTLV-1). BLV is a major animal health problem worldwide causing important economic losses. A series of attempts were developed to reduce prevalence, chiefly by eradication of infected cattle, segregation of BLV-free animals and vaccination. Although having been instrumental in regions such as the EU, these strategies were unsuccessful elsewhere mainly due to economic costs, management restrictions and lack of an efficient vaccine. This review, which summarizes the different attempts previously developed to decrease seroprevalence of BLV, may be informative for management of HTLV-1 infection. We also propose a new approach based on competitive infection with virus deletants aiming at reducing proviral loads.

Genetic heterogeneity among bovine leukemia viruses in Japan and their relationship to leukemogenicity

Bovine leukemia virus (BLV) infection in cattle causes persistent lymphocytosis, and a few percent of infected animals develop lymphoid tumors, namely enzootic bovine leukosis (EBL). In this study, a 440-bp fragment of the env gene was amplified from 204 tumor samples collected from different regions of Japan and analyzed by restriction fragment length polymorphism (RFLP) to determine the association of BLV with EBL. Of the seven RFLP types defined, types I, II, and III were dominant and found in 12.7, 75.0, and 8.3% of tumor samples, respectively. Cattle harboring type III virus were significantly older than other animals at the time of diagnosis of EBL. Type III viruses were found in approximately 33% and 5.5% of Japanese Black and Holstein cattle, respectively, with EBL. These findings indicate that genetically distinct BLV was associated with EBL in Japan and that the genetic profile may influence the leukemogenicity of the virus.