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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.

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.

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., Competing Interests: The authors have declared that no competing interests exist.

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., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Enzootic bovine leukosis (EBL) is known as bovine lymphosarcoma and normally affects the old cattle. EBL is caused by bovine leukemia virus (BLV), which is generally spread all around the world. This virus is transmitted via bovine blood products within and between cattle herds. Glycoprotein GP51 in the blood is responsible for cattle immune responses to BLV. This virus has been previously detected in cattle and even in the calf. However, to the best of my knowledge, this is the first short communication reporting the detection of EBL in calves in Iran. The samples of the present study were obtained from two calves of different mothers within the same dairy farm of 2000 cattle, located near Tehran, Iran. General clinical signs of the two calves, such as heart rate, respiratory rate, and body temperature were recorded. The clinical observations were confirmed by hematological tests, histopathological examination, enzyme-linked immunosorbent assay, polymerase chain reaction, and phylogenetic analyses. The originality of the detected virus was assessed by blasting against the other strains of BLV available on the NCBI web page. Regarding the clinical symptoms, bulging eyes were noticeable. Moreover, atypical and malignant lymphocytes were detected in the circulatory system and periorbital tissue. It should be noted that the presence and expression of GP51 in both calves and cattle was similar to the previously detected ones in Korea and Japan. The latter result confirms the originality of retrovirus structural subunit GP51. Similar observations were reported in a six-month follow-up., (Copyright © 2019, Archives of Razi Institute. Published by Kowsar.)

Bovine leukemia virus (BLV) causes enzootic bovine leucosis and is widely spread worldwide, except several European countries, Australia and New Zealand. Although BLV is highly prevalent in China, information about the genetic diversity and evolutionary dynamics of BLV among Chinese dairy herds is still lacking. To determine the genetic variability of BLV, 219 cows from four cities of Ningxia province of China were screened for BLV infection by fluorescence resonance energy transfer (FRET)-PCR and sequencing, 16 selected positive samples were subjected to molecular characterization. Phylogenetic analysis using the neighbor-joining (NJ) method on complete sequences of envelope (env) gene of BLV obtained from China and those available in GenBank (representing BLV genotypes 1-10) revealed that those Chinese strains belonged to genotypes 4 and 6. Totally, 23 mutations were identified and 16 of them were determined to be unique mutations among Chinese strains. Alignment of the deduced amino acid sequences demonstrated six mutations in glycoprotein 51 (gp51) and three mutations in glycoprotein 30 (gp30) located in the identified neutralizing domain (ND), CD8 + T cell epitope, E-epitope, B-epitope, gp51N12 and cytoplasmic domain of transmembrane protein. This study reported for the first time the BLV genotype 4 in China, and further studies are warranted to compare its immunogenicity and pathogenicity with other BLV genotypes.

Purpose: Enzootic bovine leucosis (EBL) is a silent disease caused by a retrovirus [bovine leukaemia virus (BLV)]. BLV is classified into almost 10 genotypes that are distributed in several countries. The present research aimed to describe two BLV gp51 env sequences of strains detected in the state of São Paulo, Brazil and perform a phylogenetic analysis to compare them to other BLV gp51 env sequences of strains around the world., Methodology: Two bovines from different herds were admitted to the Bovine and Small Ruminant Hospital, School of Veterinary Medicine and Animal Science, University of São Paulo, Brazil. In both, lymphosarcoma was detected and the presence of BLV was confirmed by nested PCR. The neighbour-joining algorithm distance method was used to genotype the BLV sequences by phylogenetic reconstruction, and the maximum likelihood method was used for the phylogenetic reconstruction. The phylogeny estimates were calculated by performing 1000 bootstrap replicates., Results: Analysis of the partial envelope glycoprotein (env) gene sequences from two isolates (25 and 31) revealed two different genotypes of BLV. Isolate 25 clustered with ten genotype 6 isolates from Brazil, Argentina, Thailand and Paraguay. On the other hand, isolate 31 clustered with two genotype 5 isolates (one was also from São Paulo and one was from Costa Rica). The detected genotypes corroborate the results of previous studies conducted in the state of São Paulo, Brazil. The prediction of amino acids showed substitutions, particularly between positions 136 and 150 in 11 out of 13 sequences analysed, including sequences from GenBank., Conclusion: BLV is still important in Brazil and this research should be continued.

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.

Bovine leukemia virus (BLV) is a retrovirus that causes enzootic bovine leucosis. To investigate the presence and genetic variability of BLV in the Caribbean for the first time, we preformed fluorescence resonance energy transfer (FRET)-PCR for the pol of BLV on DNA from whole blood of cattle from Dominica, Montserrat, Nevis and St. Kitts. Standard PCRs with primers for the env were used for phylogenetic analysis of BLV in positive animals. We found FRET-PCR positive cattle (12.6%, 41/325) on Dominica (5.2%; 4/77) and St. Kitts (19.2%; 37/193) but not on Montserrat (0%, 0/12) or Nevis (0%, 0/43). Positive animals were cows on farms where animals were raised intensively. Phylogenetic analysis using the neighbor-joining (NJ) method on partial and full-length env sequences obtained for strains from Dominica (n = 2) and St. Kitts (n = 5) and those available in GenBank (n = 90) (genotypes 1-10) revealed the Caribbean strains belonged to genotype 1 (98-100% sequence homology). Ours is the first molecular characterization of BLV infections in the Caribbean and the first description of genotype 1 in the region., Competing Interests: The authors have declared that no competing interests exist.

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.

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.

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.

Previous studies have classified the env sequences of bovine leukemia virus (BLV) provirus from different locations worldwide into between two and four genetic groupings. These different studies gave unique names to the identified groups and no study has yet integrated all the available sequences. Thus, we hypothesized that many of the different groups previously identified actually correspond to a limited group of genotypes that are unevenly distributed worldwide. To examine this hypothesis, we sequenced the env gene from 28 BLV field strains and compared these sequences to 46 env sequences that represent all the genetic groupings already identified. By using phylogenetic analyses, we recovered six clades, or genotypes, that we have called genotypes 1, 2, 3, 4, 5 and 6. Genotypes 1-5 have counterparts among the sequence groupings identified previously. One env sequence did not cluster with any of the others and was highly divergent when compared with the six genotypes identified here. Thus, an extra genotype, which we named 7, may exist. Similarity comparisons were highly congruent with phylogenetic analyses. Furthermore, our analyses confirmed the existence of geographical clusters.

Bovine leukemia virus (BLV) is an oncogenic virus widespread in cattle. It belongs to the genus Deltaretrovirus of the family Retroviridae along with human and simian T-lymphotropic viruses. Here we report the addition of 28 new sequences to the current literature of 16 full-length BLV envelope gene sequences. The phylogenetic clustering, genotyping, and geographic distribution of BLV env variations corresponded in most cases. Most natural variations are mapped to the surface of the proposed conformational models of BLV gp51 N-terminus and gp30 external domain, overlapping with or adjacent to immunogenic epitopes. Analyses for evidence of possible selection pressures suggest the BLV env is under stringent negative selection overall, while strong positive selection is indicated for immunogenic epitope G. Natural env deletions bounded by similar flanking sequences were observed in multiple isolates and would result in truncated signal peptides, missing gp51, and aberrant coding frames for other proteins.

DNA was extracted from the peripheral blood of a seropositive, PCR-positive, BLV-infected Holstein cow (No. 38) from Argentina. The DNA was amplified via PCR with a series of overlapping primers encompassing the entire BLV proviral DNA. The amplified BLV ARG 38 DNA was cloned, sequenced, and compared phylogenetically to three other full-length BLV sequences. Characterization of its deduced proteins and its relationship to other members of the PTLV/BLV genus of retroviruses are discussed., (Copyright 2000 Academic Press.)

After describing the results of BIV research during the past years experimental data are presented which indicate that BIV does not cause any clinical symptoms after infection and that no correlation exists with the other widely spread retrovirus in the bovine, the bovine leukosis virus (BLV). Since contact obviously did not lead to a horizontal transmission it is suggested that transmission occurs, as in the cat, vertically from dam to offspring. It was also found that a long period of time after infection can elapse before antibodies against BIV can be detected. It is also quite clear that HIV and BIV do not have much in common except that both are lentiviruses.

Degenerate and specific PCR assays were developed for bovine leukaemia virus (BLV) and/or primate T cell leukaemia/lymphoma viruses (PTLV). The degenerate assays detected all major variants of the BLV/PTLV genus at a sensitivity of 10-100 copies of input DNA; the specific systems detected 1-10 copies of input target. Sensitivity was 100% in specific DNA-PCR assays done on peripheral blood from seropositive BLV-infected cattle and HTLV-I- or HTLV-II-infected humans, and 62% in RNA/DNA-PCR assays on sera from BLV seropositive cattle. The pol fragments from 21 different BLV strains, isolated from cattle in North and Central America, were cloned and sequenced, and compared to other published BLV and PTLV pol sequences. BLV and PTLV sequences differed by 42%. Sequence divergence was up to 6% among the BLV strains, and up to 36% among the PTLV strains (with PTLV-I and PTLV-II differing among themselves by 15% and 8%, respectively). Some cows were infected with several BLV strains. Among retroviruses, BLV and PTLV sequences formed a distinct clade. The data support the interpretation that BLV and PTLV evolved from a common ancestor many millennia ago, and some considerable time before the PTLV-I and PTLV-II strains diverged from each other. The dissemination of the BLV strains studied probably resulted from the export of European cattle throughout the world over the last 500 years. The relatively similar mutation rates of BLV and PTLV, after their various points of divergence, suggest that there could be a much wider genetic range of BLV than has currently been defined.

Bovine leukemia virus belongs to a small subfamily of exogenous retroviruses that includes the human retroviruses HTLV-1, HTLV-II and the simian virus, STLV-1. Like other retroviruses, infection with BLV results in deregulation of the host immune system at both humoral and cellular levels. An approach which might help in the elucidation of some immune impairment phenomena is the investigation of the role that cytokines play in the pathogenesis and immune response of BLV infected animals. Here we describe our findings on IL-6 and TNF. We have found that the levels of IL-6 in the sera of BLV infected cows which show persistent lymphocytosis (BLV+ PL+) were significantly higher than those of BLV infected with no lymphocytosis (BLV+ PL-) or BLV negative cows (BLV-). The same results were obtained by measuring the spontaneous production of IL-6 in peripheral blood mononuclear cells (PBMC). Furthermore, PBMC derived from BLV+PL+ cows secrete higher levels of IL-6 and TNF alpha than those derived from BLV+PL- and BLV- ones following in vitro exposure to the BLV gp51 antigen, bacterial endotoxins (LPS) and ConA. Similar results were obtained when supernatants from stimulated adherent (monocytes, macrophages) and non-adherent cells (B and T lymphocytes) were tested. When exogenous IL-6 and TNF alpha were added to BLV infected cells in vitro, the expression of viral antigens was strongly suppressed. Thus, the possibility exists that the elevated production of IL-6 and even more than that of TNF alpha play a role as contributing factors to the latency of the clinical expression in BLV infection.

The human T-cell lymphotropic virus (HTLV) family includes members associated with T-cell cancers (HTLV-I and HTLV-II) as well as the etiological agent of the acquired immunodeficiency syndrome (HTLV-III). Molecular clones of these viruses were used in heteroduplex mapping experiments to study their structural and evolutionary relationships. The HTLV-I subgroup, despite some restriction enzyme site polymorphism, demonstrated a high degree of sequence conservation. Heteroduplexes of HTLV-I and HTLV-II demonstrated a significant amount of sequence homology, with the strongest region of conservation occurring in the 3'-most coding sequences, designated pX, and to a lesser, although substantial extent in the rest of the genome. Thus, the genomic organization of HTLV-II appears to be very similar to that of HTLV-I. All HTLV-III molecular clones appeared to be identical, with a single exception, which showed heterogeneity in the env gene region. In heteroduplexes between HTLV-I and HTLV-III, very little homology was observed, being confined to the gap/pol region. In contrast to the latter result, a striking amount of homology was detected between HTLV-III and a morphologically similar, pathogenic, nononcogenic lentivirus, visna virus. These data provide strong evidence for a close taxonomic and thus evolutionary relationship between HTLV-III and the lentivirus subfamily of retroviruses. A taxonomic tree, based on the genetic relatedness and biological properties of the HTLV family, is proposed.

Short-term cultures of bovine leukemic lymphocytes release virus particles with biochemical properties of RNA oncogenic viruses. These particles, tentatively called bovine leukemia virus (BLV), have a high molecular weight RNA-reverse transcriptase complex and a density of 1.155 g/ml in sucrose solutions. Molecular hybridizations between BLV/[3H]cDNA and several viral RNAs show that BLV is not related to Mason-Pfizer monkey virus, simian sarcoma associated virus, feline leukemia virus, or avian myeloblastosis virus. These results were confirmed by hybridization between BLV 70S RNA and [3H]cDNA synthesized in the various viruses tested. The high preference of BLV reverse transciptase for Mg++ as the divalent cation suggests that BLV might be an atypical mammalian leukemogenic "type C" virus. DNA-DNA hybridization studies using BLV [3H]cDNA as a probe strongly suggest that the DNA of bovine leukemic cells contains viral sequences that cannot be detected in normal bovine DNA.

The bovine leukemia virus is the etiological agent of a chronic lymphatic leukemia in cows, sheep, and goats. The same virus seems to induce a kind of wasting disease in experimentally infected rabbits. Antibodies to highly purified bovine leukemia viral Mr 51,000 glycoprotein and Mr 24,000 protein cross-react with human T-lymphotropic virus III/lymphadenopathy-associated virus antigens present in cultured lymphocytes of African patients suffering from acquired immune deficiency syndrome. Bovine leukemia virus has many structural and functional characteristics in common with the human T-lymphotropic viruses. The most striking feature of these retroviruses is the existence of a long open reading frame located at the 3' side of the provirus between the right end of the 3' side of env gene and the left end of the long terminal repeat. It is believed that the long open reading frame protein product acts in trans upon a number of genes to account for the biological effects of the virus.

Ovine leukemia virus (OLV) could not be distinguished from bovine leukemia virus (BLV) morphologically. Both C-type RNA tumor virus-producing cells induced syncytia when cocultivated with several indicator cells including XC and KC cells. Syncytia formation could be blocked by antibodies against OLV and BLV, indicating that fusion was virus mediated. Furthermore, antibodies against OLV and BLV exhibited reciprocal syncytial blocking properties, suggesting that OLV and BLV were closely related viruses.