Prevalence Of Bovine Leukemia Virus Research Articles

Breeding beef bulls as a source of BLV transmission

Bovine leukosis is a chronic lymphoproliferative disorder caused by bovine leukemia virus (BLV) that leads to economic losses in the beef and dairy industries. The USDA NAHMS Beef 1997 study estimated that 38% of cow-calf beef herds and 10.3% of individual adult cows in the US are BLV seropositive. Most BLV infected animals are asymptomatic carriers of the virus while a proportion of infected cattle develop lymphocytosis (30-40%) and lymphosarcoma (5-10%). The major route of virus transmission is iatrogenic through the transfer of blood from infected cattle. BLV proviral DNA has also been identified in nasal secretions, saliva, milk, colostrum, and semen, which could serve as potential sources for virus transmission. In the US, natural service accounts for 90% of the breeding in beef cow-calf operations. During natural breeding, minor genital tract trauma resulting from copulation may lead to blood transfer and transmission of BLV. Alternatively, BLV transmission may occur via transfer of infected secretions including semen and smegma during copulation. Little is known about the prevalence of BLV in breeding bulls and few studies have evaluated semen or smegma as a potential route of BLV transmission. The objectives of this study were to 1) determine the prevalence of BLV in breeding bulls presented for breeding soundness exams (BSE) at Michigan State University (MSU), 2) compare lymphocyte counts in BLV positive and negative bulls, and 3) evaluate the presence of BLV proviral DNA in bull smegma and semen.

Lifetime effects of infection with bovine leukemia virus on longevity and milk production of dairy cows

Enzootic bovine leukosis (EBL) is an economically important disease of dairy cattle caused by bovine leukemia virus (BLV). The economic impacts of the infection have been debated in the literature. The present study was conducted to determine the lifetime effects of BLV infection on longevity and milk production of dairy cows in Canada.The data were aggregated from a combination of two data sets: 1) BLV serum-ELISA test results from Canada-wide surveys of production limiting diseases, which took place between 1998 and 2003 in 8 provinces, and 2) longitudinal production data for all cows in the former study, extracted from the Canadian dairy herd improvement database. All participant cows had been culled or died by the onset of this study. A historical cohort study was designed, including cows which tested positive to BLV-antibodies in their first lactation (positive cohort, n=1858) and cows which tested negative in their second or later lactations (negative cohort, n=2194). To assess the impacts of infection with BLV on longevity (the number of lifetime lactations), a discrete-time survival analysis was carried out. The effect of BLV on the lifetime milk production (the sum of all life 305-day milk production) was evaluated using a multilevel linear regression model.Overall, 4052 cows from 348 herds met the eligibility criteria and were enrolled in the study. In the longevity model, the interaction term between time (lactation number) and BLV-status was highly significant. Cows which were positive to BLV had consistently greater probabilities of being culled (or dying) than the test-negative cows. In the milk production model, the interaction term between BLV-status and longevity of the cows was highly significant; indicating that lifetime BLV effects on the total milk production was dependent on the lactation in which the study cows were culled/died. Infected cows with 2 and 3 lactations showed significantly lower life milk productions [−2554kg (−3609 to −1500) and −1171kg (−2051 to −292), respectively] compared with their negative counterparts with 2 and 3 lactations. As the cows lived longer (>3 lactations), the differences in life milk production between the two cohorts were no longer significant. Overall, it was predicted that the test-positive cows produced substantially lower milk compared to the test-negative cows throughout their study lifespans. With the high prevalence of BLV in Canadian dairy cows and its detrimental economic impacts, pursuing broad-based control programs in Canada should be evaluated.

Molecular epidemiological and serological studies of bovine leukemia virus (BLV) infection in Thailand cattle

BLV is the etiological agent of enzootic bovine leucosis. BLV has negative effects on animal health and causes economic losses worldwide. However, epidemiological studies on BLV are relatively unknown in many parts of Asian countries. Thus, this study sought to explore BLV infections in cattle in Thailand to determine the extent of the geographic distribution of BLV and to measure its prevalence rates. For this study, 744 cattle from 11 farms in 9 provinces of Thailand were screened in 2013 and 2014 by ELISA and nested PCR. Of those cattle, 41 BLVs were genetically characterized using 188 BLV gp51 env gene sequences available in GenBank. The BLV prevalence in Thailand was high, ranging from 5.3% to 87.8%, as determined by PCR and 11.0% to 100% as determined by ELISA, according to geographical region. Phylogenetic analysis showed that Thailand BLVs belonged to genotypes 1 and 6 and a new genotype 10, which are sporadically observed across Thailand with a prevalence of 31.7%, 19.5%, and 48.8%, respectively. A significant number of amino acid substitutions were also found in the gp51 sequences, of which unique changes in genotype 10 have not been reported previously. Briefly, the majority of substitutions were confined to CD4+/CD8+ T-cell epitopes, neutralizing domains, and E-D-A epitopes. Those observations indicate that BLV infections in Thailand cattle are prevalent and that the geographic distribution of BLV is dynamic, with a high level of genetic diversity. This distribution implies a long-term BLV infection in cattle populations and the movement of infected cattle. In sum, this study suggests that intensive surveillance and effective prevention strategies are required to determine the prevalence of BLV in Thailand and control continuous infections with BLVs.

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

BackgroundBovine 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.ResultsIn 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.ConclusionsOur 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.Electronic supplementary materialThe online version of this article (doi:10.1186/s12977-016-0239-z) contains supplementary material, which is available to authorized users.

Detection of bovine leukemia virus and identification of its genotype in Mongolian cattle.

Epidemiological studies have indicated that bovine leukemia virus (BLV) infection is globally distributed. However, no information regarding the disease and genetic diversity of the virus in the cattle of Mongolia is currently available. In this study, the prevalence of BLV was assessed using PCR, and the genetic diversity was analyzed through DNA sequencing. Of the 517 samples tested, 20 positives were identified. Phylogenetic analysis showed that six, one, and four isolates were classified into genotype 4, 7, and 1, respectively. Most isolates were clustered with isolates from Eastern Europe and Russia. This study is the first to investigate the BLV genotype in Mongolia.

Diagnóstico molecular del virus de Leucosis Bovina en una población de vacas Holstein, Colombia

El virus de la leucosis bovina (BLV) posee un genoma RNA de cadena sencilla, pertenece a la familia Retroviridae y presenta ocho genotipos diferentes. Los linfocitos B son la célula blanco del BLV, lo cual tiene un impacto negativo sobre el sistema inmune de los bovinos ya que los hace más susceptibles a otras enfermedades de origen infeccioso además las vacas lecheras presentan una menor producción respecto al hato (2,5 a 5 %). Esta enfermedad se transmite a través del consumo de leche y principalmente de forma iatrogénica. El objetivo del presente trabajo fue diagnosticar BLV por medio de la técnica PCR anidada, para lo cual se tomaron 500 muestras de sangre de vacas Holstein pertenecientes a varios hatos ubicados en los principales municipios con carácter lechero en el departamento de Antioquía, durante los meses de febrero a junio de 2013. Se realizó una PCR anidada para detectar el provirus amplificando una región del gen env viral. Se obtuvo un fragmento de 444 pb y se comprobó la identidad de la secuencia a través de la aplicación BLAST ®. La prevalencia para el departamento de Antioquía fue del 44 % (219/500). Uno de los productos de PCR fue secuenciado y clasificado como genotipo 1; se encontró un 99 % de identidad con la secuencia FJ808575.1. Se evaluaron tres subregiones lecheras Oriente, Norte y Valle de Aburra. La presencia del virus fue de 70 %, 45 %, 31 % respectivamente. La prevalencia molecular de BLV varió entre 16 y 88 % por municipio. Se encontró diferencia estadísticamente significativa (p

Molecular detection of bovine leukemia virus in peripheral blood of Iranian cattle, camel and sheep.

Bovine leukemia virus (BLV) is a deltaretrovirus which infects and induces proliferation of B-lymphocytes in the peripheral blood circulation and in lymphoid organs primarily of cattle, leading to leukemia/lymphoma. This study was carried out to investigate the presence of BLV in cattle, sheep and camels from the Chaharmahal va Bakhtiary and Isfahan provinces in Iran. A total of 874 blood samples collected from cattle, sheep and camels were used in this study to detect BLV using a nested-PCR. The results from this study indicated that 17.2% (n=874) of all blood samples collected were positive for BLV. The percentages of blood samples positive for BLV from cattle, sheep and camels were 22.1 (n=657), 5.3 (n=95) and 0 (n=122) respectively. The results from this study showed that BLV infected cattle and sheep. Camels seemed to be resistant to BLV infection. This study contributes to the nationwide effort to obtain baseline information on the prevalence of BLV, which will assist in planning the control strategy for the disease in Iran.

Predicting within-herd prevalence of infection with bovine leukemia virus using bulk-tank milk antibody levels

Enzootic bovine leukosis (EBL) is an economically important infection of dairy cattle caused by bovine leukemia virus (BLV). Estimating the prevalence of BLV within dairy herds is a fundamental step towards pursuing efficient control programs. The objectives of this study were: (1) to determine the prevalence of BLV infection at the herd level using a bulk-tank milk (BTM) antibody ELISA in the Maritime region of Canada (3 provinces); and (2) to develop appropriate statistical models for predicting within-herd prevalence of BLV infection using BTM antibody ELISA titers.During 2013, three monthly BTM samples were collected from all dairy farms in the Maritime region of Canada (n=623) and tested for BLV milk antibodies using a commercial indirect ELISA. Based on the mean of the 3 BTM titers, 15 strata of herds (5 per province) were defined. From each stratum, 6 herds were randomly selected for a total of 90 farms. Within every selected herd, an additional BTM sample was taken (round 4), approximately 2 months after the third round. On the same day of BTM sampling, all cows that contributed milk to the fourth BTM sample were individually tested for BLV milk antibodies (n=6111) to estimate the true within-herd prevalence for the 90 herds. The association between true within-herd prevalence of BLV and means of various combinations of the BTM titers was assessed using linear regression models, adjusting for the stratified random sampling design.Herd level prevalence of BLV in the region was 90.8%. In the individual testing, 30.4% of cows were positive. True within-herd prevalences ranged from 0 to 94%. All linear regression models were able to predict the true within-herd prevalence of BLV reasonably well (R2>0.69). Predictions from the models were particularly accurate for low-to-medium spectrums of the BTM titers. In general, as a greater number of the four repeated BTM titers were incorporated in the models, narrower confidence intervals around the prediction lines were achieved. The model including all 4 BTM tests as the predictor had the best fit, although the models using 2 and 3 BTM tests provided similar results to 4 repeated tests. Therefore, testing two or three BTM samples with approximately two-month intervals would provide relatively precise estimates for the potential number of infected cows in a herd. The developed models in this study could be applied to control and eradication programs for BLV as cost-effective tools.

Carryover of bovine leukemia virus antibodies in samples from shared milk meters

Screening for infectious diseases of cattle using milk from the dairy herd improvement (DHI) sampling process is very convenient. However, when samples from shared milk meters are used, carryover of antibodies or other diagnostic targets can complicate the interpretation of the diagnostic test results for diseases, including bovine leukosis. The objectives of this study were (1) to assess the potential for carryover of antibodies against bovine leukemia virus (BLV) in milk samples obtained from shared meters, and (2) to determine if adjustment of the diagnostic test cut-off value would improve the test characteristics for meter-collected milk ELISA results. Eight dairy farms were randomly selected from herds with a wide range of BLV prevalence levels in Prince Edward Island, Canada. Within each chosen farm, 2 to 4milk meters were randomly selected. During the routine procedures of DHI sampling, 2 simultaneous milk samples, 1 hand-collected at the beginning of milking (after udder preparation) and the other from the corresponding milk meter, were taken from all lactating cows (n=236) that were milked at the selected meters (n=26). The sequence of cows using each meter was recorded. All samples were tested for BLV antibodies using a commercial indirect ELISA. Antibody carryover potential was assessed in meter-collected samples which were preceded by other cows using the same meters. Applying the hand-collected sample results as our reference standard, a new cut-off was defined for meter-collected samples to optimize the test characteristics. At the standard cut-off value of the diagnostic test, 110 (46.6%) of the hand-collected and 136 (57.6%) of the meter-collected samples were positive. For low-titer cows (e.g., true negatives), the likelihood of antibody carryover significantly increased as the titer of preceding cows increased, whereas this change was not substantial for high-titer cows. The odds of obtaining false diagnoses in meter-positive samples became larger with increase in the titer of preceding cows. A suspicious category for meter ELISA results was defined, and a retest was recommended for the cows falling into this category. This strategy effectively assisted in reducing the number of consequent false-positive results. When DHI-collected samples are used, carryover can affect the interpretation of dichotomous test results and may require adjustment of assay cut-off values.

Options for the control of bovine leukemia virus in dairy cattle

The subclinical impact of bovine leukemia virus (BLV) on the sustainability of the US dairy industry is only now being fully recognized. Findings of recent longitudinal studies conducted in Michigan dairy herds were consistent with the results of previous studies in showing that within-herd prevalence of BLV-infected cattle was negatively associated with milk production and cow longevity. Risk factors relating to routes of hematogenous transmission such as the use of shared hypodermic needles, shared reproductive examination sleeves, and natural breeding were associated with BLV within-herd prevalence. Few US dairy producers know the prevalence of BLV-infected cattle in their herds or are aware of the insidious economic impact of BLV or the options for BLV control. As an increasing number of countries eradicate BLV from their cattle populations, restrictions on the movement of US cattle and cattle products will likely increase. Veterinarians should be aware of recent developments for screening serum and milk samples for antibodies against BLV and the results of research regarding the economic impact of BLV so they can advise their dairy clients of available alternatives for monitoring and controlling BLV infection.

Incidence of Enzootic Bovine Leukosis and Prevalence of Bovine Leukemia Virus in Slaughtered Cattle in Tokyo

1997～2012年度の東京都におけると畜牛の地方病性牛白血病（EBL）の発生状況を調査した結果，1997年度のEBL発生率は0.003％であったが，2005年度から増加し，2012年度は0.03％となった．EBL牛163頭のうち，生体所見で異常があったものは14.6％にすぎず，19.8％では白血球が7,500個/μl 未満であり，牛白血病ウイルス（BLV）抗体検査で陰性が1.85％認められた．一方，リアルタイムPCR法では全例からBLV遺伝子を検出したことから，本法はEBL診断に有効と考えられた．また，と畜牛のBLV抗体調査を行ったところ，陽性率は2005～2007年度には肥育用乳用種で23.8％，肥育用肉用種で19.8％であった．2012年度には肥育用肉用種のみを調査したが，12.7％に低下した．交雑種の陽性率は黒毛和種に比較して2回の調査ともに有意に高かった．

Bovine leukemia virus in Michigan beef bulls

Bovine leukemia virus (BLV) is a contagious retrovirus of cattle. Little is known about the prevalence, risk factors, or potential health impacts of BLV in beef cattle. Most cattle remain asymptomatic, but 30 to 40% will develop persistent leukocytosis. Fewer than 5% of infected cattle develop malignant lymphosarcoma, which in the US, is the most common neoplastic disease of cattle discovered at slaughter. Recent studies have suggested that BLV prevalence has increased dramatically, and has a previously unappreciated economic impact on the cattle industry. The objectives of this study were to determine the crude and age-specific prevalence of BLV among beef bulls in Michigan, assess the possible impact of BLV infection on bull fertility, and identify the major risk factors for BLV transmission among beef bulls.

Association of bovine leukemia virus and Mycobacterium avium subsp paratuberculosis with shedding of Shiga toxin-producing Escherichia coli (STEC)

Cattle are a reservoir of Shiga toxin-producing Escherichia coli (STEC), which is an important cause of foodborne illness and hemolytic uremic syndrome (HUS) in children. Bovine leukemia virus (BLV) is a retrovirus that causes enzootic bovine leukosis. Most animals infected with BLV never develop clinical signs, but 30% of BLV carriers will develop a persistent lymphocytosis, and< 5% will develop malignant lymphosarcoma. Johnes' disease (JD) is caused by Mycobacterium avium subsp paratuberculosis (MAP), and cattle generally become infected with MAP at a very early age ( < 30 days old), although clinical disease does not typically manifest itself until 3-5 years of age. Recent studies have estimated the herd prevalence of BLV and JD in Michigan dairy herds to be approximately 85% and 50%, respectively. Both diseases are chronic in nature and can have a negative impact on the health and productivity of cattle from both clinical and subclinical effects. Because of their chronic and potentially debilitating nature, secondary health issues are often associated with BLV and JD. Our hypothesis was that cattle infected with BLV and JD were more likely to shed STEC than were uninfected cattle. Our rationale for this hypothesis was that BLV affects the immune system of cattle, which might favor the colonization and shedding of STEC. In the case of JD, the alteration of normal intestinal function and structure could potentially favor the colonization and shedding of STEC in infected cattle. Our objective was to determine whether cattle that test positive for JD and BLV could be a potential target population for the implementation of control strategies to reduce STEC contamination in the human food chain.

牛白血病対策の現状と課題

牛白血病対策の現状と課題

경기도 북부지역 젖소 사육농장의 bovine leukemia virus 감염 실태 조사

This study was carried out to investigate the prevalence of bovine leukemia virus (BLV) infection and to compare the results of enzyme-linked immunosorbent assay (ELISA) and nested polymerase chain reaction (nPCR) in dairy farms in northern Gyeonggi province from August through December 2011. A total of 625 dairy cattle from 14 dairy farms were tested for antibodies against BLV using commercially available ELISA test kit. The overall seroprevalence of BLV infection was 76.3%. The seroprevalence of diary cattle according to age was the highest at 61~72 months (88.0%, P<0.001). Two hundred fifty one dairy cattle from 7 diary farms were tested ELISA and nPCR. The kappa value of BLV between ELISA and nPCR was 0.765. The results indicate that BLV infection spread widely in dairy farms and the nPCR is rapid method for the early detection of BLV infection.

Trends in export of dairy Holstein heifers from the United States

A general overview of the export and testing requirements is given for bred female dairy cattle from the United States (US) to Turkey, Mexico, and Russia, the top three export markets. Disease-by-disease serologic test results are presented for 7,339 Holstein dairy heifers tested in quarantine in southeastern Chester County, Pennsylvania for export to Turkey from June 2010 to October 2011. Management practices to decrease the prevalence of diseases of importance to the export market are reviewed. Prevalence rates of serologically positive heifers tested during the quarantine period using the enzyme linked-immunosorbent assay were: bluetongue virus (BTV) 4.9%, Mycobacterium avium subsp. paratuberculosis (MAP) 0.65%, and bovine leukemia virus (BLV) 5.7%. Prevalence rate of bovine viral diarrhea virus - persistently infected (BVDV PI) heifers using pooled polymerase chain reaction was 0.68%. Because of the high prevalence of BLV in US cattle, most if not all heifers are routinely screened for BLV prior to purchase and entry into quarantine. Some of the heifers in this data set originating from endemic BTV states were pre-screened for BTV. Because of pre-screening for BLV prior to purchase for export, the prevalence of BLV reported was lower than would be expected in the general population of heifers in the US. The prevalence of BTV would depend on whether heifers came from BTV free or endemic states. Dairy herds wanting to market heifers for export should apply management practices known to decrease the prevalence of BTV, BLV, MAP, and BVDV PI.

Prevalence of Bovine Leukemia Virus (BLV) Antibodies in Bulk Tank Milk of Dairy Cattle Herds of Mashhad Area, North-East of Iran

Prevalence of Bovine Leukemia Virus (BLV) Antibodies in Bulk Tank Milk of Dairy Cattle Herds of Mashhad Area, North-East of Iran

Association between bovine leukemia virus, production, and population age in Michigan dairy herds

The objective of this study was to determine the herd-level effect of bovine leukemia virus (BLV) infection on dairy production, culling, and cow longevity. During routine herd testing, Dairy Herd Improvement Association technicians collected milk samples from about 40 cows from each of 104 randomly selected Michigan dairy herds averaging ≥120 milking cows and 11,686kg of milk/yr. Milk samples were analyzed for the presence of anti-BLV antibodies by ELISA, and herd- and lactation-specific estimates of BLV prevalence were computed to determine which were the most predictive of herd milk production, culling rate, and cow longevity (proportion of cows in their third or greater lactation). On this basis, the herd BLV index (an unweighted mean BLV prevalence rate for lactation number 1, 2, 3, and ≥4) was selected as the measure of BLV prevalence that was the most highly associated with BLV economic impact. Step-down multivariate analysis was used to determine the extent to which any of 19 herd-level management variables may have confounded the association of BLV index and measures of herd economic impact (milk production and cow longevity). The BLV index was not associated with the 12-mo culling rate, but was negatively associated in the final multivariable model with the proportion of cows that were ≥third lactation, and was negatively associated with herd milk production. In summary, increased prevalence of BLV within Michigan dairy herds was found to be associated with decreased herd milk production and decreased cow longevity. Our results provide evidence that BLV infection is associated with herd-level economic impacts in high-performing dairy herds.

Using a Herd Profile to Determine Age-Specific Prevalence of Bovine Leukemia Virus in Michigan Dairy Herds

Enzootic bovine leukosis is a contagious disease of cattle caused by the retrovirus, bovine leukemia virus (BLV) and is the most common cause of malignant neoplasm in cattle. In order to facilitate surveillance of this disease in dairy herds, we developed a method to combine ELISA of milk collected during routine production testing with a prescribed sampling of cows that is independent of the proportion of cows within each lactation. In 113 Michigan dairy herds, milk samples from ten cows in each of the 1st, 2nd, 3rd, and ≥4th lactations were analyzed for anti-Bovine Leukemia Virus (BLV) antibodies by milk ELISA. For each herd, a BLV herd profile (BHP) was calculated as the simple average of the percent of BLV-positive cows within each of the four lactation groups. The mean BHP for all herds was 32.8%, with means of 18.5, 28.8, 39.2, and 44.8% of 1st, 2nd, 3rd, and ≥4th lactation animals infected, respectively. In eight herds, we determined the correlation between the BHP, and true herd prevalence by testing the entire lactating herd (r = 0.988, P < 0.0001). The BHP allows discrimination of lactation-specific BLV prevalence within a dairy herd, to help identify risk factors and management plans that may be important in transmission of BLV.

Bovine leukemia virus proviral load in naturally-infected animals

Due to the high prevalence of Bovine Leukemia Virus (BLV) in argentinean dairy herds, classical control based on the individual level of infection do not constitute a feasible strategy. An approach based on selective segregation of animals according to their peripheral-blood proviral load (PVL) could be a practical alternative. We conducted this work to get information about the frequency distribution of PVL under natural conditions with the purpose to describe the proportion of animals that could be selected as potential low transmitters. We analyzed the PVL on 332 cattle from four infected farms with different levels of seroprevalence (Farm A: 30%, Farm B: 66%, Farm C: 87% and Farm D: 92%) together with the correlation of PVL with BLV antibody levels. In all farms, more than 50% of the seropositive animals showed a proviral load lower than our internal calibrator sample which corresponds to 1 infected cell in 100 non-infected cells according to: Farm A: 78%, Farm B: 79%, Farm C: 55% and Farm D: 79%. The analysis of antibodies showed that low p24-ELISA reactivity is associated with low PVL in the vast majority of animals. The present study provides biological data that could contribute with the design of a management program that includes a selective segregation policy for BLV control based on the level of proviral load, which represents a measure of integrated viral genome in host cells and a surrogate marker of the level of BLV transmission under natural conditions.