Timing of seroconversion and acquisition of positive polymerase chain reaction assay results in calves experimentally infected with bovine leukemia virus

BLV-CoCoMo Dual qPCR Assay Targeting LTR Region for Quantifying Bovine Leukemia Virus: Comparison with Multiplex Real-Time qPCR Assay Targeting pol Region

The proviral load (PVL) of the bovine leukemia virus (BLV) is a useful index for estimating disease progression and transmission risk. Real-time quantitative PCR techniques are widely used for PVL quantification. We previously developed a dual-target detection method, the "Liquid Dual-CoCoMo assay", that uses the coordination of common motif (CoCoMo) degenerate primers. This method can detect two genes simultaneously using a FAM-labeled minor groove binder (MGB) probe for the BLV long terminal repeat (LTR) region and a VIC-labeled MGB probe for the BoLA-DRA gene. In this study, we evaluated the diagnostic and analytical performance of the Dual-CoCoMo assay targeting the LTR region by comparing its performance against the commercially available Takara multiplex assay targeting the pol region. The diagnostic sensitivity and specificity of the Liquid Dual-CoCoMo assay based on the diagnostic results of the ELISA or original Single-CoCoMo qPCR were higher than those of the Takara multiplex assay. Furthermore, using a BLV molecular clone, the analytical sensitivity of our assay was higher than that of the Takara multiplex assay. Our results provide the first evidence that the diagnostic and analytical performances of the Liquid Dual-CoCoMo assay are better than those of commercially available multiplex assays that target the pol region.

Pilot implementation of a newly developed bovine leukemia virus control program on 11 Alberta dairy farms

We developed a custom bovine leukemia virus (BLV) control program for the Alberta dairy industry, consisting of a risk assessment and a comprehensive list of best management practices (BMP) aimed at prevention of BLV transmission between cattle. This control program was implemented on 11 farms for approximately 1 yr. Blood samples were collected from all cattle ≥12 mo old, and serum was tested with a commercial ELISA. Risk assessments were performed on each farm, risk-connected on-farm management was identified, and management changes expected to prevent transmission of BLV between cattle were suggested by the first author and agreed upon with each farmer. Throughout the following year, all participating farmers were visited multiple times to identify and overcome barriers to implementation and to monitor progress. After approximately 1 yr of implementing BLV control, all cattle ≥12 mo old on farm with a negative or no previous test result were sampled, and the within-herd prevalence was determined. The median number of cattle on farm that were ≥12 mo was 195 (range 110-524). The initial prevalence averaged 39% (13-66%). On average, 5 BMP (3-7) were suggested to each farmer. On average, 4 BMP (1-7) were implemented. At the second sampling, the average within-herd prevalence of all animals that tested positive (including the previous sampling) was 36% (12-62%). Eight farms reduced their within-herd BLV prevalence, within-herd prevalence stayed constant on 1 farm, and it increased on 1 farm. The remaining farm terminated their participation before the second sampling. The number of seroconversions per farm ranged from 3 to 109, highlighting the success of some producers to minimize new infections. The risk assessment was proven to be a valuable tool to identify flaws in on-farm management, although risk assessment score was unrelated to the within-herd BLV prevalence. Finally, it appeared that implementation of BMP aimed at prevention of BLV transmission between cattle could reduce within-herd BLV prevalence when farmers committed to their implementation.

Bovine leukemia virus detection and dynamics following experimental inoculation

Bovine leukemia virus (BLV) infects more than 40% of the United States cattle population and impacts animal health and production. Control programs aiming to reduce disease prevalence and incidence depend on the ability to detect the BLV provirus, anti-BLV antibodies, and differences in blood lymphocyte counts following infection. These disease parameters also can be indicative of long-term disease progression. The objectives of this study were to determine the timing and to describe early fluctuations of BLV-detection by qPCR, ELISA, and lymphocyte counts. Fifteen Holstein steers were experimentally inoculated with 100 μL of a blood saline inoculum. Three steers served as in-pen negative controls and were housed with the experimentally infected steers to observe the potential for contract transmission. Five additional negative controls were housed separately. Steers were followed for 147 days post-inoculation (DPI). Infections were detected in experimentally infected steers by qPCR and ELISA an average of 24- and 36 DPI, respectively. Significant differences in lymphocyte counts between experimentally infected and control steers were observed from 30 to 45 DPI. Furthermore, a wide variation in peak proviral load and establishment was observed between experimentally infected steers. The results of this study can be used to inform control programs focused on the detection and removal of infectious cattle.

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

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

Evaluation of a serum ELISA for detection of bovine leukemia viral antibodies in milk samples

Bovine leukemia virus (BLV) infection has worldwide distribution in both dairy and beef herds. Our study was initiated in order to encourage control of BLV infection by using milk samples, in lieu of serum samples, to readily test lactating animals prior to dry-off and calving. Two Holstein dairy herds (A and B), with known status of BLV infection as determined by serology, were sampled by the collection of serum and fresh milk samples. Selected samples were tested using a USDA-licensed BLV antibody ELISA kit (Bovine leukemia virus antibody test kit; VMRD, Pullman, WA) for serum. Forty-one lactating cows from each herd were sampled. Herd A was confirmed to have endemic BLV infection; herd B was confirmed to be free of BLV infection. The milk ELISA results demonstrated 100% identification of positive and negative animals compared with the serum results. The correlation of the ELISA values between serum and milk samples was 97%, which supports the use of this BLV ELISA on milk samples.

Short communication: Evaluation of 5 different ELISA for the detection of bovine leukemia virus antibodies

Although Canadian dairy herds have been infected with bovine leukemia virus (BLV) for years, recent research has put new emphasis on the potential negative effects of this infection. Consequently, BLV control is becoming more favorable; however, BLV control cannot be successful without identifying infected animals. Bovicheck BLV (Biovet, Saint-Hyacinthe, QC, Canada) is currently the only assay licensed by the Canadian Centre for Veterinary Biologics. The first goal of this study was, therefore, to determine the reproducibility of the Bovicheck BLV assay for serum samples derived from Canadian cattle. The second goal was to evaluate and compare 5 different ELISA and determine their test characteristics using serum samples from Canadian herds. The considered ELISA were Bovicheck BLV, ID Screen BLV Competition (IDvet, Grabels, France), Idexx Leukosis Serum X2 Ab Test (Idexx Europe B.V., Hoofddorp, the Netherlands), Svanovir BLV gp51-Ab (Svanova, Uppsala, Sweden), and the Serelisa BLV Ab Mono Indirect (Synbiotics, Lyon, France). Eighty serum samples from Canadian cattle provided by Prairie Diagnostic Services (PDS; Saskatoon, SK, Canada) and an additional 80 serum samples from Canadian dairy and beef herds were used for the study. The Bovicheck BLV assay yielded the same results for all PDS-derived samples, implying a high level of reproducibility and robustness of this assay. Additionally, the comparison of the assays' results showed high agreement between assays, with Cohen's kappa values between κ = 0.91 and κ = 1. Furthermore, using original test results of the field samples as true status, relative diagnostic sensitivity and specificity were calculated. Relative diagnostic sensitivity of all tests was 100%. False-positive results were probable; therefore, the following relative diagnostic specificities were determined: 100% for Bovicheck BLV, Idexx Leukosis Serum X2, and Svanovir BLV; 95% for ID Screen BLV; and 97% for Serelisa BLV. When considering other test characteristics, ID Screen BLV is exceptional due to considerable practical advantages.

Detection and genotyping of bovine leukemia virus in Mexican cattle

Bovine leukemia virus (BLV) was detected and genotyped in a population of 201 dairy cattle from central Mexico. Using a commercial indirect enzyme-linked immunosorbent assay (iELISA) kit, 118 polymerase chain reaction (PCR)-positive and BLV antibody-positive samples were identified; the concordance between tests was substantial. A phylogenetic study of 27 partial sequences of the env gene gp30 was performed. Four mutations were detected involving the PXXP motif in the cytoplasmic domain of the transmembrane protein. This study provided evidence of the efficacy of PCR for the detection of BLV and demonstrated the presence of genotype 1 BLV in Mexico.

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

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

Serological and molecular detection of bovine leukemia virus in cattle in Iraq

Bovine leukemia virus (BLV) is highly endemic in many countries, including Iraq, and it impacts the beef and dairy industries. The current study sought to determine the percentage of BLV infection and persistent lymphocytosis (PL) in cattle in central Iraq. Hematological, serological, and molecular observations in cross breeds and local breeds of Iraqi cattle naturally infected with BLV were conducted in the peripheral blood mononuclear cells of 400 cattle (340 cross breed and 60 local breed) using enzyme-linked immunosorbent assay and polymerase chain reaction (PCR). On the basis of the absolute number of lymphocytes, five of the 31 positive PCR cases had PL. Among these leukemic cattle, one case exhibited overt neutrophilia. Serum samples were used to detect BLV antibodies, which were observed in 28 (7%) samples. PCR detected BLV provirus in 31 samples (7.75%). All 28 of the seropositive samples and the 3 seronegative samples were positive using PCR. Associations were observed between bovine leukosis and cattle breed, age and sex. Age-specific analysis showed that the BLV percentage increased with age in both breeds. Female cattle (29 animals; 7.34%) exhibited significantly higher infectivity than male cattle (two animals; 4.34%). In conclusion, comprehensive screening for all affected animals is needed in Iraq; programs that segregate cattle can be an effective and important method to control and/or eliminate the BLV.

A randomized controlled trial on preweaning morbidity, growth and mortality in Holstein heifers fed a lacteal-derived colostrum replacer or pooled maternal colostrum

BACKGROUND

The objective of this randomized controlled trial was to determine the effect of feeding a commercial lacteal-derived colostrum replacer (CR) or pooled maternal colostrum (MC) on preweaning morbidity, growth and mortality in Holstein heifer calves. A total of 568 calves were randomly assigned to be fed either 3.8 L of pooled MC or two doses (200 g IgG) of a CR. Calves were monitored daily for preweaning morbidity until weaning at 60 d old. Birth and weaning weights were measured to estimate growth rates.

RESULTS

Calves fed CR were significantly less likely to be affected with a diarrhea event (OR = 0.58; 95% CI, 0.38 to 0.88; P value = 0.011) and had a higher rate of daily weight gain (0.051 kg/day; 95% CI, 0.03 to 0.08; P value <0.001) compared to calves fed pooled MC. Use of lacteal-derived colostrum replacer was not significantly associated with respiratory disease (OR = 1.01; 95% CI 0.67 to 1.51; P value = 0.974 ), omphalitis (OR = 0.93; 95% CI 0.06 to 14.86; P value = 0.956), or mortality (HR = 0.71; 95% CI 0.27 to 1.92; P value = 0.505) in the study calves.

CONCLUSIONS

The lacteal-derived CR fed at the study dose was a viable colostrum alternative in the event of poor quality pooled MC for the prevention of preweaning diarrhea and resulted in higher growth rates in comparison to calves fed pooled MC in the study herd.

Development of an improved real time PCR for the detection of bovine leukaemia provirus nucleic acid and its use in the clarification of inconclusive serological test results

With the aim to erradicate Enzootic Bovine Leukosis from Poland, a more sensitive real-time polymerase chain reaction was required and developed to detect proviral Bovine leukaemia virus (BLV) DNA, the causative agent of Enzootic Bovine Leukosis (EBL). This new method proved more sensitive for our needs, than the current protocols available in the public domain. DNA was extracted from peripheral blood leukocytes of 51 cattle, which had given rise to doubtful serological test results by ELISA, and from mesenteric lymph nodes of six cattle that were slaughtered as EBL suspect cases. Additionally, fourteen DNA samples were obtained from animals with a strong BLV antibody response by ELISA. All real-time data were compared to results obtained from three different nested PCR methods. All 14 strongly positive ELISA samples were positive in all PCR tests. The real-time assay in comparison to the conventional PCR methods detected 7.8% (4/51) more specimens positive for BLV nucleic acid and showed a detection limit down to one copy. These observations represent the first report in the value of using a real-time method to help elucidate the disease status of animals when inconclusive ELISA results are obtained in the diagnostic laboratory. Thus, this method should be recommended for use in countries which have implemented an EBL-eradication programme, where a low level of BLV infection is evident.

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 Infection in Dairy Cattle: Effect on Serological Response to Immunization against J5 Escherichia coli Bacterin

Thirteen bovine leukemia virus- (BLV-) negative and 22 BLV-positive Holstein cows were immunized with J5 Escherichia coli bacterin at dry off, three weeks before calving, during the second week after calving, and three weeks after the third immunization. Serum was collected before the initial immunization, immediately before the third and fourth immunizations, and 21 days after the fourth immunization. Anti-J5 E. coli IgM, IgG1, and IgG2 titers were determined by ELISA. Anti-J5 E. coli IgM titers did not differ significantly (P = .98) between groups. Increases in anti-J5 E. coli IgG1 titers were higher in the BLV-negative cows (P = .057). Geometric mean anti-J5 E. coli IgG2 titers increased fourfold in the BLV-negative cows, which was significantly higher (P = .007) than the twofold increase in the BLV-positive cows. Cattle infected with BLV may have impaired serologic responses following immunization with J5 bacterin, and response may differ according to antibody isotype.

Coinfection of a Cow with Bovine Leukemia Virus and Mycobacterium Bovis

Bovine leukosis associated with infection with the delta retrovirus Bovine leukemia virus (BLV) is endemic in many cattle herds in the United States. Infection has been associated with immunosuppression and decreased productivity. Cases of tuberculosis in cows due to infection with Mycobacterium bovis reemerged in Michigan in 1998, and despite intensive eradication attempts, new cases of bovine tuberculosis are sporadically identified. The present report details a coinfection with BLV and M. bovis in a Holstein cow from Michigan that presented as part of a bovine tuberculosis screening program. Peripheral and visceral lymph nodes of this animal were markedly enlarged, homogeneously pale white, and bulged on the cut surface. The submandibular, mesenteric, and caudal mediastinal lymph nodes contained multifocal to coalescing caseogranulomas that ranged from 1 to 5 cm in diameter. Histologically, dense sheets of monomorphic populations of neoplastic lymphocytes obliterated the normal architecture of all lymph nodes. Caseogranulomas were characterized by central pools of amorphous degenerate eosinophilic and occasionally mineralized granular debris surrounded by thick rims of epithelioid macrophages, occasional Langhan's type giant cells, and fibrosis. Polymerase chain reaction assay was positive for BLV. Cultures of affected lymph nodes yielded growth of M. bovis.