Evaluation of a bulk-milk ELISA test for the classification of herd-level bovine leukemia virus status

The results of a commercial bulk-milk enzyme-linked immunosorbent assay (ELISA) test for herd-level bovine leukemia virus (BLV) status were compared to results obtained from individual agar-gel immunodiffussion (AGID) testing on sampled cattle. A positive herd was defined as a herd having one or more AGID-positive animals. The estimated true herd status was based on the sensitivity and specificity of the AGID test and the number of cattle sampled per herd. Ninety-seven herds were used, with a mean of 13 cows sampled per herd. The AGID test indicated an apparent herd prevalence of 70.1%. After accounting for the number of cows sampled and the sensitivity and specificity of the AGID test, the estimated true herd prevalence of BLV was 52.3%. The ELISA test identified 79.4% of herds as positive for BLV, and had an apparent sensitivity and specificity of 0.97 and 0.62, respectively. However, after accounting for the sensitivity and specificity of the AGID test in individual animals, the specificity of the ELISA test was 0.44. The ELISA test was useful for identifying BLV-negative herds (i.e., ruling out the presence of BLV infection in test negative herds). With the moderately low specificity, herds identified as positive by the ELISA test would require further testing at the individual or herd level to definitively establish their BLV status.

Detection of bovine leukemia viruses (BLV) in mammary tissues of BLV antibody-positive cows affected by subclinical mastitis

The mammary tissues of 6 cows with bovine leukemia virus (BLV) antibody and subclinical mastitis were investigated histopathologically, and their organ cultures were ultrastructurally observed. Numerous BLV particles, 110 to 120 nm in diameter, were seen around lymphocytes, which had infiltrated into mammary alveoli and showed blastogenesis under culture. Particles budding from the cell membrane were also found.

[The bovine leukosis virus and the diagnosis of infected animals]

The results of selection of primers and amplification of fragments of provirus genes Gag and Env of bovine leukemia virus (BLV) in artificially infected cell line and in cattle leukocytes are presented. The distinctions between diagnoses of BLV infected animals by the testing their blood for antibodies to viral antigens and by the presence of provirus genes in DNA of their leukocytes were revealed.

Experiences with the New York State Bovine Leukosis Virus Eradication and Certification Program

The Diagnostic Laboratory at the Veterinary College at Cornell University has offered a bovine leukosis virus (BLV) eradication/ certification program since 1985. The program has been popular with purebred breeders since its inception. Recently, many commercial dairymen have also begun participating to reduce the high incidence of clinical leukosis observed in their heavily infected herds. Eradication is achieved through a management and testing program designed to meet the needs of each farm. Over the years, experiences and observations have elucidated a number of factors that significantly affect the rate of progress and time required for a herd to become "BLV-Free."

Serological evidence of the occurrence of enzootic bovine leukosis (EBL) virus infection in cattle in Tanzania

Antibodies specific to bovine leukosis (leukemia) virus (BLV) were examined by enzyme-linked immunosorbent, assay (ELISA) in 2,849 serum sample from exotic/improved and indigenous cattle originating from 6 regions of Tanzania, 2,047 from dairy cattle and 802 from beef cattle. An overall infection rate (prevalence) of approximately 36% was detected. The infection rates were 41% and 21.4% in dairy and beef herds respectively. In the dairy herds the infection rate varied from herd to herd. The possible sources of infection and methods of spread within Tanzania are discussed.

[Examination of antigen preparations from the virus of enzootic bovine leukosis with regard to suitability for immunoblotting]

8 different leukosis antigen preparations for immunodiffusion from 6 European countries were examined regarding their suitability for immunoblotting. Emphasis was made on demonstration of immunoreactions at 51 kDa beside reactions at 24 kDa. The whole protein content of the different preparations varied very much. gp 51 could be detected only by monoclonal antibodies at a molecular weight of 53 kDa, in one preparation at 26 kDa. With sera from field or with the European reference serum E4 none or only weak reactions could be observed at these molecular weights. All antigen preparations contained bovine IgG that is divided in L- and H-chains by SDS-electrophoresis under reducing conditions. These single chains reacted also with anti-bovine-IgG-biotin conjugate und disturbed the detection of the leukosis specific reactions. The bovine IgG could be removed by a Protein-G-column. One of all tested leukosis antigens, which are prepared for immunodiffusion, was suitable for immunoblotting. This antigen preparation contained only few bovine IgG and had strong reactions at 24 kDa. A special antigen preparation for detection of other reliable immunoreactions in the immunoblot has to be developed.

Evaluation of polymerase chain reaction (PCR) application in diagnosis of bovine leukaemia virus (BLV) infection in naturally infected cattle

The practical application of polymerase chain reaction (PCR) for the diagnosis of bovine leukaemia virus (BLV) infections in naturally infected cattle was evaluated. Compared to serological tests the PCR was definitely found to be a more sensitive method, yielding the highest number of positive results (10% more compared to enzyme-linked immunosorbent assay, (ELISA), and 17.7% more compared to agar-gel immunodiffusion, (AGID)). In testing cattle from herds with BLV incidence under 5%, out of 52 provirus positive cattle only 43 were correctly identified by ELISA. When compared to AGID only 37 of the 52 PCR positive animals were correctly identified. Of 18 cattle imported from the Slovak Republic and kept in a quarantine stable, four were found to be BLV provirus positive by PCR, while serological tests indicated one animal positive and three negative. Therefore, it is impossible to prevent the spread of the infection from one country to another by serological testing only. Moreover, it is feasible to identify animals with changing antibody titres correctly by PCR. Using PCR we were also able to distinguish BLV infected from uninfected calves that were serologically positive due to colostral antibodies. Higher sensitivity of BLV provirus detection by PCR was achieved using env gene rather than tax gene specific primers. Negative results by PCR in cases of positive serological reactions are still possible, as shown in case of one adult animal. These findings indicate that PCR is a highly sensitive method and might be successfully used and economically advantageous for different practical applications in detection of BLV infection in naturally infected cattle.

Direct use of cell lysates in PCR-based diagnosis of bovine leukemia virus infection

Polymerase chain reaction (PCR) has been used for direct detection of bovine leukemia virus (BLV) proviral DNA in cattle, but it is still mainly used for experimental research. One bottleneck for routine diagnosis of BLV by PCR has always been the isolation and purification of DNA. We compare the use of not purificated with highly-purified DNA in the PCR-based diagnosis of BLV infection. DNA extracted from whole blood by chloroform extraction (CP-DNA) and DNA prepared only by osmotic shock, washing, heating and freezing procedures (RPoS-DNA), were utilized. Fifteen cattle well characterized serologically were investigated for BLV-provirus with PCR using this different DNA preparations. With both methods all but one investigated animal were correctly identified. It was estimated that in case of CP-DNA PCR 10 BLV-provirus copies were sufficient to obtain a positive result. The sensitivity of RPoS-DNA PCR was similar. Because of the greater practicability of the latter technique we used it in a small field study with ten cattle. All serologically positive animals were correctly identified by the PCR. In addition one seronegative animal was found to carry BLV-provirus. Therefore RPoS-DNA PCR might be a good tool for the routine diagnosis of BLV-infected cattle.

Serological survey to determine the prevalence of bovine leukaemia virus antibodies in dairy cattle on selected farms in the Gauteng and Mpumalanga provinces

Cattle from a farm where enzootic bovine leukosis had been diagnosed were tested to determine the prevalence of bovine leukaemia virus antibodies. Farmers who had bought cattle from this farm were identified and their herds also tested. Of 381 adult dairy cattle tested, 14 animals reacted positively (3.67%). Cattle (n = 81) from 3 selected herds, not associated with the affected farm were also bled and 7 animals reacted positively (8.64%).

[Preparation of monoclonal antibodies to gp51 antigen and their use for early diagnosis of bovine leukemia]

A strain BLV-gp51-V7 of hybrid cells has been obtained that is characterised by high specificity as to antigen (glycoprotein gp51). Ascitic tumour appear in syngenic mice inoculated with hybrid cells of strain BLV-gp51-V7. Monoclonal antibodies were isolated from the ascitic fluid of mice. These antibodies were used with the purpose of early diagnosis of cattle leucosis.

Bovine leukaemia-virus infection in Costa Rica

The geographical distribution of bovine leukaemia-virus (BLV)-infected herds in Costa Rica and the isolation of BLV from cases of enzootic bovine leukosis are presented. It was found that BLV is prevalent throughout the country, affecting mostly dairy cattle (Bos taurus) located in the highlands around the central valley of Costa Rica. By contrast, beef cattle (Bos indicus) raised in the lowlands were affected to a lesser extent. One out of four isolates of BLV obtained from local cattle with lymphosarcomatous tumours was analysed and had similar physical, serological and biological characteristics to a reference strain of BLV isolated in the USA. However, different patterns of recognition of BLV-protein p24 were observed among naturally infected cattle using the western-blotting technique.

Serological and haematological diagnosis of enzootic bovine leukosis in cattle in Turkey

A serological study of enzootic bovine leukosis in the Bursa Region of Turkey showed that of 459 cattle (282 Holstein, 127 Brown-Swiss and 50 native Boz breed) 42 (9.15 per cent) were seropositive. The seropositive cattle had higher IgG1 (P < 0.001) and lower IgM (P < 0.01) levels than the seronegative cattle. In addition, the seropositive cattle with persistent lymphocytosis had higher IgG1 levels (P < 0.001), total leucocyte counts (P < 0.001) and lymphocyte counts (P < 0.001), than the cattle in the seropositive group without persistent lymphocytosis and the cattle in the seronegative group. There was a positive correlation (P < 0.05) between the IgG1 levels and the lymphocyte counts of the seropositive cattle with persistent lymphocytosis.

Epidemiological evaluation of a monoclonal ELISA detecting antibodies against bovine leukaemia virus in serum pools

Sensitivity, specificity and predictive values of an enzyme-linked immunosorbent assay (ELISA) for detecting antibodies against bovine leukaemia virus (BLV) were evaluated using a representative sample of 145 serum pools, comprising from 3 to 48 individual sera. The sample was constituted according to the frequency distribution of the negative and positive pools analysed during a screening involving the whole cattle population of Belgium. Sensitivity and specificity were estimated to 88.9% and 100% and the predicted negative and positive values were 99.9% and 100%, respectively. These results indicate the use of serum pools is suitable for the detection of BLV infected herds in eradication campaigns.

[Detection of antibodies against the virus of enzootic bovine leukosis in serum and milk samples using an immunoblot]

A method for detection of antibodies against the enzootic bovine leukemia virus in single serum and milk samples, as well as in pooled milk samples, is described. After the electrophoretical separation of the antigen proteins with SDS-PAGE, a transfer onto an immobilizing membrane is done, followed by immunodetection based on the reaction of the leukemia virus antibodies with antigen proteins. The antibody-antigen complex is visualized by a second, biotinylated antibody and peroxidase-avidin. This system is developed for verification of ELISA-results in surveillance of official admitted herds free of leukosis. The threshold for positive samples is tested by the European reference serum E-4, Copenhagen, the dilution 1:25,000 is recognized as positive. The detected antigen-antibody reactions are determined by scanning and comparison with the MW-markers.

Evaluation of an enzyme-linked immunosorbent assay for detection of antibodies to bovine leukemia virus in serum and milk

The results of examination of 375 bovine serum and 150 bovine milk samples for detection of bovine leukemia virus infection by the immunodiffusion technique were compared with those of an enzyme-linked immunosorbent assay (ELISA). It was concluded that the ELISA is another useful method for the detection of antibodies to bovine leukemia virus in serum and milk. The ELISA provides a quantitative result and has the advantage of being more sensitive and less time-consuming than the conventional immunodiffusion technique.

Use of polymerase chain reaction to diagnose bovine leukemia virus infection in calves at birth

A specific polymerase chain reaction (PCR) assay was devised, allowing detection of 1 bovine leukemia virus (BLV)-infected cell in 10(4) bovine lymphocytes. The efficacy of field application of the developed method was verified by evaluating the rate of viral transmission to calves from infected cows, whether they have persistent lymphocytosis. With this objective, 43 calves were simultaneously tested at birth and at 6 months of age for viral antibodies in serum and for proviral DNA in lymphocytes. At birth, 36 calves were BLV-negative and 3 were BLV-positive by results of serologic and DNA-based assays. Conversely, results for 4 calves had lack of correlation between the diagnostic methods. In particular, 2 calves were DNA-positive and antibody-negative for BLV and 2 other calves had the opposite test results. At 6 months of age, when the immunologic pattern more closely reflects the status of calves' immune response, independent of maternal antibodies, all calves DNA-negative for BLV at birth (n = 38), were consistently PCR- and antibody-negative for BLV. On the contrary, the cattle DNA-positive for BLV at birth (n = 5), whether seropositive or not, were PCR- and antibody-positive for BLV, at the time of the second screening. Thus, these results indicate reliability of the PCR to diagnose perinatal BLV infection. Furthermore, the observation that all calves found to be infected at birth were born to BLV-positive cows with persistent lymphocytosis, indicates that the persistent lymphocytosis status of the cow may represent a factor associated with BLV infection in utero.

Early detection of bovine leukemia virus in cattle by use of the polymerase chain reaction

A study was performed to determine whether experimentally induced bovine leukemia virus (BLV) infection in cattle could be detected earlier by use of polymerase chain reaction (PCR) amplification of genomic DNA extracted from leukocytes than by use of conventional agar-gel immunodiffusion (AGID). The PCR primers were designed to amplify a 375-base-pair region of the proviral gag gene. Five cows were identified that were BLV-negative on the basis of AGID and PCR results. At day 0, these cows were inoculated IM with blood pooled from 3 naturally infected cows. Blood samples were taken on days 0, 1, and 7, and every 2 weeks thereafter until 3 months after inoculation. Three of the cows were BLV-positive by AGID test results 3 weeks after inoculation, and the remaining 2 seroconverted at 5 weeks. In contrast, all 5 cows were BLV-positive by PCR results 7 days after inoculation and remained positive for the duration of the study. Five cows that were BLV-positive by AGID test and PCR results on day 0 and from which samples were obtained at the same times as those from the other 5 cows, remained BLV-positive by results of both tests during the course of the study. Results indicate that under experimental conditions, BLV infection in cattle can be detected as much as 2 to 4 weeks earlier by use of PCR than by use of the AGID test.

Application of the polymerase chain reaction (PCR) in veterinary diagnostic virology

The polymerase chain reaction has become an important diagnostic tool for the veterinary virologist. Conventional methods for detecting viral diseases can be laborious or ineffective. In many cases PCR can provide a rapid and accurate test. In this article we explain the basic principles of PCR and supply a reference list of its uses in diagnostic veterinary virology.

Tumor-associated M(r) 34,000 and M(r) 32,000 membrane glycoproteins that are serine phosphorylated specifically in bovine leukemia virus-induced lymphosarcoma cells

Tumor-associated antigens that are expressed in tumor cells of cattle with enzootic bovine leukosis (EBL) were analyzed previously by use of 13 monoclonal antibodies. We biochemically identified one of the tumor-associated antigens, which is recognized by the c143 monoclonal antibody, as two glycoproteins, each having an apparent molecular weight of 32,000 or 34,000. These glycoproteins were found in the plasma membrane of peripheral blood lymphocytes of both bovine leukemia virus (BLV)-free normal and BLV-infected cattle. With the progression of EBL, the proportion and the absolute number of cells positive for the tumor-associated antigen increased. Moreover, the level of the M(r) 34,000 component, which was susceptible to cell-surface labeling, increased over the level of the M(r) 32,000 component. Partial proteolytic peptide mapping with V8 protease and deglycosylation analysis revealed that the two glycoproteins most likely have an identical M(r) 30,000 polypeptide portion but have different N-linked oligosaccharide portions. Both glycoproteins were found to be phosphorylated at serine residue(s) in EBL-derived B-lymphoid cell lines and in tumor cells and peripheral blood lymphocytes from cattle with EBL, but not in peripheral blood lymphocytes from BLV-free normal cattle and BLV-infected cattle without any evidence of tumor, suggesting that the phosphorylation of these glycoproteins is related to the transformed state of the BLV-infected B-lymphoid cells.

Estimation of the sensitivity and specificity of the agar gel immunodiffusion test for bovine leukemia virus: 1,296 cases (1982-1989)

A retrospective study of the results of 12,549 agar gel immunodiffusion tests for bovine leukemia virus, conducted on 1,296 dairy bulls over an 8-year period, was performed to estimate the sensitivity and specificity of the test. The number of tests performed on each bull ranged from 5 to 35, with a mean of 9.7 tests per bull. Bulls were categorized by their agar gel immunodiffusion test responses; 1,069 (82.5%) were noninfected and 227 (17.5%) were infected. Eighteen false-positive results were reported from the noninfected bulls. Test specificity was estimated to be 99.8%. Thirty-one false-negative results were reported from the infected bulls. Test sensitivity was estimated to be 98.5%. Fifty-six bulls had 1 or more positive responses when less than 6 months old. In 26 (46%), these results were thought to be attributable to colostral immunity.

Early detection of bovine leukemia virus by using an enzyme-linked assay for polymerase chain reaction-amplified proviral DNA in experimentally infected cattle

Bovine leukemia virus is the causative agent of bovine leukosis and has been described in many countries throughout the world. We describe here a sensitive and readily applicable assay for the detection of bovine leukemia proviral DNA. Detection relies on initial amplification of proviral DNA by using polymerase chain reaction (PCR) followed by an enzyme-linked assay (PCR-ELA). Amplification is carried out by using one biotinylated primer and a second primer containing the GCN4 protein binding site. DNA is detected by a colorimetric assay after it is coupled to GCN4-coated plates and subsequently incubated with horseradish-streptavidin peroxidase and the appropriate substrate to produce a chromogenic reaction. It was possible to detect proviral DNA for all of eight bovine leukemia virus-infected calves by 2 weeks postinfection. Use of the more conventional agar gel immunodiffusion assay failed to reveal the presence of the virus in any of the animals up to 4 weeks postinfection. The PCR-ELA detected as little as 0.1 to 0.2 ng of amplified DNA per well, which compares very favorably with ethidium bromide staining of gels, by which 1 to 2 ng per lane was detected. This method lends itself to mass screening, is carried out in a similar way to an enzyme-linked immunosorbent assay, and does not require gel electrophoresis or the use of radioactive gene probes.

Thymic and mammary lymphosarcoma in a three-year-old heifer

Ventral edema, dyspnea, fever, tachycardia, bloat and muffled heart sounds were identified in a 3-year-old heifer. Attempts to relieve the bloat by passing an orogastric tube were unsuccessful. The heifer was bovine leukemia virus-negative by the agar gel immunodiffusion test, and had normocytic, normochromic anemia, mature neutrophilia, and hyperproteinemia. Pleural effusion was identified by thoracic ultrasonography. Cytologic examination of pleural fluid revealed an increased number of atypical lymphocytes. The heifer died, and at necropsy, thymic and metastatic mammary lymphosarcoma was confirmed.

Direct detection of bovine leukemia virus infection: practical applicability of a double polymerase chain reaction

A double polymerase chain reaction (PCR) assay has been devised for the direct detection of bovine leukemia virus (BLV). The assay was directly performed on blood leukocytes, avoiding the DNA-purification procedures. The PCR products were identified by gel-electrophoresis and the specificity of the test was confirmed by hybridization with a biotinylated oligonucleotide probe. When testing the sensitivity of PCR, less than eight genome copies of the provirus were detected in the background of two million negative lymphocytes. In a BLV infected herd 22 animals of various age groups were examined by the indirect (serological) diagnostic tests of agar-gel immunodiffusion and indirect ELISA as well as by the direct detection method of PCR. The tests were repeated at monthly intervals on five occasions. When examining the specimens from cows and heifers, a close agreement was found between the results of the various methods. The newborn calves, which were the offspring of BLV infected mothers, were consequently negative in PCR throughout the experimental period. However, in the indirect tests the calves were positive during the first samplings and became negative only around four months of age. Since the indirect tests can not discriminate infection from colostral immunity, PCR proved to be a useful complementary assay for the safe diagnosis of BLV infection in young calves.

Infectivity of bovine leukaemia virus infected cattle: an ELISA for detecting antigens expressed in in vitro cultured lymphocytes

A simple ELISA is described for quantifying expression of bovine leukaemia virus (BLV) antigens in short-term cultures of peripheral blood lymphocytes (PBL) isolated from infected cattle. The PBL-ELISA demonstrated that antigen expression levels in infected cattle could vary by more than 50-fold. Inoculation of sheep with dilutions of lymphocytes from two BLV-infected cattle, differentiated in the PBL-ELISA by 50 to 100-fold, suggested that antigen expression levels were correlated with infectivity. Haematological data indicated that increased antigen expression in PBL cultures was associated with an increased number of circulating B-lymphocytes, irrespective of whether or not an animal had lymphocytosis. This supported the hypothesis that BLV-infected cattle that are PBL-ELISA positive are more infectious and may present a greater risk of transmitting the disease. The applicability of the PBL-ELISA to a field situation was assessed with 98 BLV-infected cattle from three commercial dairy herds with infection prevalences of 11%, 23% and 47%. Similar percentages (49%, 50% and 52%) of PBL-ELISA positive cattle were identified among those infected cattle available for testing in the three herds. An additional 22 infected cattle from an experimental herd were tested to assess the stability of antigen expression levels over an 8 month period. Fewer (27%) of these cattle were identified as PBL-ELISA positive and antigen expression levels were generally lower than those observed in the commercial herds. Antigen expression levels in the experimental herd remained stable over the period of the study. The potential of the PBL-ELISA to assist in BLV eradication programs by identifying those seropositive cattle with the greatest potential to transmit infection is discussed.

Bovine leukemia virus infection in Taiwan: evaluation of the enzyme-linked immunosorbent assay and agar gel immunodiffusion test

I evaluated an enzyme-linked immunosorbent assay (ELISA) and agar gel immunodiffusion (AGID) test simultaneously for the detection of bovine leukemia virus (BLV) antibodies. Total 1,293 serum samples were tested for ELISA and AGID test and the results were compared. The results of ELISA and AGID agreed by 1,156 out of 1,293 (89.4%). All of AGID-positive 356 sera were positive by ELISA. However, of 451 ELISA-positive sera, 95 sera were either negative or equivocal by AGID test. Eleven animals which showed ELISA-positive but AGID-negative or equivocal became AGID-positive in a year. It may be inferred that ELISA detects infected cattle earlier and with greater sensitivity than AGID.

Diagnosis of enzootic bovine leucosis in single and pooled samples

Diagnosis of enzootic bovine leucosis is based on detection of antibodies against bovine leukemia virus, BLV. Some ELISA modifications have proved sensitive enough for use in the examination of pooled blood samples from slaughterhouses, milk and pooled milk samples. Suggestions for the standardisation of different ELISA modifications using a common reference serum are presented.

Animal diseases caused by retroviruses: enzootic bovine leukosis, equine infectious anaemia and caprine arthritis-encephalitis

This article presents the essential features of three retroviral infections of animals: enzootic bovine leukosis, equine infectious anaemia and caprine arthritis-encephalitis. A unique feature of these diseases is persistent infection, maintained throughout the life of the host animal by the presence of a provirus integrated into the cells of the host, thereby making an infected animal a continuous source of the pathogen. Information currently available on the epidemiology and detection of these diseases is sufficient to institute effective disease control measures.