Early detection of bovine leukosis virus DNA in infected sheep using the polymerase chain reaction

Absence of Bovine Leukosis Virus in Semen of Seropositive Bulls

A polymerase chain reaction (PCR)-based detection system was established to identify the presence of bovine leukosis virus (BLV) DNA in bovine semen. Seventy-nine bulls were included in the study. Serum, peripheral blood leukocytes, and semen were collected from each of the 79 bulls. The BLV-specific antibody was detected in serum by agar gel immunodiffusion and viral DNA in blood and semen by PCR. Serologically, 29 of the 79 bulls were BLV positive. Twenty-seven of the 29 seropositive bulls and 1 of the seronegative bulls had BLV DNA in peripheral blood leukocytes. All 79 bulls tested PCR negative for the presence of BLV in semen. This data is strong evidence that properly collected semen from BLV seropositive bulls will not contribute to dissemination of this viral 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.

Development and application of modern agricultural biotechnology in Botswana: the potentials, opportunities and challenges

In Botswana, approximately 40% of the population live in rural areas and derive most of their livelihood from agriculture by keeping livestock and practising arable farming. Due to the nature of their farming practises livestock and crops are exposed to diseases and environmental stresses. These challenges offer opportunities for application of biotechnology to develop adaptable materials to the country's environment. On the other hand, the perceived risk of genetically modified organisms (GMOs) has dimmed the promise of the technology for its application in agriculture. This calls for a holistic approach to the application of biotechnology to address issues of biosafety of GMOs. We have therefore assessed the potentials, challenges and opportunities to apply biotechnology with specific emphasis on agriculture, taking cognisance of requirement for its research, development and application in research and teaching institutions. In order to achieve this, resource availability, infrastructure, human and laboratory requirements were analyzed. The analysis revealed that the country has the capacity to carry out research in biotechnology in the development and production of genetically modified crops for food and fodder crops. These will include gene discovery, genetic transformation and development of systems to comply with the world regulatory framework on biosafety. In view of the challenges facing the country in agriculture, first generation biotech crops could be released for production. Novel GM products for development may include disease diagnosis kits, animal disease vaccines, and nutrient use efficiency, drought, and pest and disease resistant food and fodder crops.

Molecular characterization of the env gene from Brazilian field isolates of Bovine Leukemia Virus

Molecular characterization of Bovine leukemia virus (BLV) isolates from Brazil using the env gene sequences revealed a high conservation of this gene. In most cases the substitutions corresponded to silent transitions. In addition, cystein residues, potential glycosylation sites, neutralization domains and other critical residues involved with the envelope structural domains and viral infectivity were conserved. Most of the substitutions found in the aminoacid sequences of the gp51 protein were localized in the G and H epitopes. Using the SIFT software, it was predicted that they should not alter the protein functions. Phylogenetic analyses showed that partial or complete env gene sequences grouped in three or four phylogenetic clusters, respectively. The sequences from the Brazilian isolates had similar mutation rates as compared to samples from other countries, and belonged to at least two phylogenetic clusters.

Development of a PCR to diagnose BLV genome in frozen semen samples

The sanitary and economic impact of BLV infection is associated with the interference in the international movement of cattle and their germ plasm. Although experimental data support the improbability that semen from BLV-positive bulls could infect recipient cows, restriction for commercialization of semen from infected animals is still present. The objective of this work was to standardize a PCR assay to diagnose the presence of BLV genome in frozen semen samples. The developed methodology involves the amplification of an internal fragment of gag gene. The limit of detection of this technique was six viral particles, using gag-PCR followed by hybridization analysis. Frozen semen samples from seropositive bulls were analyzed. It was possible to detect proviral DNA in 9 out of 173 samples. Additionally, a biological test in susceptible sheep was performed in order to evaluate the transmission of BLV genome by semen from seropositive animals. This data strongly suggest that semen from seropositive bulls that resulted negative by PCR can be used for artificial insemination (AI), accompanied by proper collection protocols. The development of this PCR assay constitutes a valuable diagnostic tool to determine the BLV-free status of frozen semen samples used for AI.

Establishment of a new bovine leukosis virus producing cell line

Due to the prevalence of different bovine leukosis virus (BLV) species in the cattle population in Europe, problems may arise in the serological diagnosis of BLV infections. In addition, earlier investigations demonstrated that contamination of the BLV antigen-producing cell culture systems by bovine viral diarrhea virus (BVDV) may give rise to misinterpretation of serological test results after BVDV vaccination of cattle. By co-cultivation of peripheral leukocytes of a BLV-infected cow with a permanent sheep kidney cell line, a new BLV-producing cell line named PO714 was established. This line carries a BLV provirus of the Belgian species and has been tested to be free of a variety of possibly contaminating viruses and mycoplasms. Investigations of a panel of well-characterised sera by agar gel immunodiffusion (AGID) and capture ELISA (cELISA) tests using antigen prepared from this new cell line in comparison with antigen of the well-known cell line FLK/BLV yielded comparable results. False positive results caused by BVDV cross-reactions could be eliminated when tests were carried out with antigen derived from the new cell line.

Partial sequencing of env gene of bovine leukaemia virus from Brazilian samples and phylogenetic analysis

Analysis of the partial bovine leukaemia virus (BLV) env gp51 gene sequences obtained from three BLV strains isolated in three different regions of Brazil was carried out. The Brazilian BLV env gp51 sequences were compared with seven other corresponding sequences of BLV strains isolated in different countries and with consensus sequence as well. The obtained data point on qualitative and quantitative differences among the analysed strains as far as the occurrence of single point mutations is concerned. Two Brazilian strains show significantly higher mutation rate than other analysed strains. Amino acid analysis did not show, however, any substantial changes of the primary protein structure coded by well conserved region of BLV env gp51 gene. Based on the obtained data, the putative dendogram image of possible phylogenetic relations among the studied BLV strains is presented as well.

Comparative study of PCR as a direct assay and ELISA and AGID as indirect assays for the detection of bovine leukaemia virus

The choice of a diagnostic method depends on the characteristics of the herd to be analysed. Two herds with different prevalences of enzootic bovine leukaemia were chosen to study the concordance between agar gel immunodiffusion (AGID), enzyme-linked immunosorbent assay (ELISA) and polymerase chain reaction (PCR) methods. PCR, an increasingly used virological method, was performed with four sets of primers, amplifying different genomic regions (env, pol and tax), from DNA extracted either from peripheral blood monocytes (PBMCs) or milk leucocytes. The highest percentage of positive animals was obtained using PCR performed with DNA extracted from PBMCs using primers which amplified either env or pol, followed by PCR using PBMCs and primers which hybridized with tax, then ELISA using serum and finally AGID. The results of PCR were more consistent with PBMCs than when milk leucocytes were used.

A rapid and sensitive diagnosis of bovine leukaemia virus infection using the nested shuttle polymerase chain reaction

Bovine leukaemia virus (BLV) is the causative agent of enzootic bovine leukosis (EBL). In Argentina, where a program to eradicate EBL has been introduced, sensitive and reliable diagnosis has attained high priority. Although the importance of the agar gel immunodiffusion test remains unchanged for routine work, an additional diagnostic technique is necessary to confirm cases of sera with equivocal results or of calves carrying maternal antibodies.Utilizing a nested shuttle polymerase chain reaction, the proviral DNA was detected from cows experimentally infected with as little as 5 ml of whole blood from BLV seropositive cows that were nonetheless normal in haematological terms. It proved to be a very sensitive technique, since it rapidly revealed the presence of the provirus, frequently at 2 weeks postinoculation and using a two-round procedure of nested PCR taking only 3 hours. Additionally, the primers used flanked a portion of the viral genome often employed to differentiate BLV type applying BamHI digestion. It is concluded that this method might offer a highly promising diagnostic tool for BLV infection.

Provirus variants of the bovine leukemia virus and their relation to the serological status of naturally infected cattle

Infection of cattle with the bovine leukemia virus (BLV) results in a strong permanent antibody response to the BLV antigens some weeks after infection. However, cattle may carry provirus and not have detectable antibody titers. To prove the occurrence of different BLV provirus variants in German cattle and to study the influence of special BLV variants on the immunoreaction, a 444-bp fragment of the env gene of 35 naturally BLV infected animals was analyzed. Seven different groups of BLV provirus variants were found on the basis of restriction fragment length polymorphism. Three BLV provirus variant groups and five additionally sequenced BLV isolates showed a high similarity to BLV provirus isolates from other geographical areas. The variation in nucleotide sequence of the five BLV isolates compared with nine previously sequenced BLV isolates ranged up to 5. 3%. While BLV provirus variant groups A, C, D, E, F, and G were clearly related to agar-gel immunodiffusion test (AGID)- and enzyme-linked immunosorbent assay (ELISA)-positive animals, BLV provirus variant group B was solely found in permanent AGID- and ELISA-negative or in transient ELISA-positive animals. Altogether, these results indicate that special BLV provirus variants may be responsible for atypical forms of BLV infection in cattle.

Quantitative competitive polymerase chain reaction for detection and quantification of infectious bursal disease virus cDNA and RNA

A polymerase chain reaction (PCR)-based method to measure complementary DNA (cDNA) and RNA levels of infectious bursal disease virus (IBDV) was developed. Quantification was achieved by quantitative competitive PCR (QC-PCR) amplification. A competitor, a deletion mutant of the wild type IBDV cDNA, was 10-fold serially diluted and co-amplified with IBDV cDNA after being reversely transcribed from the viral RNA. After agarose gel electrophoresis, staining, and densitometric scanning, the bands on the digitized images were analyzed and quantified by computer-assisted image analysis. Complementary DNA of standard, as well as variant strains, of serotype 1 IBDV was detected and quantified using the same QC-PCR procedures. The assay could measure IBDV cDNA levels ranging from 1 microgram to 45 fg and RNA levels ranging from 9 micrograms to 45 fg. The results indicated that QC-PCR is sensitive, easy to perform, and suitable for routine quantitation of IBDV cDNA or RNA levels.

Laboratory diagnostic tests for retroviral infections in dairy and beef cattle

Detection of bovine retroviruses stretches our diagnostic creativity to its limits. The nucleic acid-based, PCR-amplified assays are finding increased clinical use as the veterinary and livestock industry seek earlier detection of infection for eventual corrective management decisions. We are evolving from a point of disease diagnosis by tumor identification through conventional histopathology, to molecular diagnostics for early identification of retroviral nucleic acid (provirus). The clinical use of antibody-based assays lies in the simplicity of testing large numbers of animals, the relative sensitivity of the assays, and the low cost of testing. Although the pathogenicity of bovine leukemia virus (BLV) for cattle has been well documented, the disease potential for bovine immunodeficiency-like virus (BIV) for cattle is still being determined. Nevertheless, pressure to test for retroviral infections of livestock and, when feasible, removal of these infected animals from the herd will be increased.

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.

Bovine leukaemia virus: rapid detection of proviral DNA by nested PCR in blood and organs of experimentally infected calves

The early stage of bovine leukaemia virus (BLV) infection was studied in experimentally infected calves in order to assess the diagnostic applicability of a double polymerase chain reaction (PCR). In addition, the kinetics of infection and virus distribution were evaluated. To simulate the natural route of virus transmission, the calves were infected by transferring two different infectious doses of whole blood from a BLV infected cow. The establishment of infection was determined by the double PCR and syncytia formation assay and by indirect serological methods including indirect ELISA, gp51/p24 ELISA, agar gel immunodiffusion (AGID) and Western blotting. BLV antibodies were first detected in ELISA on post infection (p.i.) day 26. Close agreement was found between the results of the various indirect methods. BLV infection was first detected in peripheral blood lymphocytes (PBL) by the PCR on p.i. day 7. No animal became seropositive to BLV prior to direct detection of BLV infection by the PCR. At slaughter, urine and saliva specimens as well as various organs were collected from the calves and tested by the double PCR. Several of the organs yielded positive results: e.g. spleen, uterus, liver, kidney, abomasum, and lymph nodes. Nine out of eleven spleen suspensions were positive by the PCR, including the spleen from one calf, which otherwise remained negative in all tests throughout the experiment. This phenomenon indicates that an animal may be infected without detectable levels of BLV proviral DNA in PBLs and without circulating antibodies, further emphasizing the diagnostic importance of the PCR. The findings indicate that the PCR is the most rapid method for the early detection of BLV infection in cattle and a valuable tool for studying the tropism of the virus.

Importance of primer selection in the application of PCR technology to the diagnosis of bovine leukemia virus

The polymerase chain reaction (PCR) was used to detect bovine leukemia virus in bovine blood samples. When applied to leucocytes extracted from the blood samples, the standard method of DNA extraction gave good correlation with agar gel immunodiffusion, but a method in which 5 microliters of blood was the starting material was unreliable. Selection of the primers was important, and differences in results were observed when the PCR method was applied to blood samples from different geographic areas. The sensitivity varied from 50% to 90%, depending on the primer set applied to the gag gene of proviral nucleic acid. This variation was based on geographic origin of the cattle, suggesting an influence of viral strain. In some areas, more than 1 primer may needed to optimize results.

Recombinant viral vaccines for enzootic bovine leucosis

Recently published studies on the development and use of recombinant vaccinia virus (VV) vaccines incorporating either the complete envelope (env) gene or only a fragment of the env gene consisting of the coding sequence for the env glycoprotein 51 (gp51) and part of gp30 of the bovine leukaemia virus (BLV) are described. It has been reported that vaccination of sheep with recombinant VV vaccines containing the complete env gene appears to protect sheep against challenge infection with BLV. The evidence for this protection is based on the lack of persistence of high titres of anti-gp51 antibodies compared with unvaccinated BLV infected controls, on the enhanced CD4 proliferative responses to specific BLV gp51 synthetic peptides in the vaccinated sheep, and on the inability to detect BLV pro-virus by polymerase chain reaction in the vaccinated sheep after 4 months following challenge infection compared with continual detection in unvaccinated sheep over a 16 month trial period. It has been suggested that cell-mediated immune responses may be an important aspect of protective immunity against BLV infection and it has been reported that large tracts of amino acid sequences within the env and pol genes are highly conserved in different isolates from different countries which is of importance in designing peptide derived vaccines.

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.

Detection of bovine leukemia virus RNA in serum using the polymerase chain reaction

A method was developed for detecting bovine leukemia virus (BLV) RNA in serum samples using a pair of primers from the BLV polymerase gene in the polymerase chain reaction (PCR). The PCR was able to detect 3800-7600 molecules of BLV RNA. At this level of sensitivity eleven pools of adult and one fetal bovine serum appeared free from BLV contamination.

Two immunodominant regions revealed by monoclonal antibodies on the main structural protein p24 of bovine leukemia virus

Eleven different monoclonal antibodies (Mabs) directed against the main structural protein p24 of bovine leukemia virus (BLV) were prepared. All Mabs reacted with p24 in Western blot and in radioimmunoprecipitation. Competition antibody binding assays with the prepared Mabs distinguished three independent groups of Mabs. Two immunodominant regions (IDRs) of p24 BLV were defined by these Mabs. The Mabs were induced preferentially against two immunodominant regions on the native form of p24 BLV (BLVp24 IDR-1 and BLVp24 IDR-2). Mab of the third group was directed against a different immunogenic epitope of p24 BLV. A model of the IDRs based on the differences in the fine epitope specificity of Mabs defining these immunodominant regions is proposed.

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.

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.