Evaluation of alternative methods for the detection of bovine leukaemia virus in cattle

Molecular frequency of bovine leukemia virus in Creole cattle of Eastern Colombia

Enzootic Bovine Leukosis (EBL), caused by the bovine leukosis virus (BLV), is a global infectious disease affecting livestock. This study focuses on studying the frequency and genetic traits of BLV in three Creole breeds including Chino Santandereano (Chino), Casanareño (CAS), and Sanmartinero (SM) in Eastern Colombia. We implemented a cross-sectional survey between 2019 and 2020 across four departments (Arauca, Casanare, Santander and Meta) in Eastern Colombia to assess the molecular characteristics of BLV infection in these breeds. A total of 253 cattle were analyzed, of which 42.6 %, 28.8 %, and 28.4 % belonged to the Chino, CAS, and SM breeds, respectively. BLV provirus was detected using nested polymerase chain reaction (n-PCR) targeting the conserved region of the env viral gene. Subsequently, the obtained amplicons were sequenced and subjected to phylogenetic analyses. The overall BLV infection frequency was 26.48 % (95 % CI: 21.01 - 31.98 %), with Chino exhibiting the highest frequency (35.1 %) following by SAM and CAS, respectively (P < 0.05). Other epidemiological variables associated with the infection included age, department, and season (P < 0.05). BLV-positive animals exhibited elevated levels of total serum proteins (P < 0.05), while molecular characterization revealed the exclusive circulation of BLV genotype 1 within these breeds. This study provides an updated assessment of BLV infection in Creole breeds from the eastern of Colombia, underscoring their lower infection frequency compared to introduced breeds and their reduced susceptibility to developing clinical signs. The epidemiological and molecular characteristics observed should be considered in developing control programs aimed at improving genetic resistance to BLV in Colombian cattle.

Retrospective analysis of the epizootic situation of enzootic bovine leukosis in Ukraine in 1994–2019

In Ukraine, prevention and control measures of bovine leukosis were regulated by relevant legislation, regulations and instructions developed in the period 1960–1992. Currently, the instruction on prevention and rehabilitation of this disease in cattle is used (approved in 2007). According to it, the identification of infected animals is carried out from 6 months of age by serological (first scheme in AGID or another in ELISA) and genomic (PCR) methods. The infected animals are removed from the herd or slaughtered. In general, because of the diagnostic and preventive measures carried out during the analyzed period (26 years) 10 519 farms were rehabilitated from leukosis (2 346 affected farms remained from previous years). At the same time, more 4 million infected cattle were slaughtered. The majority of affected farms (more than 1 000) were registered in the period between 1994 (2 346 farms) and 2003 (1 247 farms). Since 2014, the number of affected farms has remained mostly below 10 and the number of infected cattle has decreased to 2 000 animals per year. At the same time, the number of rehabilitated farms also decreased (from 1 307 farms in 1998 to 4 farms in 2014). The same trend was registered with the dynamics of the number of animals that were removed from the herd due to leukosis. Thus, in 1995 and 1997 their numbers were 321 178 and 558 649 animals, respectively, and in 2014 it was 1 124. The obtained indicators of intensity and extensiveness of the epizootic process show that the incidence rate was maximal during 1998–2000 and amounted to 3.7–4.3%. The maximum indicators of the coefficient of affection were recorded in 1997–2000 and equal 11.8–15.3%. The rate of foci remained on the level of 90–270 throughout the all analyzed period. During 2008–2019, specialists of the Ukrainian veterinary laboratories investigated more than 47 million samples of cattle blood sera for enzootic bovine leukosis by AGID and ELISA. However, despite the significant diagnostic work, the important factor in the decrease of the number of affected farms and infected animals is the decrease in the total number of cattle in Ukraine (almost 22 million animals in 1994 against 3 million in 2019). Graphic trends of these indicators are comparable and agree with the decrease in the number of cattle in our country by analyzed period. After 2014, the number of affected farms ranged 10–17 per year (mostly in private households). However, the full recovery of cattle in Ukraine from bovine leukosis has not taken place, although our country is closer than ever to this.

The Development of an Efficient Test Automation and Sample Tracking System in a High-Throughput Veterinary Laboratory to Improve the Emergency Response to Outbreaks of Highly Infectious Disease in Great Britain

Serological surveillance testing for bovine brucellosis, Enzootic Bovine Leukosis, and Warble fly reaches approximately 1.7 million tests every year in the UK. An automated system was developed to improve the quality of data handling for all three surveys. The objectives were to make improvements in efficiency, sample tracking, and test reporting and to develop an epidemiological database. The developed system of bar-coded samples and test plates processed on automated equipment capable of scanning barcodes and transferring information across a computer network met the data objectives. Consolidating and automating the highly repetitive liquid-handling steps met the cost objectives. To reduce the impact of temporary bottlenecks in the system, a philosophy of ‘islands of automation’ was adopted to maximize the throughput. The creation of this system was driven largely by the need to have in place an effective emergency response capability to handle the serological needs of an outbreak of an acute and highly infectious veterinary disease such as Foot and Mouth Disease (FMD) in the UK.

Moving past serology: Diagnostic options without serum

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Traditional serology has moved beyond blood as a test medium.

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A number of different samples and tissues are now frequently used in veterinary diagnosis.

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Testing can be robust and accurate and opens up the field to a variety of new opportunities.

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Molecular testing allows direct testing for the agent on a variety of tissues and samples, and pools.

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Pooling of samples can allow for more efficient and cost-effective testing.

Detecting antibodies formed in serum in response to infection is the traditional function of serology. Diagnostic modalities have included complement fixation tests, agar gel immune-diffusion, radioimmunoassay, ELISA and immunofluorescence. More recent technology now allows for the direct detection of pathogens by PCR. This review details the options for diagnostic testing using specimen types other than serum, identifying the advantages and disadvantages of these options and providing evidence for more widespread use of these techniques and specimen types.

Peroxidase-linked assay for detection of antibodies against bovine leukosis virus

A peroxidase linked assay (PLA) was designed to screen bovine sera for the presence of specific antibodies against bovine leukosis virus (BLV). Out of 201 samples of bovine sera analyzed, 52.2% were considered positive by PLA, 26.4% by AGID, and 38.9% by ELISA. Western blotting analyses excluded 27 samples found to be positive by PLA. PLA showed 100% of sensitivity when compared with AGID and ELISA. Specificity was 64.8% and 78%, respectively (kappa coefficients were 0.70 and 0.83). These findings indicate that PLA can be used as an alternative method for the diagnosis of BLV infection in cattle.

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.

Bovine leukemia virus can be classified into seven genotypes: evidence for the existence of two novel clades

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

The detection of bovine leukemia virus proviral DNA by PCR-ELISA

A sensitive non-radioactive microplate hybridization assay for the detection of proviral DNA of bovine leukemia virus (BLV)-specific polymerase chain reaction (PCR) product is described. The PCR products are labeled by adding digoxigenin-dUTP to the nested PCR reaction and are captured by a microtitre plate coated with oligonucleotide probe, which is complementary to the inner region of the amplification product. Captured products are reacted with an anti-DIG Fab fragment conjugated to peroxidase, and detected using a colorimetric reaction. The PCR-enzyme linked immunosorbent assay (ELISA), detecting as low as 10(-4) ng of proviral DNA in a background of 1 microg of BLV-negative DNA, was up to 100-fold more sensitive than ethidium bromide staining, and showed equal sensitivity to Southern blot hybridization. Using this method it was possible to monitor the presence of proviral DNA in four sheep infected experimentally with BLV, over a 10 months postinfection period, as well as in 29 cattle infected naturally. The test is rapid and highly sensitive and is a useful additional tool for the detection of BLV-infected animals.

Seroprevalence of bovine leukemia virus in dairy cattle in Argentina: comparison of sensitivity and specificity of different detection methods

Bovine leukemia virus (BLV) is a retrovirus that induces a chronic infection in cattle, which develop in three possible pathological forms: asymptomatic course, persistent lymphocytosis (PL) and lymphosarcoma. Once infected, cattle remain virus carriers for life and start to show a serological reaction within a few weeks after infection. Eradication and control of the disease is based on early diagnostic and segregation of the carriers. The agar gel immunodiffusion (AGID) test has been the serological test of choice for routine diagnosis of serum samples. Nevertheless, in more recent years, the enzyme-linked immunosorbent assay (ELISA) has replaced the AGID for large scale testing. Although Argentina has over 60 million cattle population, no nationwide studies have been conducted yet to determine the prevalence of the infection. To estimate the rate of BLV infection in dairy cattle in Argentina, a survey for specific antibodies in >10,000 serum samples from animals over 18 months old, belonging to 363 different herds from the largest dairy production areas of the country, was carried out in our laboratory, along 1999. For this purpose, we developed an ELISA to detect serum antibodies against the BLV virus. The cut-off of the ELISA was established over 339 serum samples, using polymerase chain reaction and southern blot (PCR-SB) as confirmatory test. The sensitivity and specificity of the ELISA was of 97.2 and 97.5%, respectively, while the local official AGID test showed a sensitivity of 79.7% and specificity of 99.0%. To know the seroprevalence of BLV on dairy herds, and also the incidence of the infection within the herd, the serological survey was based on individual serum samples. The results show that the prevalence of infected individuals is of 32.85%, while the percentage of infected herds, harboring one or more infected animals, is of 84%. These results indicate a medium level of seropositive animals when taken individually, but a high prevalence of infected farms, which has been notoriously increased in the last 15 years as shown when compared with previous data from particular geographic areas, indicating that BLV constitutes a serious sanitary problem for dairy producers in Argentina. They also indicate the poor sensitivity of the official AGID test used in the country.

Polymerase chain reaction amplification and gene sequence analysis of a calicivirus from a feral rabbit

A reverse transcription-polymerase chain reaction (RT-PCR) amplification assay was used to detect calicivirus gene sequences in a liver tissue derived from a feral rabbit which died of a recent outbreak of rabbit haemorrhagic disease (RHD) in New Zealand. Five pairs of primers were designed to amplify five complementary DNA genomic sequence stretching from nucleotide positions 1594 to 7071, yielding amplified fragments of 361, 340, 805,670 and 386 bp for the primer pairs RC-1/RC-2, RC-3/RC-4, RC-5/RC-6, RC-7/RC-8 and RC-9/RC-10 respectively. The identity of the amplified fragments was confirmed by chemiluminescence Southern blot hybridization and direct cycle sequencing. The nucleotide sequences of the five amplified fragments were determined and comparisons of the nucleotide and deduced amino acid sequences revealed a close genetic relationship of the New Zealand isolate 97-10372 with overseas strains of RHD virus.

Detection of antibodies against the core protein p24 of the bovine leukaemia virus in cattle for confirmatory serological testing

An electrophoretic immunoblotting technique which was developed recently was evaluated for the identification of serum antibodies against the bovine leukaemia virus core protein p24 by using 167 sera from a bovine leukaemia virus-negative herd, and 144 sera from herds naturally infected with the virus. The sensitivity of the immunoblot was 97.4%, relative to sera which were positive in the polymerase chain reaction and in a commercial EBL-ELISA. The specificity of the immunoblot was 99.4%, for the sera from a cattle herd in which all animals were negative by a commercial EBL-ELISA, and it was 96.7% relative to sera which were negative by the polymerase chain reaction and by the agar gel immunodiffusion test from bovine leukaemia virus-infected cattle herds. A p24-specific ELISA was developed, using a monoclonal anti-p24 antibody for coating microtitre plates, a crude antigen preparation, and a monoclonal anti-bovine IgG-horse radish peroxidase conjugate as components. All reagents were commercially available. While the p24-ELISA worked well with sera from serial bleeds from calves infected experimentally with the bovine leukaemia virus and its sensitivity with sera from the naturally-infected cattle was 96.5%, its specificity was relatively low at 85.0 or 53.3%, respectively for the two negative sera groups.