PCR detection of amplified 132 bp repeats in Marek's disease virus type 1 (MDV-1) DNA can serve as an indicator for critical genomic rearrangement leading to the attenuation of virus virulence

Newly detected mutations in the Meq oncogene and molecular pathotyping of very virulent Marek's disease herpesvirus in Tunisia

Marek's disease (MD) is a contagious avian viral disease that is responsible for large economic losses to farmers. The disease is caused by Marek's disease virus (species Gallid alphaherpesvirus 2), which causes neurological lesions, immune suppression, and tumor proliferation of lymphoid cells that invade a large number of organs and tissues. Despite widespread vaccination, Marek's disease virus (MDV), has shown a continuous increase in its virulence and has acquired the ability to overcome immune responses induced by vaccines. In the present study, the oncogenic serotype MDV-1 was detected by real-time PCR in DNA samples extracted from organs developing tumor infiltrations. Identification of the pathotype based on a 132-bp tandem repeat and sequencing and phylogenetic analysis of the Meq gene and its encoded protein allowed classification of the isolated viruses as "very virulent", with two new and unique mutations in the Meq gene resulting in amino acid substitutions. Sequencing of pp38, vIl-8, UL1 and UL44 genes did not reveal any new mutations that were characteristic of the Tunisian isolates or correlated with virulence. These results raised concerns about the ability of HVT and CVI988 vaccines, which are currently used in Tunisia and other countries, to protect chickens against highly virulent virus strains.

Outbreak of Avian Tuberculosis in Commercial Domestic Pekin Ducks ( Anas platyrhynchos domestica)

Avian tuberculosis is a contagious disease affecting various domestic and wild bird species, and is caused by Mycobacterium avium . It is reported extremely rarely in commercial poultry flocks and has not been reported in commercial domestic ducks to date, with domestic ducks reported to be moderately resistant to M. avium infection. Here, we report the outbreak of avian tuberculosis in commercial Pekin duck ( Anas platyrhynchos domestica) flocks. Postmortem and histopathologic findings included nodules presenting in the visceral organs of ducks, and granulomas with central caseous necrosis surrounded by infiltrating lymphocytes. The M. avium pathogen was isolated and further identified by Ziehl-Neelsen staining and PCR based on insert sequence IS901 and the 16S rRNA gene. We highlight that avian tuberculosis not only has economic significance for the duck industry, but also presents a potential zoonotic hazard to humans.

Rapid detection of the Marek's disease viral genome in chicken feathers by loop-mediated isothermal amplification

A loop-mediated isothermal amplification (LAMP) method for the rapid detection of serotype 1 Marek's disease virus (MDV) was developed. The method used a set of three pairs of primers to amplify the MEQ gene for detecting serotype 1 MDV. The MDV LAMP method did not cross-react with serotype 2 and serotype 3, nor did the LAMP primers have binding sites for the common avian DNA viruses (reticuloendotheliosis virus, chicken anemia virus, subgroup J of the avian leukosis virus). Additionally, the assay could detect up to 10 copies of the MEQ gene in the MD viral genome, and it had 10 times higher sensitivity than the traditional PCR methods. The LAMP master mix was stable for 90 days at -20°C. Furthermore, the efficiency of LAMP for detection of serotype 1 MDV in clinical samples was comparable to those of PCR and viral isolation. The LAMP procedure is simple and does not rely on any special equipment. The detection of serotype 1 MDV by LAMP will be useful for detecting and controlling oncogenic Marek's disease.

Characterization of 132 bp Repeats BamH1-H Region in Pathogenic Marek's Disease Virus of Poultry in Gujarat, India, Using PCR and Sequencing

A total of 34 clinical samples and four Marek's disease virus (MDV) vaccines were tested using primer BamH1/BamH2 in layer birds of poultry. Out of 34 samples tested for detection of MDV, 32 samples produced approximately 434 bp product. All the three HVT vaccines as well as SB-1 (MDV-2) vaccine failed to produce the expected amplicons, there by proving negative for the targeted 132 bp repeats of MDV genome by the primers BamH1/BamH2. Resultant PCR products of the field samples were purified and sequenced and resulted in 378 bp long sequences. PCR was found very satisfactory in detecting the presence of MDV either in feather follicle or in tissue samples. Sequencing study has proved beyond doubt that the two representative samples contained two 132 bp repeats indicating the virulent nature of the field virus.

Relationship between Marek's disease virus load in peripheral blood lymphocytes at various stages of infection and clinical Marek's disease in broiler chickens

Vaccination with herpesvirus of turkey (HVT) vaccine provides protection against clinical Marek's disease (MD) but does not preclude infection with wild-type MD virus (MDV). The quantity of MDV detected in circulating lymphocytes during the early period after infection may be a useful predictor of subsequent clinical MD later in the life. A study was designed to quantify MDV and HVT copy number in peripheral blood lymphocytes (PBL) using real-time polymerase chain reaction between days 5 and 35 post-challenge and to relate this to subsequent development of gross MD lesions. Female commercial broiler chickens were vaccinated with HVT or were sham-vaccinated at hatch, then challenged with MDV strain MPF-57 at day 2 post-vaccination and reared in positive-pressure isolators up to 56 days post-challenge, when all survivors were euthanized. All dead and euthanized chickens were examined post mortem for gross MD lesions. Birds were scored for MD lesions and mortality. MDV and HVT genome copy numbers were determined for each PBL sample. There was an increase in HVT load in PBL between days 7 and 37 post-vaccination, with marked increases between days 7 and 16 and again between days 30 and 37. There was a steady increase in MDV load to 35 days post-challenge. The mean MDV copy number (log(10)) was greater in chickens subsequently exhibiting gross MD lesions (5.05 +/- 0.21) than in those that did not (2.88 +/- 0.223), with the largest difference at 14 and 21 days post-challenge (P < 0.001). Quantification of MDV during early infection is therefore a potential tool for monitoring MD in broiler flocks.

Replication kinetics of Marek's disease vaccine virus in feathers and lymphoid tissues using PCR and virus isolation

CVI988 (Rispens), an avirulent strain of Marek's disease virus, is the most widely used vaccine against Marek's disease. The kinetics of replication of CVI988 was examined in tissues of chickens vaccinated at either 1 day or 14 days of age and sampled regularly up to 28 days post-vaccination. Age at vaccination had no significant effect on the kinetics of CVI988 virus replication. During the cytolytic phase of infection (1-7 days), virus levels peaked in the spleen, bursa and thymus with very close correlation among these organs. Virus load in peripheral blood lagged behind and did not reach high levels. Significant numbers of virus genomes were detected in the feather tips only after 7 days, but subsequently rose to levels almost 10(3)-fold greater than in the other tissues. This is the first accurate quantitative data for kinetics of CVI988 replication in a variety of tissues. There was good correlation between data from virus isolation and PCR, with real-time PCR being the preferred method for rapid, accurate and sensitive quantification of virus. Feathers were ideal for non-invasive sampling to detect and measure CVI988 in live chickens and, from 10 days onwards, virus load in feather tips was predictive of virus load in lymphoid tissues where immune responses will occur. The potential for real-time PCR analysis of feather samples for further investigation of the mechanism of vaccinal protection, and to assist optimization of vaccination regimes, is discussed.

Absolute quantitation of Marek's disease virus and Herpesvirus of turkeys in chicken lymphocyte, feather tip and dust samples using real-time PCR

The further development of Taqman quantitative real-time PCR (qPCR) assays for the absolute quantitation of Marek's disease virus serotype 1 (MDV1) and Herpesvirus of turkeys (HVT) viruses is described and the sensitivity and reproducibility of each assay reported. Using plasmid DNA copies, the lower limit of detection was determined to be 5 copies for the MDV1 assay and 75 copies for the HVT assay. Both assays were found to be highly reproducible for Ct values and calculated copy numbers with mean intra- and inter-assay coefficients of variation being less than 5% for Ct and 20% for calculated copy number. The genome copy number of MDV1 and HVT viruses was quantified in PBL and feather tips from experimentally infected chickens, and field poultry dust samples. Parallelism was demonstrated between the plasmid-based standard curves, and standard curves derived from infected spleen material containing both viral and host DNA, allowing the latter to be used for absolute quantification. These methods should prove useful for the reliable differentiation and absolute quantitation of MDV1 and HVT viruses in a wide range of samples.

A DNA vaccine containing an infectious Marek's disease virus genome can confer protection against tumorigenic Marek's disease in chickens

A DNA vaccine containing the infectious BAC20 clone of serotype 1 Marek's disease virus (MDV) was tested for its potential to protect against Marek's disease (MD). Chickens were immunized at 1 day old with BAC20 DNA suspended either in PBS, as calcium phosphate precipitates, incorporated into chitosan nanoparticles, in Escherichia coli DH10B cells, or bound to gold particles for gene-gun delivery. Challenge infection with MDV strain EU1 was performed at 12 days old, and four out of seven birds immunized with BAC20 DNA in saline by the intramuscular route remained free of MD until day 77 after challenge infection. A delay in the development of the disease could be observed in some animals vaccinated with other BAC20 DNA formulations, but clinical MD and tumour formation were evident in all but one bird. Five out of seven animals immunized with the vaccine virus CVI988 were protected against MD, but none out of seven birds survived EU1 challenge infection after injection of negative-control plasmid DNA. In a second animal experiment, five out of 12 chickens immunized with BAC20 DNA and six out of eight birds immunized with virus reconstituted from BAC20 DNA remained free of MD after challenge infection. In contrast, none out of 12 chickens survived challenge infection after immunization with BAC20 DNA lacking the essential gE gene or with gE-negative BAC20 virus. The results suggested that an MDV BAC DNA vaccine has potential to protect chickens against MD, but that in vivo reconstitution of vaccine virus is a prerequisite for protection.

Molecular identification of the Marek's disease virus vaccine strain CVI988 in vaccinated chickens

The study describes three polymerase chain reaction (PCR) systems for the CVI988 vaccine virus: the meq gene, the MDV BamHI-D/H 132 bp tandem repeat fragment and the MDV-gB gene. Whereas the PCR product of virulent MDV strains and of the CVI988 virus strain with the meq and the 132 bp primer sets differed for the two templates, the MDV-gB PCR products were similar. The sensitivity of the three PCRs was determined for the two templates: the CVI988 DNA was detected up to 2.48 plaque forming units, and a MDV-1 DNA, was amplified with the 132 bp primers up to the 10(-3) DNA dilution, and up to the 10(-2) with the MDV-gB and meq gene primers. As conventional detection for the CVI988 vaccine virus is by tissue culture, the aim was to analyse the feasibility of the molecular detection of the vaccine virus in the vaccinated chick. In two experimental trials employing specific pathogen free and commercial Lohmann chicks, respectively, the vaccine virus replicated to a limited extent; it was detected only in the spleen of up to 60% chicks at 2-4 weeks and in one chick at 3 weeks, respectively. The survey of three commercial Lohmann flocks, kept in biosecurity conditions, revealed the vaccine virus only in the spleen of 40% of 30-day-old chicks. The present study shows that CV1988 DNA is present in vaccinated chicks in a low quantity and it is difficult to detect directly from the chick, probably because vaccine viruses are latent in vivo. For an efficient detection it is pertinent to cultivate the vaccine virus on chicken embryo fibroblasts (CEF), as then the virus escapes the latent state, enters into the productive mode of replication, and a high viral copy number is produced.

The genome of turkey herpesvirus

Here we present the first complete genomic sequence of Marek's disease virus serotype 3 (MDV3), also known as turkey herpesvirus (HVT). The 159,160-bp genome encodes an estimated 99 putative proteins and resembles alphaherpesviruses in genomic organization and gene content. HVT is very similar to MDV1 and MDV2 within the unique long (UL) and unique short (US) genomic regions, where homologous genes share a high degree of colinearity and their proteins share a high level of amino acid identity. Within the UL region, HVT contains 57 genes with homologues found in herpes simplex virus type 1 (HSV-1), six genes with homologues found only in MDV, and two genes (HVT068 and HVT070 genes) which are unique to HVT. The HVT US region is 2.2 kb shorter than that of MDV1 (Md5 strain) due to the absence of an MDV093 (SORF4) homologue and to differences at the UL/short repeat (RS) boundary. HVT lacks a homologue of MDV087, a protein encoded at the UL/RS boundary of MDV1 (Md5), and it contains two homologues of MDV096 (glycoprotein E) in the RS. HVT RS are 1,039 bp longer than those in MDV1, and with the exception of an ICP4 gene homologue, the gene content is different from that of MDV1. Six unique genes, including a homologue of the antiapoptotic gene Bcl-2, are found in the RS. This is the first reported Bcl-2 homologue in an alphaherpesvirus. HVT long repeats (RL) are 7,407 bp shorter than those in MDV1 and do not contain homologues of MDV1 genes with functions involving virulence, oncogenicity, and immune evasion. HVT lacks homologues of MDV1 oncoprotein MEQ, CxC chemokine, oncogenicity-associated phosphoprotein pp24, and conserved domains of phosphoprotein pp38. These significant genomic differences in and adjacent to RS and RL regions likely account for the differences in host range, virulence, and oncogenicity between nonpathogenic HVT and highly pathogenic MDV1.

Reconstitution of Marek's disease virus serotype 1 (MDV-1) from DNA cloned as a bacterial artificial chromosome and characterization of a glycoprotein B-negative MDV-1 mutant

The complete genome of Marek's disease virus serotype 1 (MDV-1) strain 584Ap80C was cloned in Escherichia coli as a bacterial artificial chromosome (BAC). BAC vector sequences were introduced into the U(S)2 locus of the MDV-1 genome by homologous recombination. Viral DNA containing the BAC vector was used to transform Escherichia coli strain DH10B, and several colonies harboring the complete MDV-1 genome as an F plasmid (MDV-1 BACs) were identified. DNA from various MDV-1 BACs was transfected into chicken embryo fibroblasts, and from 3 days after transfection, infectious MDV-1 was obtained. Growth of MDV-1 recovered from BACs was indistinguishable from that of the parental virus, as assessed by plaque formation and determination of growth curves. In one of the MDV-1 BAC clones, sequences encoding glycoprotein B (gB) were deleted by one-step mutagenesis using a linear DNA fragment amplified by PCR. Mutant MDV-1 recovered after transfection of BAC DNA that harbored a 2.0-kbp deletion of the 2.6-kbp gB gene were able to grow and induce MDV-1-specific plaques only on cells providing MDV-1 gB in trans. The gB-negative virus reported here represents the first MDV-1 mutant with a deletion of an essential gene and demonstrates the power and usefulness of BACs to analyze genes and gene products in slowly growing and strictly cell-associated herpesviruses.

The genome of a very virulent Marek's disease virus

Here we present the first complete genomic sequence, with analysis, of a very virulent strain of Marek's disease virus serotype 1 (MDV1), Md5. The genome is 177,874 bp and is predicted to encode 103 proteins. MDV1 is colinear with the prototypic alphaherpesvirus herpes simplex virus type 1 (HSV-1) within the unique long (UL) region, and it is most similar at the amino acid level to MDV2, herpesvirus of turkeys (HVT), and nonavian herpesviruses equine herpesviruses 1 and 4. MDV1 encodes 55 HSV-1 UL homologues together with 6 additional UL proteins that are absent in nonavian herpesviruses. The unique short (US) region is colinear with and has greater than 99% nucleotide identity to that of MDV1 strain GA; however, an extra nucleotide sequence at the Md5 US/short terminal repeat boundary results in a shorter US region and the presence of a second gene (encoding MDV097) similar to the SORF2 gene. MD5, like HVT, encodes an ICP4 homologue that contains a 900-amino-acid amino-terminal extension not found in other herpesviruses. Putative virulence and host range gene products include the oncoprotein MEQ, oncogenicity-associated phosphoproteins pp38 and pp24, a lipase homologue, a CxC chemokine, and unique proteins of unknown function MDV087 and MDV097 (SORF2 homologues) and MDV093 (SORF4). Consistent with its virulent phenotype, Md5 contains only two copies of the 132-bp repeat which has previously been associated with viral attenuation and loss of oncogenicity.

Control strategies for Marek's disease: a perspective for the future

Marek's disease virus is an evolving pathogen, acquiring virulence in response to increasingly effective vaccines. Although vaccine efficacy is generally good, industry has placed a high priority on more effective products. The search for better vaccines has been conducted mainly in the arena of molecular biology, and has been thus far disappointing. Various conditions prevail that currently limit the potential to develop suitable long-term solutions. A new paradigm based on reduction of early exposure, multiple levels of host resistance, and improved cooperation among stakeholders is proposed for consideration.

Detection of animal pathogens by using the polymerase chain reaction (PCR)

The polymerase chain reaction (PCR) is a nucleic acid-based technique that enables the rapid and sensitive detection of specific micro-organisms. Although this technique is widely used in veterinary research, it has not yet found applications in routine microbiological analysis of veterinary clinical samples. However, advances in sample preparation together with the increasing availability of specific gene sequences will probably lead to the more widespread diagnostic use of PCR in the future. PCR is likely to have a strong impact in the epidemiology, treatment and prevention of animal infectious diseases.

Applications of DNA amplification techniques in veterinary diagnostics

An overview of the principles of the polymerase chain reaction, ligase chain reaction, self-sustained sequence replication and Q beta replicase is given. The application of these methods for the diagnosis of veterinary infectious and hereditary diseases as well as for other diagnostic purposes is discussed and comprehensive tables of reported assays are provided. Specific areas where these DNA-based amplification methods provide substantial advantages over traditional approaches are also highlighted. With regard to PCR-based assays for the detection of viral pathogens, this article is an update of a previous review by Belák and Ballagi-Pordány (1993).

Open reading frames in a 4556 nucleotide sequence within MDV-1 BamHI-D DNA fragment: evidence for splicing of mRNA from a new viral glycoprotein gene

A DNA segment of the MDV-1 BamHI-D fragment was sequenced, and the open reading frames (ORFs) present in the 4556 nucleotide fragment were analyzed by computer programs. Computer analysis identified 19 putative ORFs in the sequence ranging from a coding capacity of 37 amino acids (aa) (ORF-1a) to 684aa (ORF-1). The special properties of four ORFs (1a, 1, 2, and 3) were investigated. Two adjacent ORFs, ORF-1a and ORF-1, were found by computer analysis to have the properties of two introns encoding a glycoprotein: ORF-1a encodes an aa sequence with the properties of a signal peptide, and ORF-1 encodes a polypeptide with a membrane anchor domain and putative N-glycosylation sites in the aa sequence. ORF-1a and ORF-1 were found to be transcribed in MDV-1-infected cells. Two RNA transcripts were detected: a precursor RNA and its spliced form. Both are transcribed from a promoter located 5' to ORF-1a, and splice donor and acceptor sites are used to splice the mRNA after cleavage of a 71-nucleotide sequence. This finding suggest that ORF-1a and ORF-1 are two introns of a new MDV-1 glycoprotein gene. The DNA sequence containing ORF-1 was transiently expressed in COS-1 cells, and the viral protein produced in these cells was found to react with anti-MDV serotype-1 Antigen B-specific monoclonal antibodies. These studies indicate that the protein encoded by ORF-1 has antigenic properties resembling Antigen B of MDV-1. A gene homologous to ORF-1 was detected in the genome of both MDV-2(SB1) and MDV-3(HVT), which serve as commercial vaccine strains. Two additional ORFs were noted in the 4556 nucleotide sequence: ORF-2, which encodes a 333 aa polypeptide initiating in the UL and terminating in the TRL prior to the putative origin of replication, and ORF-3, which encodes a 155 aa polypeptide that is partly homologous to the phosphoprotein pp38 encoded by the BamHI-H sequence. The 65 N-terminal aa of the two gene products are identical, both being derived from the nucleotide sequences in the TRL and IRL, respectively. Additional homologous aa sequences are the hydrophobic aa domain in the middle of both proteins. The functions of ORF-2, ORF-3, and additional ORFs are under study.