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

Molecular characterization of Marek's disease virus in a poultry layer farm from Colombia

Marek's disease (MD) is a lymphoproliferative disease caused by an Alphaherpesvirus, genus Mardivirus, serotype 1 (Gallid Herpesvirus 2, GaHV-2) that includes all known pathogenic strains. In addition to Marek's disease virus (MDV) serotype 1, the genus includes 2 distinct nonpathogenic serotypes: serotype 2 (GaHV-3) and serotype 3 (Meleagridis Herpesvirus 1, MeHV-1) which are used in commercially available vaccines against MD. As a result of vaccination, clinical signs are not commonly observed, and new cases are usually associated with emerging variant strains against which the vaccines are less effective. In this study, a commercial layer farm showing clinical signs compatible with MDV infection was evaluated. Histological lesions and positive immunohistochemistry in the sciatic nerve and thymus were compatible with cytolytic phase of MD. GaHV-2, GaHV-3 and MeHV-1 were identified by PCR and qPCR in blood samples from 17 birds with suspected MD. Analysis of the Meq gene of the Colombian GaHV-2 isolate revealed a 99% sequence identity with Asian strains, and in the phylogenetic analysis clustered with vv+ MDV. The analysis of amino acid alignments demonstrated an interruption of the proline rich region in P176A, P217A and P233L positions, which are generally associated with vv+ strains. Some of these changes, such as P233L and L258S positions have not been reported previously. In addition, primary cell cultures inoculated with lymphocytes isolated from the spleen showed typical cytopathic effect of GaHV-2 at 5 d post infection. Based on the molecular analysis, the results from this study indicate the presence of vv+ MDV infection in commercial birds for the first time in Colombia. It is recommended to perform further assays in order to demonstrate the pathotype characteristics in vivo.

Differential amplification of Marek's disease CVI988 vaccine and of wild-type isolates from organs of commercial chickens using single or duplexed probes in real-time PCR

The differentiation of Marek's disease virus (MDV)-infected and vaccinated animal (DIVA) test, based on the MDV pp38 gene was described by Baigent et al. [(2016). Real-time PCR for differential quantification of CVI988 vaccine and virulent MDV strains. Journal of Virological Methods, 233, 23-36], using similar primers and alternate probes for virulent MDV-1 and the vaccine CVI988 virus. We explored the assay's applicability for commercial vaccines and commercial chickens, as the above-mentioned study employed tissue-cultured MDV strains and tissues from experimental trials. DNA of visceral organs and feathers of vaccinated or naturally infected chickens was used. Further, the applicability of the DIVA assay was evaluated using single or duplexed probes for the two viruses in the same amplification tube. Due to the high viral content in the commercial vaccines and in the clinical cases of MDV-1 infected commercial chickens, their examination by the MDV-1 DIVA real-time PCR was performed in one step. However, for the feather DNAs of commercially vaccinated birds, a step of pre-amplification was required. The MDV-1 DIVA real-time PCR performed as single probe in separate tubes using the Vir3 probe was very sensitive for virulent MDV-1 strains, but not very specific, as it also gave a clear signal with CVI988 vaccine virus. In contrast, the CVI vaccine probe was specific for CVI988, and did not recognize the MDV-1 strains. When both probes were present in one tube, the CVI probe showed a greater sensitivity for CV1988, while the Vir3 probe showed a much better specificity for virulent MDV-1.

Real-time PCR for differential quantification of CVI988 vaccine virus and virulent strains of Marek's disease virus

CVI988/Rispens vaccine, the 'gold standard' vaccine against Marek's disease in poultry, is not easily distinguishable from virulent strains of Marek's disease herpesvirus (MDV). Accurate differential measurement of CVI988 and virulent MDV is commercially important to confirm successful vaccination, to diagnose Marek's disease, and to investigate causes of vaccine failure. A real-time quantitative PCR assay to distinguish CVI988 and virulent MDV based on a consistent single nucleotide polymorphism in the pp38 gene, was developed, optimised and validated using common primers to amplify both viruses, but differential detection of PCR products using two short probes specific for either CVI988 or virulent MDV. Both probes showed perfect specificity for three commercial preparations of CVI988 and 12 virulent MDV strains. Validation against BAC-sequence-specific and US2-sequence-specific q-PCR, on spleen samples from experimental chickens co-infected with BAC-cloned pCVI988 and wild-type virulent MDV, demonstrated that CVI988 and virulent MDV could be quantified very accurately. The assay was then used to follow kinetics of replication of commercial CVI988 and virulent MDV in feather tips and blood of vaccinated and challenged experimental chickens. The assay is a great improvement in enabling accurate differential quantification of CVI988 and virulent MDV over a biologically relevant range of virus levels.

An outbreak of lymphomas in a layer chicken flock previously infected with fowlpox virus containing integrated reticuloendotheliosis virus

Visceral lymphomas occurred in a 236-day-old layer flock previously diagnosed with reticuloendotheliosis virus (REV)-integrated fowlpox virus (FPV) infection at the age of 77 days. Common pathologic lesions were multiple neoplastic nodules of homogeneous lymphocytes in the livers and spleens of all submitted chickens. All neoplastic tissues were positive for the REV envelope (env) gene by PCR. In a retrospective molecular study of FPV-infected 77-day-old chickens from the same flock, we identified nearly full-length REV provirus integrated into the genome of FPV as well as the REV env gene in trachea samples, whereas only the REV LTR region was present in the FPV strain used to vaccinate this flock. The 622-bp REV env gene nucleotide sequence derived from the trachea and neoplastic tissues was identical. Commercial ELISA of serum samples revealed that all chickens aged between 17 and 263 days in this flock were positive for REV but not for avian leukosis virus. Taken together, the evidence suggests that the visceral lymphomas were caused by a REV-integrated FPV field strain. FPV infections of commercial chickens should be followed up by careful monitoring for manifestations of REV infection, including lymphomas and immune depression, considering the ease with which the REV provirus appears to be able to integrate into the FPV genome.

Vaccination against Marek's disease reduces telomerase activity and viral gene transcription in peripheral blood leukocytes from challenged chickens

We investigated whether telomerase activity and viral gene transcription were associated with protection against the RB-1B strain of Marek's disease virus (MDV) in chickens vaccinated with Rispens CVI988 or the herpes virus of turkey (HVT). Telomerase activity in peripheral blood leukocytes (PBLs) seemed to be an appropriate marker of lymphoma and levels of viral transcription were correlated with the virulence of MDV strains. Vaccinated protected birds had lower levels of telomerase activity and RB-1B viral gene transcription than unvaccinated chickens infected with RB-1B. The decrease in RB-1B viral transcription was more marked in chickens vaccinated with CVI988 than in those vaccinated with HVT. Indeed, RB-1B viral transcription was not detectable after 14 days post-challenge. In conclusion, telomerase activity and gene transcription in challenge MDV strains are potential new reliable criteria of protection in vaccinated chickens.

Molecular evaluation of responses to vaccination and challenge by Marek's disease viruses

A real-time quantitative polymerase chain reaction was utilized to study the in vivo replication of Marek's disease vaccine viruses and of virulent oncogenic strains. In the first of four experiments, the growth of the herpes virus of turkeys (HVT) vaccine was detectable in various organs of infected chicken embryos, with the highest viral loads being present in the spleen. No evidence was obtained for replication of serotype-1 Marek's disease viruses in embryos. In the second experiment, viral loads were measured in several organs of chickens after administration of the Rispens and HVT vaccines immediately after hatching. Lowest levels were noted for the Rispens strain after 1 to 8 weeks. By contrast, HVT vaccine grew well in all tested organs, with the highest loads being present in the spleen. Highest loads were observed in unvaccinated birds after challenge with the highly virulent strain MPF57 at day 8, especially in lymphoid organs. A positive relationship was observed between viral load and clinical signs, including tumour formation. In a third study, viral loads were measured in the organs of chickens administered the Rispens vaccine on the day of hatch and challenged at day 8 with MPF57. High levels of clinical protection were afforded against MPF57 by the Rispens vaccine but, in confirmation of earlier findings, sterilizing immunity was not induced. In a fourth study, two experiments were conducted--in which viral loads were measured after challenge of chickens vaccinated with HVT in ovo or at day 1 after hatching. Similar protection was achieved in birds vaccinated in ovo on embryonic days 11 and 17, although protection was slightly, but not significantly, lower than for birds vaccinated at day 1.

Absolute quantification using real-time polymerase chain reaction of Marek's disease virus serotype 2 in field dust samples, feather tips and spleens

Methods for Taqman quantitative real-time PCR (qPCR) assays to detect the three serotypes of Marek's disease virus (MDV) are available, and absolute quantification has been developed for MDV serotype 1 and serotype 3. The development of a method for absolute quantification of Marek's disease virus serotype 2 (MDV2) is described in this paper. Using plasmid DNA, the lower detection limit of the MDV2 assay was determined to be 10 copies. Three independent assay runs showed highly reproducible Ct values and calculated copy numbers, with mean intra- and inter-assay coefficients of variation of less than 3% for Ct and less than 21.5% for calculated copy number. Absolute quantification of MDV2 was performed successfully on dust samples collected from poultry farms across Australia, material from infectious spleens and feather tips from chickens vaccinated with an attenuated strain of MDV2. Thus, it is now possible to use qPCR assays for absolute quantification of all three serotypes of MDV in a sample.

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.

An enzyme-linked immunosorbent assay (ELISA) for detection of Marek's disease virus-specific antibodies and its application in an experimental vaccine trial

An enzyme-linked immunosorbent assay (ELISA) for the detection of Marek's disease virus (MDV)-specific antibodies was developed. Chicken embryo cells (CEC) or chicken kidney cells (CKC) were infected with MDV vaccine strain CVI988/Rispens, and infected-cell lysates were prepared at day 5 post-infection by freeze-thawing. Uninfected-cell lysates served as negative controls. Sera were used at a 1 : 100 dilution and were added in parallel to wells containing the infected and uninfected cell lysates. The optical densities at 492 nm (OD(492 nm)) were measured after detection of bound chicken antibodies with anti-chicken IgG peroxidase conjugate and colour reactions using o-phenylenediamine (OPD) as a substrate. The best results concerning the signal-to-noise ratio were obtained by using CKC cells rather than CEC for antigen preparation. The OD(492 nm) of plasma or serum samples with infected CKC was <0.02 when samples of unvaccinated and unchallenged maternal antibody-negative white leghorn chickens were tested. Sera and plasma samples of positive control birds exhibited OD(492 nm) of <0.01 when tested with uninfected CKC. The assay was used to monitor a trial that compared experimental BAC DNA vaccines and a commercial vaccine. Sustained seroconversion and antibody titers that were constantly rising until day 84 after vaccination (71 days after challenge) was observed only when chickens did not develop Marek's disease. In contrast, chickens developing the disease mounted marginal and short-lived antibody titers only. We conclude that the developed ELISA may be a valuable tool for the evaluation of the efficacy of MDV vaccination under experimental but possibly also under field conditions.

Replication-competent bacterial artificial chromosomes of Marek's disease virus: novel tools for generation of molecularly defined herpesvirus vaccines

Marek's disease (MD), a highly infectious disease caused by an oncogenic herpesvirus, is one of the few herpesvirus diseases against which live attenuated vaccines are used as the main strategy for control. We have constructed bacterial artificial chromosomes (BACs) of the CVI988 (Rispens) strain of the virus, the most widely used and effective vaccine against MD. Viruses derived from the BAC clones were stable after in vitro and in vivo passages and showed characteristics and growth kinetics similar to those of the parental virus. Molecular analysis of the individual BAC clones showed differences in the structure of the meq gene, indicating that the commercial vaccine contains virus populations with distinct genomic structures. We also demonstrate that, contrary to the published data, the sequence of the L-meq of the BAC clone did not show any frameshift. Virus stocks derived from one of the BAC clones (clone 10) induced 100 percent protection against infection by the virulent strain RB1B, indicating that BAC-derived viruses could be used with efficacies similar to those of the parental CVI988 vaccines. As a DNA vaccine, this BAC clone was also able to induce protection in 6 of 20 birds. Isolation of CVI988 virus from all of these six birds suggested that immunity against challenge was probably dependent on the reconstitution of the virus in vivo and that such viruses are also as immunogenic as the in vitro-grown BAC-derived or parental vaccine viruses. Although the reasons for the induction of protection only in a proportion of birds (33.3%) that received the DNA vaccine are not clear, this is most likely to be related to the suboptimal method of DNA delivery. The construction of the CVI988 BAC is a major step towards understanding the superior immunogenic features of CVI988 and provides the opportunity to exploit the power of BAC technology for generation of novel molecularly defined vaccines.

Marek's disease virus genome separation from feather tip extracts by pulsed field gel electrophoresis

Marek's disease virus is an oncogenic herpes virus of poultry that is highly cell associated. In the infected tissues and tumors the virus replicates in a low copy number. The propagation and dissemination of the virus takes place at the feather follicle epithelium, where the viral genome is produced in high copy number. As the viral genome is a large circular DNA molecule (200 kbp), pulsed field gel electrophoresis was used for separation of the viral genome directly from the infected chicken. DNA was extracted from tumors or feather tips by the phenol:chloroform technique or by low melting agar technique. It was found that feathers, being the site of virus productive replication, are useful for separation of free Marek's disease virus DNA from in vivo infections.