Efficacy of a bivalent vaccine against Marek's disease

A bivalent vaccine was prepared by combining inactivated Marek's disease virus and turkey herpesvirus. The efficacy of this vaccine, compared to turkey herpesvirus and inactivated Marek's disease virus separately, was studied in unsexed White Leghorn chicks which were vaccinated at one day old and then challenged at 21 days old with fowl blood infected with virulent Marek's disease virus. The bivalent vaccine appreciably delayed mortality resulting from Marek's disease and elicited the highest protective efficacy as judged on the basis of Marek's disease-specific mortality and percentage occurrence of lesions. The occurrence, extent and severity of gross lymphomas and microscopic lymphoproliferative lesions in various organs of the bivalent vaccinated birds were less than in the other challenged groups. In addition, the level of viraemia remained consistently and significantly lower in the bivalent vaccinated birds.     

Early posthatch protection against Marek's disease in chickens vaccinated in ovo with a CVI988 serotype 1 vaccine

CVI988, a serotype 1 Marek's disease virus (MDV), was used as an in ovo vaccine in specific-pathogen-free chickens to determine if this virus induces early posthatch protection against Marek's disease as has been shown previously for turkey herpesvirus. MDV CVI988 was injected at embryonation day (ED) 17 (group 1) or at hatch (group 2). A third group (group 3) was left unvaccinated. At 1, 2, 3, 4, 5, and 7 days of age, chickens from each group were sampled and examined as follows: a) single-cell suspensions of spleen were inoculated onto chicken embryo fibroblast monolayers to isolate the virus; b) sections of bursal tissues were stained by indirect immunofluorescence assays with anti-pp38 monoclonal antibody to identify viral antigen expression; and c) chickens were exposed intra-abdominally to MDV RB1B, a virulent serotype 1 MDV. Results revealed that in chickens given MDV CVI988 at ED 17, virus and virus-encoded protein were not detected until chickens were 3 and 2 days old after hatching, respectively. Results also indicated that during the first 4 days after hatch, the chickens given MDV CVI988 at ED 17 were better protected against virulent MDV than those given MDV CVI988 at hatch (P < or = 0.001). These results suggested that MDV CVI988 proteins were adequately expressed in the embryo to initiate prehatch immunologic response. Additional efforts with more sensitive techniques than used in this study are needed to identify the nature of viral expression in embryos.     

On the specificity of the immunoglobulin G produced by chickens experimentally infected with Marek's disease virus

In chickens experimentally infected with Marek's Disease virus (MDV) an increased amount of immunoglobulin G is produced. Using a technique of quantitative crossed immunoelectrophoresis it has been shown, that 70% of this immunoglobulin G is non-specific. Only 18% could be absorbed with MDV strain CPRL VII-infected chicken kidney cells, and only 5% with MDV-induced lymphoblastoid cells of the MDCC-MSB1 cell line. It is hypothesized that the production the unspecific immunoglobulin G is caused by a polyclonal stimulation of B-cells.     

Effects of vaccine dose, virus challenge dose and interval from vaccination to challenge on protection of broiler chickens against Marek's disease virus challenge

OBJECTIVES: To examine the effects of varying the doses of turkey herpesvirus (HVT) vaccine and Marek's disease virus (MDV) challenge at two intervals after vaccination on the protection of chickens against challenge with MDV. DESIGN AND PROCEDURE: Experiment 1, a dose response study, consisted of 11 doses of HVT vaccine administered at hatch followed by challenge with 100 plaque forming units (pfu) of MDV 5 days post vaccination. Experiment 2, a 2 x 6 x 2 factorial design, included two HVT vaccine types, six different doses of HVT vaccine and 50 pfu and 200 pfu of MDV challenge 2 days post vaccination. All chickens were reared up to day 56 post challenge when all survivors were killed humanely. Dead and killed chickens were examined for gross MD tumours. RESULTS: Experiment 1 showed a significant positive linear relationship between dose of HVT vaccine and protective index in chickens challenged 5 days post vaccination. However the range of protective index observed was limited. In Experiment 2 neither HVT vaccine provided significant protection at any dose. There was no significant effect of vaccine type or MDV challenge dose on overall protection against challenge. Chickens challenged with 200 pfu of MDV had significantly higher mortality and MD incidence than those with 50 pfu. CONCLUSIONS: HVT vaccine dose had a significant impact on protective index, but vaccination to challenge interval appeared to have greater impact on the protective efficacy of vaccination. A fourfold increase in challenge dose increased mortality rate and incidence of MD.     

Cellular and cytokine responses in feathers of chickens vaccinated against Marek's disease

In Marek's disease virus infection, feather follicle epithelium (FFE) constitutes the site of formation of infectious virus particles and virus shedding. The objective of this study was to characterize cellular and cytokine responses as indicators of cell-mediated immune response in FFE and associated feather pulp following immunization against Marek's disease. Analysis of feather tips collected between 4 and 28 days post-immunization (d.p.i.) from chickens vaccinated post-hatch with either CVI988/Rispens or herpesvirus of turkeys revealed that replication of these vaccine viruses started at 7d.p.i., peaked by 21d.p.i., and subsequently, showed a declining trend. This pattern of viral replication, which led to viral genome accumulation in feather tips, was associated with infiltration of T cell subsets particularly CD8+ T cells into the feather pulp area and the expression of cytokine genes such as interferon-gamma, which is an indication of elicitation of cell-mediated immune responses at the site of virus shedding.     

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.     

Derivation, safety and efficacy of a Marek's disease vaccine developed from an Australian isolate of very virulent Marek's disease virus

Objective To develop a serotype 1 Marek's disease (MD) vaccine from a very virulent MDV (vvMDV) pathotype and demonstrate safety and efficacy against early challenge with very virulent field strains in the presence of maternal antibody.
Study design Strain BH 16 was isolated and attenuated by serial cell culture passage. One of two cloned passages was selected for vaccine development following early laboratory-scale protection trials in commercial birds. Comparative protection trials were carried out on the BH 16 vaccine and on a CVI 988 Rispens vaccine using commercial and SPF chickens. Challenge viruses used were either a low passage strain BH 16 virus, the Woodlands No. 1 strain or MPF 57 strain of MDV. The BH 16 vaccine was back-passaged in SPF chickens six times and virus recovered from the final passage and the original vaccine virus were tested for safety. The immunosuppressive potential of the BH 16 and Rispens vaccines was also assessed in parallel.
Results The BH 16 and Rispens vaccines induced comparable levels of protection when used as monovalent or multi-valent vaccines, although protection achieved with the mono-valent vaccines was lower. No gross tumour formation was evident in any birds receiving the BH 16 vaccine or bird-passaged virus, although microscopic lesions were present in 2/12 birds that received the bird-passaged virus. In tests for immunosuppression, there was no histological evidence of damage to either the bursa of Fabricius or the thymus.
Conclusion The BH 16 vaccine was shown to be safe and at least as protective as the Rispens vaccine against three highly virulent MD challenge viruses.     

Derivation,safety and efficacy of a Marek's disease vaccine developed from an Australian isolate of very virulent Marek's disease virus

OBJECTIVE: To develop a serotype 1 Marek's disease (MD) vaccine from a very virulent MDV (vvMDV) pathotype and demonstrate safety and efficacy against early challenge with very virulent field strains in the presence of maternal antibody. STUDY DESIGN: Strain BH 16 was isolated and attenuated by serial cell culture passage. One of two cloned passages was selected for vaccine development following early laboratory-scale protection trials in commercial birds. Comparative protection trials were carded out on the BH 16 vaccine and on a CVI 988 Rispens vaccine using commercial and SPF chickens. Challenge viruses used were either a low passage strain BH 16 virus, the Woodlands No. 1 strain or MPF 57 strain of MDV. The BH 16 vaccine was back-passaged in SPF chickens six times and virus recovered from the final passage and the original vaccine virus were tested for safety. The immunosuppressive potential of the BH 16 and Rispens vaccines was also assessed in parallel. RESULTS: The BH 16 and Rispens vaccines induced comparable levels of protection when used as monovalent or multivalent vaccines, although protection achieved with the monovalent vaccines was lower. No gross tumour formation was evident in any birds receiving the BH 16 vaccine or bird-passaged virus, although microscopic lesions were present in 2/12 birds that received the bird-passaged virus. In tests for immunosuppression, there was no histological evidence of damage to either the bursa of Fabricius or the thymus. CONCLUSION: The BH 16 vaccine was shown to be safe and at least as protective as the Rispens vaccine against three highly virulent MD challenge viruses.     

Protection of chickens from Newcastle and Marek's diseases with a recombinant herpesvirus of turkeys vaccine expressing the Newcastle disease virus fusion protein.

Recombinant strains of herpesvirus of turkeys (HVT) were constructed that contain either the fusion protein gene or the hemagglutinin-neuraminidase gene of Newcastle disease virus (NDV) inserted into a nonessential gene of HVT. Expression of the NDV antigens was regulated from a strong promoter element derived from the Rous sarcoma virus long terminal repeat. Recombinant HVT strains were stable and fully infectious in cell culture and in chickens. Chickens receiving a single intra-abdominal inoculation at 1 day of age with recombinant HVT expressing the NDV fusion protein had an immunological response and were protected (> 90%) against lethal intramuscular challenge at 28 days of age with the neurotropic velogenic NDV strain Texas GB. Recombinant HVT expressing the NDV hemagglutinin-neuraminidase provided partial protection (47%) against the same challenge. Chickens vaccinated with recombinant HVT vaccines had low levels of protection against NDV replication in the trachea when challenged ocularly. Recombinant HVT vaccines and the parent HVT strain provided similar levels of protection to chickens challenged with the very virulent RB1B strain of Marek's disease virus, indicating that insertion of foreign sequences into the HVT genome did not compromise the ability of HVT to protect against Marek's disease.     

Field trials with a bivalent vaccine (HVT and SB-1) against Marek's disease

White leghorn chickens on five farms were given a bivalent Marek's disease (MD) vaccine consisting of turkey herpesvirus (HVT) and SB-1 (a nononcogenic MD virus); other chickens received only HVT. The farms had histories of "vaccination failures," presumably owing to an exceptionally virulent challenge MD virus. The bivalent vaccine uniformly protected chickens better than HVT alone between 12 and 16-20 weeks of age, when serious MD losses occurred. During that period, total mortality in groups given both viruses ranged from 0.39 to 1.26% (mean 0.86%), whereas that in HVT-vaccinated groups not exposed to SB-1 varied from 1.92 to 7.44% (mean 3.43%). Chickens in pens or rows with close contact to those given bivalent vaccine also had low MD mortality rates (0.46-1.06%, mean 0.77%), probably from the spread of SB-1.