Insertion of reticuloendotheliosis virus long terminal repeat into the genome of CVI988 strain of Marek's disease virus results in enhanced growth and protection.

Marek's disease (MD) is a lymphoproliferative disease of chickens caused by serotype 1 MD virus (MDV). Vaccination of commercial poultry has drastically reduced losses from MD, and the poultry industry cannot be sustained without the use of vaccines. Retrovirus insertion into herpesvirus genomes is an efficient process that alters the biological properties of herpesviruses. RM1, a virus derived from the virulent JM strain of MDV, by insertion of the reticuloendotheliosis (REV) long terminal repeat (LTR), was attenuated for oncogenicity but retains properties of the parental virus, such as lymphoid organ atrophy. Here we show that insertion of the REV LTR into the genome of vaccine strain CVI988 resulted in a virus (CVRM) that replicated to higher levels than parental CVI988 in cell culture and that remained apathogenic for chickens. In addition, CVRM showed protection indices similar or superior to those afforded by CVI988 virus in laboratory and field protection trials, indicating that it could be developed as a safe and efficacious vaccine to protect against very virulent plus MDV.

Improving the outcome of a Marek's disease challenge in multiple lines of egg type chickens.

A challenge test following inoculation with a standard amount of a vv+ strain of the Marek's disease (MD) virus in multiple lines and multiple generations of egg type chicken and the corresponding phenotypic trend are described. This program significantly reduced mortality of progeny from selected sires for three to 11 generations in eight of the nine elite lines studied herein. In brown egg lines, a retrospective analysis of DNA indicated an association between the blood type B (major histocompatibility complex) of the sire and the MD mortality in the challenge of its progeny. As a result of the multigeneration stock amplification and crossbreeding processes used in the commercial breeding industry, improvement in survival after challenge at the elite level will translate to improved welfare for millions of birds at the commercial production level.

Comparative evaluation of vaccine efficacy of recombinant Marek's disease virus vaccine lacking Meq oncogene in commercial chickens.

Marek's disease virus (MDV) oncogene meq has been identified as the gene involved in tumorigenesis in chickens. We have recently developed a Meq-null virus, rMd5 Delta Meq, in which the oncogene meq was deleted. Vaccine efficacy experiments conducted in Avian Disease and Oncology Laboratory (ADOL) 15I(5) x 7(1) chickens vaccinated with rMd5 Delta Meq virus or an ADOL preparation of CVI988/Rispens indicated that rMd5 Delta Meq provided superior protection than CVI988/Rispens when challenged with the very virulent plus MDV 648A strain. In the present study we set to investigate the vaccine efficacy of rMd5 Delta Meq in the field compared to several commercial preparations of CVI988/Rispens. Three large-scale field experiments, in which seeder chickens were inoculated with a very virulent plus strain of 686, vv+ MDV, were conducted in a model developed by Hy-Line International. In addition, comparisons were made with bivalent vaccine (HVT+SB-1), HVT alone and several serotype 3 HVT-vectored vaccines individually or in combination with CVI988/Rispens. Experimental results showed that addition of HVT to either of the two commercial CVI988/Rispens preparations tested (A or B) did not enhance protection conferred by CVI988/Rispens alone and that rMd5 Delta Meq was a better or equal vaccine compared to any of the CVI988/Rispens vaccines tested under the conditions of the field trials presented herein. Our results also emphasized the complexity of factors affecting vaccine efficacy and the importance of challenge dose in protection.

Serotype 1 viruses modified by backpassage or insertional mutagenesis: approaching the threshold of vaccine efficacy in Marek's disease.

Improved vaccines to control Marek's disease (MD) in chickens are desired by the poultry industry but have been difficult to develop. Studies were conducted to evaluate strategies for deriving MD vaccines of high protective efficacy, irrespective of virulence. Candidate viruses from parent strains representing v and vv+ pathotypes were modified by cell culture passage, backpassage in chickens, or insertional mutagenesis following cocultivation with retroviruses. Ten strains considered most likely to exhibit high protective efficacy were selected for further study. The ability of these modified viruses to protect commercial or maternal antibody-positive (ab+) chickens against virulent MD virus (MDV) challenge was compared with that of strain CVI988, the standard commercial MD vaccine. Modified strains were also evaluated for the ability to induce lymphomas or other pathologic changes in ab+ and antibody-negative (ab-) chickens. Two of the 10 modified viruses, strains RM1 and CVI988/BP5, provided high levels of protection against highly virulent MDV challenge. The magnitude of protection was greater than that of one laboratory and two commercial preparations of CV1988, but was approximately equal to that of two other commercial preparations of CVI988 in laboratory and field tests. Three of the strains, including RMI and CVI988/BP5, induced lymphoid organ atrophy in ab-chicks but not in ab+ commercial chicks, a property designated here as L phenotype. Seven strains, including two L+ strains, were mildly oncogenic for ab- chicks, a property designated here as O phenotype. Five of these strains caused no tumors in ab+ chickens. The two fully attenuated strains induced neither lymphomas nor lymphoid organ atrophy. The L and O phenotypes appeared not to be linked, and both (especially the L phenotype) appeared associated with high levels of protection. These studies also illustrated differences in the protective efficacy of different preparations of CVI988 vaccine, indicating the need to choose carefully the most protective strains as controls for efficacy studies. A new vv+ strain, designated as 686, is described and appears useful as a challenge virus; it is the most virulent of the 48 field isolates of MDV thus far pathotyped in this laboratory. These findings support the conclusion that new virus strains with high levels of protective immunity comparable to that of CVI988 can be developed. However, the question of whether strains can be developed that exceed the efficacy of current CVI988-based vaccines remains unanswered. After more than 30 years of unsuccessful endeavor by many laboratories toward this goal, it now may be useful to consider whether the efficacy of MD vaccines is limited by some type of biologic threshold.

Chicken industry strategies for control of tumor virus infections.

Marek's disease (MD) and lymphoid leukosis (LL) are two distinct viral diseases that cause tumor mortality in chickens. Marek's disease, being horizontally transmitted, is controlled through biosecurity measures and vaccination. Prevention of early exposure before vaccine immunity is established is most important. Some multi-house growing farms have converted to all single-age placements to break the ongoing cycle of transmission. Vaccination against MD involves either in ovo or day-old administration of live vaccine, including single or multiple serotype products. Field viruses appear to adapt over time and become resistant to the prevalent vaccine. The Rispens vaccine (CVI-988) has shown good efficacy against recently emerging very virulent MD strains in the U.S. Genetic resistance of the host to MD and control of other immunosuppressive diseases also affects MD susceptibility. Lymphoid leukosis is primarily vertically transmitted and therefore controlled by elimination of shedder hens at the primary breeder level. Depending upon the genetic type, commercial performance of laying hens may be greatly improved by eradication of the LL virus from the breeding stock.