Evaluation and Identification of Marek's Disease Virus BAC Clones as Standardized Reagents for Research

Phenotypic Characterization of Recombinant Marek's Disease Virus in Live Birds Validates Polymorphisms Associated with Virulence

Marek's disease (MD) is a highly infectious lymphoproliferative disease in chickens with a significant economic impact. Mardivirus gallidalpha 2, also known as Marek's disease virus (MDV), is the causative pathogen and has been categorized based on its virulence rank into four pathotypes: mild (m), virulent (v), very virulent (vv), and very virulent plus (vv+). A prior comparative genomics study suggested that several single-nucleotide polymorphisms (SNPs) and genes in the MDV genome are associated with virulence, including nonsynonymous (ns) SNPs in eight open reading frames (ORF): UL22, UL36, UL37, UL41, UL43, R-LORF8, R-LORF7, and ICP4. To validate the contribution of these nsSNPs to virulence, the vv+MDV strain 686 genome was modified by replacing nucleotides with those observed in the vMDV strains. Pathogenicity studies indicated that these substitutions reduced the MD incidence and increased the survival of challenged birds. Furthermore, using the best-fit pathotyping method to rank the virulence, the modified vv+MDV 686 viruses resulted in a pathotype similar to the vvMDV Md5 strain. Thus, these results support our hypothesis that SNPs in one or more of these ORFs are associated with virulence but, as a group, are not sufficient to result in a vMDV pathotype, suggesting that there are additional variants in the MDV genome associated with virulence, which is not surprising given this complex phenotype and our previous finding of additional variants and SNPs associated with virulence.

Design of live-attenuated animal vaccines based on pseudorabies virus platform

Pseudorabies virus (PRV) is a double-stranded DNA virus with a genome approximating 150 kb in size. PRV contains many non-essential genes that can be replaced with genes encoding heterogenous antigens without affecting viral propagation. With the ability to induce cellular, humoral and mucosal immune responses in the host, PRV is considered to be an ideal and potential live vector for generation of animal vaccines. In this review, we summarize the advances in attenuated recombinant PRVs and design of PRV-based live vaccines as well as the challenge of vaccine application.

Evaluation of Protective Efficacy When Combining Turkey Herpesvirus-Vector Vaccines

Turkey herpesvirus (HVT) is widely used as a vaccine against Marek's disease in chickens and recently as a vector for foreign genes from infectious bursal disease virus, Newcastle disease (ND) virus, infectious laryngotracheitis (ILT) virus, and avian influenza virus. Advantages of HVT-vector vaccines are that the vaccines do not contain live respiratory viruses or live infectious bursal disease virus able to replicate and cause disease or embryo mortality, they can be administered at hatch or in ovo, and they are relatively insensitive to interference from maternally derived antibodies. As producers have tried to combine HVT-vector vaccines to protect against additional diseases, reports have indicated that applying two vectored vaccines using the same HVT vector is reported to reduce the efficacy of one or both vaccines. To confirm this interference, we evaluated commercial vaccines from multiple companies, including products with inserts designed to protect against ND, infectious ILT, and infectious bursal disease (IBD). Using a standard dosage, we found that the ILT product was most severely affected by the addition of other vaccines, as demonstrated by a significant increase in clinical signs, significant decrease in weight gain, and increase in quantity of challenge virus observed from tracheal swabs collected from Days 3-5 postchallenge. The ND and IBD products were also affected by the addition of other vaccines, although in most cases differences compared to vaccination with the vector alone were not statistically significant. This study demonstrates the importance of following manufacturer guidelines and the need for validating alternative strategies to benefit from the high level of protection offered by vector vaccines.