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Marek's disease (MD) is an oncogenic, lymphoproliferative, and highly contagious disease of chickens. Its etiologic agent is the alphaherpesvirus Marek's disease virus (MDV, Gallid alphaherpesvirus 2), and it is a chronic and ubiquitous problem for the poultry industry with significant economic impact in the United States and worldwide. We have previously demonstrated that MDV attenuated by dicodon deoptimization of the UL54 gene results in reduced gene product accumulation in vitro, with reduced viral genome copy number upon infection and reduced atrophy of bursa and thymus in vivo as well. In this report we detail our attempts to use the same attenuation strategy on a meq-deleted MDV mutant, rMd5B40ΔMeq. Unlike the wild-type rMd5B40 virus the rMd5B40ΔMeq is no longer oncogenic, but infected birds experience an unacceptable amount of bursa and thymus atrophy (BTA). We produced two meq-deleted MDV recombinants with a dicodon-deoptimized UL54 (rMd5B40ΔMeq/UL54deop1 and -deop2) and tested their tendency to cause BTA and to serve as a protective vaccine. We found that, although dicodon deoptimization of the UL54 gene results in a virus that spares the infected animal from atrophy of the bursa and thymus, the meq-deleted UL54-deoptimized recombinant is also less protective than the meq-deleted virus without UL54 deoptimization, the HVT + SB1 combination vaccine, or the Rispens (CVI988) vaccine.

Herpesvirus of turkeys (HVT) has been successfully used as a Marek's disease (MD) vaccine for more than 40 yr. Either alone (broiler chickens) or in combination with vaccines of other serotypes (broilers, broiler breeders, and layers), HVT is used worldwide. In recent years, several vector vaccines based on HVT (rHVT) have been developed. At present, there are both conventional HVT and rHVTs in the market, and it is unknown if all of them confer the same level of protection against MD. The objective of this study was to further characterize the protection conferred by two conventional HVTs (HVT-A and HVT-B) and three recombinant HVTs (rHVT-B, rHVT-C, and rHVT-D) against MD in broiler chickens. In a first study we evaluated the efficacy of two conventional HVTs (HVT-A and HVT-B) administered at different doses (475, 1500, and 4000 PFU) at day of age on the ability to protect against an early challenge with very virulent plus strain 645. In a second experiment we evaluated the protection ability of several HVTs (both conventional and recombinant) when administered in ovo at a dose of 1500 PFU using the same challenge model. Our results show that each HVT product is unique, regardless of being conventional or recombinant, in their ability to protect against MD and might require different PFUs to achieve its maximum efficacy. In Experiment 1, HVT-A at 4000 PFU conferred higher protection (protection index [PI] = 63) than any of the other vaccine protocols (PI ranging from 36 to 47). In Experiment 2, significant differences were found among vaccine protocols with PI varying from 66 (HVT-A) to 15 (rHVT-D). Our results show that each HVT is unique and age at vaccination and vaccine dose greatly affected vaccine efficacy. Furthermore, they highlight the need of following manufacturer's recommendations.