Efficacy of Various HVT Vaccines (Conventional and Recombinant) Against Marek's Disease in Broiler Chickens: Effect of Dose and Age of Vaccination

Abstract

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.

Genetic Heterogeneity among Chicken Infectious Anemia Viruses Detected in Italian Fowl

“…Vaccine persistence, circulation and reversion to virulence have also been frequently reported for other empirically attenuated avian vaccines developed to control immunosuppressive or respiratory diseases [52][53][54]. Next-generation vaccines, with improved stability and safety, are currently under study and development for most avian pathogens and, in the future, they will likely replace traditional vaccines [55][56][57][58].…”

Evaluation of Factors That Influence Dose Variability of Marek's Disease Vaccines

“…The requirement for licensing MD vaccines in the U.S. is a virus titer per dose of least 1,000 PFU [254]. However, most biologic manufacturers exceed this requirement as a precaution against lack of protective immunity, partly to counteract the reduction in the dose that occurs for the normally handling of the vaccine but also because every vaccine requires different levels of PFU to achieve the maximum protection [178,231]. Our results emphasize that MD vaccine doses should be given as a range of dose more than average PFUs, as dose variability will affect the dose of vaccines that chickens receive.…”

“…In the current study we have not evaluated the effect of a CV on protection but the effect of CV on the vaccine dose. However, the effect of vaccine dose on protection has been well studied [116,231]. While all the vaccines were above the required minimum PFU at maximum dose, 3 vaccines were below and 2 were at the minimum required level at the minimum dose.…”

“…The virus content assay and the titer used in the efficacy study will determine the recommended dose for each batch. In the USA, the minimum standard required dose is 1,000 PFU [254] but this does not guarantee protection when challenged with MDV of high virulence [116,231]. The vaccine seed virus' protection potential varies and must be determined using protection tests (50% protective dose test and protection index test).…”

“…route and age of vaccination, dose of vaccine administered and the combination of vaccines used[116,178,231]. Vaccine replication in vivo can be tested by obtaining samples of feather pulp at 1 week of age and testing using real time PCR[116].Factors influencing the strength of the immune responses elicited by the vaccine 3.3.1.…”