Study of Efficacy and Replication of Recombinant Vector Vaccines by Using Turkey Herpesvirus Combined with Other Marek's Disease Vaccines

Several recombinant turkey herpesviruses (rHVTs) have been developed within the past decades, and they are now used commercially worldwide. In broiler chickens, rHVTs are usually administered alone, but in long-living birds they are used in combination with Marek's disease (MD) vaccines of other serotypes (i.e., CVI988). The objectives of this work were to 1) evaluate protection against MD conferred by HVT and two rHVTs when combined with CVI988 and 2) optimize the use of rHVT in combination with CVI988 to maximize replication of rHVT without compromising MD protection. Various vaccine protocols, all using rHVT or HVT at the recommended dose (RD), were evaluated. Protocols evaluated included in ovo vaccination with HVT+CVI988 or rHVT+CVI988 (using either the double dose [DD] or the RD of CVI988), day of age vaccination of rHVT+CVI988 at DD, and revaccination protocols using rHVT in ovo followed by CVI988 at DD at day of age. Our results show that, when combined with CVI988, HVT and rHVTs confer a similar level of protection against MD (>90%) regardless of whether CVI988 was used at RD or at DD. However, the combination of rHVT with CVI988 at DD resulted in reduced replication rates of rHVT (60%-76% vs. 95%-100%). Our results show that such a negative effect could be avoided without jeopardizing MD protection by administering CVI988 at RD (if combined in ovo with rHVT) or administered rHVT first in ovo followed by CVI988 at DD at day of age.