Replication kinetics, shedding, transmission and protective efficacy of Rispens/CVI988 vaccine virus in single and combined infections with very virulent Marek's disease virus

Abstract

The attenuated MDV-1 Rispens/CVI988 vaccine is widely used to vaccinate chickens worldwide and is the most effective current vaccine against MD. Two experiments were designed to investigate transmission of the Rispens/CVI988 vaccine virus (Rispens/CVI988) between chickens, the viral kinetics and shedding profile of Rispens/CVI988 and very virulent MDV (vvMDV, isolate 02LAR) in single and mixed infections and the effect of vaccination to challenge interval on the protection provided by Rispens/CVI988 against vvMDV challenge. Experiment 1 used 70 specific pathogen free (SPF) chickens in four climate controlled rooms. In three rooms, 10 chickens were vaccinated with one of the three commercial Rispens/CVI988 vaccines at day old and left in contact with 10 unvaccinated chickens. The fourth room held 10 unvaccinated control birds. As determined by MDV-specific qPCR of weekly room dust, PBL and feather samples the commercially available Rispens/CVI988 vaccine virus strains are shed in significant quantities and transmit effectively to in-contact chickens. Experiment 2 used 600 commercial ISA Brown layers of the same age (day old) in 24 isolators. Chickens were vaccinated with Rispens/CVI988 (3200 pfu) and/or challenged with the 02LAR (400 pfu) on days 0, 5, 10 post hatching providing vaccination to challenge intervals (VCI) of -10, -5, 0, 5, 10 days with the negative values indicating challenge prior to vaccination. As determined by MDV-specific qPCR able to differentiate between the viruses, vaccination with Rispens/CVI988 greatly reduced the viral load of vvMDV in PBL and feather cells but only if birds were vaccinated prior challenge.