Replication Kinetics and Shedding of vvMDV and Rispens CV1988 Vaccine Virus during Single and Mixed Infections

The effects of co-infections at various intervals with Rispens CVI988 vaccine virus and very virulent MDV on the replication and shedding of each virus were examined in 600 ISA Brown layer chickens in 24 isolators. Chickens were infected with Rispens and/or vvMDV isolate 02LAR on days 0, 5, or 10 post hatching providing vaccination to challenge intervals (VCI) of- 10, -5, 0, 5 or 10 days with the negative values indicating challenge prior to vaccination. Peripheral blood lymphocytes (PBL), feather tips and exhaust dust were sampled between 7 and 56 days post infection (dpi) and subjected to real-time quantitative PCR tests specific for each virus. Vaccination significantly reduced the load of vvMDV in PBL, feather tips and dust only when vaccination preceded challenge. Challenge with vvMDV had a similar effect on the Rispens load in PBL and feathers but not dust. VCI was strongly negatively associated vvMDV load and positively associated Rispens load. Differences between the two viruses were greatest in PBL and feathers, and least in dust. In single infections wMDV load was 2-3 logs greater than Rispens load but in mixed infections it was only higher when challenge preceded vaccination. A previous study showed that MDV challenge increased the shedding rate of HVT and MDV serotype 2 vaccine viruses but in the present study the reverse was true for Rispens. This study expands our understanding of the interaction between pathogenic and vaccinal viruses following vaccination with imperfect vaccines, and has implications for selection of samples to assess vaccination success.