Effect of wide variation in the timing of vaccination relative to challenge on protection against MDV provided by Rispens CV1988 vaccine

As part of a broader experiment investigating interaction between vaccinal and wild-type serotype-1 MDV in co-infected hosts, we investigated the protection afforded by Rispens CVI988 vaccine in commercial IsaBrown layer chicks that were challenged with pathogenic MDV at intervals of -10, -5, 0, 5 and 10 days relative to vaccination, commencing at day 0 (day of hatch). Challenge was with 500 pfu of the isolate 02LAR and vaccination with 4000 pfu of commercial Rispens CV1988 vaccine. The 12 treatment combinations in the experiment (including controls) were replicated in separate isolators. A total of 24 isolators and 600 chicks were used. Chickens dying during the experiment or reaching the end of it at 56 days post challenge were examined for gross MD lesions and the vaccinal protective index calculated as a percentage. The incidence of MD ranged from 0-57.9% in challenged birds and the protective index for vaccination to challenge intervals of -10, -5, 0, 5 and 10 were 20.7%, -3.9%, 59.3%, 84.7% and 100% respectively. Not surprisingly vaccination after challenge provided no significant protection. Simultaneous challenge and vaccination did provide significant protection but protection was only optimal (100%) when challenge was 10 days after vaccination.