Early posthatch protection against Marek's disease in chickens vaccinated in ovo with a CVI988 serotype 1 vaccine

CVI988, a serotype 1 Marek's disease virus (MDV), was used as an in ovo vaccine in specific-pathogen-free chickens to determine if this virus induces early posthatch protection against Marek's disease as has been shown previously for turkey herpesvirus. MDV CVI988 was injected at embryonation day (ED) 17 (group 1) or at hatch (group 2). A third group (group 3) was left unvaccinated. At 1, 2, 3, 4, 5, and 7 days of age, chickens from each group were sampled and examined as follows: a) single-cell suspensions of spleen were inoculated onto chicken embryo fibroblast monolayers to isolate the virus; b) sections of bursal tissues were stained by indirect immunofluorescence assays with anti-pp38 monoclonal antibody to identify viral antigen expression; and c) chickens were exposed intra-abdominally to MDV RB1B, a virulent serotype 1 MDV. Results revealed that in chickens given MDV CVI988 at ED 17, virus and virus-encoded protein were not detected until chickens were 3 and 2 days old after hatching, respectively. Results also indicated that during the first 4 days after hatch, the chickens given MDV CVI988 at ED 17 were better protected against virulent MDV than those given MDV CVI988 at hatch (P < or = 0.001). These results suggested that MDV CVI988 proteins were adequately expressed in the embryo to initiate prehatch immunologic response. Additional efforts with more sensitive techniques than used in this study are needed to identify the nature of viral expression in embryos.