Abstract
Field trials were carried out in 20, 425 birds on 10 farms to evaluate a lyophilized vaccine prepared from herpesvirus of turkey (HVT) in prophylactic efficacy against Marek's disease (MD), in comparison with cell-associated HVT vaccine. There was no significant difference (P>0.1) in protection rate between both vaccines. So the lyophilized vaccine was proved to be equivalent to the cell-associated in the efficacy. The chickens of one farm vaccinated with the lyophilized vaccine and those of another farm with the cell-associated vaccine were examined periodically for antibody by the agar-gel precipitin test (AGP). Antibody response due to vaccination could be detected on both farms when homologous (HVT) antigen was used, but not when heterologous (MDV) antigen was used. It was suggested that AGP might be useful for surveillance of field flocks after vaccination. Two groups of chickens with and without maternal antibody, respectively, were inoculated with the lyophilized vaccine and examined for appearance and persistence of antibody by both AGP and indirect immunofluorescent antibody technique (IFA). No significant differences were found (P>0.05) between both groups in the rate of chickens positive for AGP antibody after vaccination. A quantitative study by IFA, however, indicated that antibody production was delayed to a great extent in the group with a maternal antibody titer of 320 and higher, even if antibodies were derived from MDV-infection. This delay was overtaken by the 7th week postvaccination.
