Pathogenicity and immunogenicity of infectious bursal disease virus

Studies were conducted on the Pathogenicity of recent infectious bursal disease virus (IBDV) isolates and on the use of day-of-age vaccination for this virus. Eight IBDV strains with unique reverse transcription polymerase chain reaction restriction fragment length Polymozphisms (RT/PCR-RFLP) patterns were used to challenge specific-pathogen-free (SPF) layer and commercial broiler chicks. All eight viruses were shown to replicate and were determined to be pathogenic in SPF chicks. Three strains were shown to replicate and produce bursal lesions in broilers demonstrating that the ability of these strains to cause disease in broilers is delayed but not prevented by IBDV vaccines currently used in the broiler-breeder flocks. The effects of day-of-age and four-weeks of age vaccination with IBDV vaccine stain 8903 in SPF layer chickens was investigated. Day-of-age vaccination was effective in inducing seroconversion but four weeks-of-age vaccination was not effective in eliciting an active immune response. The effects of day-of-age vaccination with IBDV vaccine strain 8903 on the protection of broiler chicks possessing maternal antibodies to IBDV against challenge at two days-of-age with IBDV variant strain Del-E was investigated. Enzyme-linked immunosorbent assay (ELISA) and virus neutralization (VN) antibody titers showed similar overall rates of decline in all groups and no seroconversion was detected indicating that the live variant IBD vaccine was ineffective in stimulating a primary serological response when administered to broilers in the presence of significant IBDV maternal antibody. The effect of day-of-age vaccination with IBDV alone or in combination with Marek's disease virus (MDV) in broiler chicks was investigated. Chicks were vaccinated with IBDV or MDV or dually vaccinated with IBDV + MDV. ELISA titers showed similar rates of decline among all groups. VN titers in the IBDV + MDV and control groups remained higher and declined more slowly, as compared to VN titers in the IBDV and the MDV vaccinated groups. Vaccination with MDV alone was associated with increased rates of VN IBDV antibody decline similar to those caused by vaccination with IBDV alone, while concurrent vaccination with IBDV + MDV was not associated with increased rates of VN IBDV antibody decline over that of non-vaccinated controls. IBDV vaccination at one day-of-age does not cause accelerated IBDV-specific maternal antibody decline but does appear to cause an accelerated decline in neutralizing IBDV-specific maternal antibody. Furthermore, vaccination with IBDV at one-day-of-age appeared to slow the accelerated rate of IBDV-neutralizing antibody decline caused by MDV vaccination.