Immunogenicity and antigenicity of very virulent strains of infectious bursal disease viruses.

The immunogenicity and antigenicity of three very virulent (vv) strains of infectious bursal disease viruses (IBDVs) from Turkey (OA), Holland (HOL), and Taiwan (PT) were investigated, and their antigenic relationships with the American classic SAL, variant IN serotype 1, and serotype 2 OH IBDVs were studied. In the present study, the vvIBDVs were passaged eight times in chicken embryos and were characterized by reverse transcription/polymerase chain reaction-restriction fragment length polymorphism with restriction enzymes MboI and BstNI. The restriction fragment length polymorphism profile patterns of these viruses did not change after eight passages in chicken embryos. Attempts were unsuccessful in adapting the vvIBDVs to replicate in vitro after six passages in primary cell cultures of chicken (chicken embryo fibroblast cells, chicken kidney embryo cells, and chicken embryo bursal cells), five passages in established cell lines (LSCC-RP9 and LSCC-RP12 B-lymphoblastoid cells, chicken macrophage [MQ-NCSU] cells, quail [QT35] cells), and 10 passages in vero and BGM-70 cells. Because it was not possible to adapt the vvIBDVs to cell culture, the antigenic relatednesses between these viruses and the American strains were established by cross-neutralization tests in chicken embryos, and the relatedness values were calculated with the Archetti and Horsfall formula. The results revealed that the vvIBDVs are antigenically related to the classic SAL strain but are antigenically different from the variant IN strain and serotype 2 OH virus. For the immunogenicity study, inactivated vaccines prepared from the vvIBDVs and containing 10(5) mean embryo lethal dose of each virus induced different levels of protection. The vaccines provided full protection against the classic STC strain but provided no protection against the variant IN strain at 5 and 10 days postchallenge.