Characterization of variant infectious bursal disease virus from a broiler farm in Japan using immunized sentinel chickens.

We attempted the isolation of variant infectious bursal disease (IBD) viruses by using sentinel chickens immunized with inactivated classical-type IBD vaccine. Immunized sentinel chickens with high levels of neutralizing antibodies and non-immunized sentinel chickens were raised together with broiler chickens in a commercial farm. Severe atrophy of the bursa of Fabricius was observed from the second week after cohabitation in non-immunized sentinel chickens. However, in immunized sentinel chickens and broiler chickens, atrophy was observed from the third week after cohabitation. The IBD virus (IBDV) isolated from the bursa of Fabricius of immunized sentinel chickens, designated as strain IBDV TY2, showed severe atrophy of the bursa in infected SPF chickens. Antiserum to the IBDV TY2 strain showed higher neutralizing activity to heterologous IBDV strains than did antiserum to the K strain vaccine virus. Phylogenetic analysis revealed that the nucleotide sequences encoding the hypervariable region of virus protein 2 of the IBDV TY2 strain did not cluster with the classical, variant or very virulent IBDV groups. Based on these results, we suggest that the IBDV TY2 strain may constitute a novel variant type of IBDV.