Genetic, antigenic and pathogenic characterization of four infectious bursal disease virus isolates from China suggests continued evolution of very virulent viruses.

Infectious bursal disease virus (IBDV) causes an economically significant disease of young chickens worldwide. The emergence of very virulent IBDV (vvIBDV) strains has brought more challenges for effective prevention and control of this disease. The aim of the present study was to characterize four IBDV isolates from various regions of China between late 1990s and recent years and to compare them with previously isolated European IBDV strains. In this study, one Chinese vvIBDV strain isolated in 1999 and three strains isolated between 2005 and 2011 were analyzed at the genetic, antigenic and pathogenic levels. Strain SH99 was closely related and clustered in the same genetic lineage as the typical vvIBDV based on the genomic sequences of segments A and B. However, the three more recent Chinese vvIBDV (HLJ0504, HeB10 and HuN11) showed several genetic changes in both segments and clustered in a distinct lineage from the typical vvIBDV and the previously known Chinese vvIBDV. Based on the binding to a panel of neutralizing monoclonal antibodies in antigen capture enzyme-linked immunosorbent assays, all Chinese vvIBDVs exhibited similar antigenicity with the European typical vvIBDV strains. Nonetheless, the pathogenicity caused by the recent Chinese vvIBDV was higher than that induced by the European typical vvIBDV. This study calls for a sustained surveillance of IBD situation in China in order to support a better prevention and control of the disease.