Antigenicity characterization of four representative natural reassortment IBDVs isolated from commercial three-yellow chickens from Southern China reveals different subtypes co-prevalent in the field

The antigenic relationships between the natural reassortment field strains of infectious bursal disease virus (IBDV), and between the field strains and the vaccine strains are poorly understood. In the present study, the antigenicity of four representative natural reassortment IBDV isolates designated JS7, GD10111, NN1005 and NN1172 from southern China during the years 2005-2011 and their antigenic relationship with the most commonly used vaccine strain B87 were investigated in vivo. For this purpose, cross-challenge studies were performed on 28-day-old birds, which were 2 weeks post-vaccination by oil-emulsion vaccines (OEVs) prepared from the four field viruses and B87, respectively. The protection related values (PRV) were evaluated based on the protection rate measured by clinical signs and mortality, bursa/body weight (B/BW) ratio and the viral load in the bursal samples at 3 and 7 days post challenge. As a result, the PRV showed that the isolates NN1172 and GD10111 belonged to the same antigenic subtype, while the isolates NN1005 and JS7 belong to another subtype. The vaccine strain B87 was grouped with the isolates NN1005 and JS7 but actually belongs to another small subgroup and provided only 60-80% protection against the challenge of the four field strains. The results demonstrated that different antigenic subtypes co-existed among the field natural reassortment IBDV strains and the commonly used vaccine strain B87 was antigenically different from the prevalent IBDVs in southern China.