Antigenic and sequence heterogeneity of infectious bursal disease virus strains isolated in Australia

Six recently isolated field strains of infectious bursal disease virus (IBDV) were compared to vaccine strains at the antigenic and genetic level to ascertain the level of heterogeneity among Australian IBDV strains. Five strains, 01/94, 02/95, 03/95, 04/95 and 08/95, isolated at four locations in the state of Victoria, were antigenic variants. They failed to react with monoclonal antibodies directed against two different epitopes on the VP2 protein which were present in vaccine strains and one field isolate (06/95) from the state of New South Wales. Serum neutralization tests confirmed that these strains were antigenic variants as they were of a different subtype to that of vaccine strains. Sequence comparison of the hypervariable region of the VP2 proteins showed that the five Victorian strains had between 13 and 16 amino acid substitutions in comparison with vaccine strains. Four to six of these substitutions were in the two hydrophilic domains previously identified as being of importance in the formation of protective virus neutralizing antibodies. Comparison of these five variants to those isolated previously in the USA revealed little similarity at both the antigenic and genetic level. Phylogenetic analysis showed that Australian IBDV strains belong to a separate and distinct genetic group which is considerably heterogeneous. Overall the results indicate that the current Australian IBDV situation resembles that seen in the USA, with the existence of classical and variant IBDV strains, but neither the classical nor the variant strains found in Australia are closely related to those prevalent in the USA.