Antigenic characterization of an Arkansas isolate of infectious bursal disease virus.

The s977 strain of infectious bursal disease virus (IBDV) was isolated in northwest Arkansas in 1977 from the bursae of young broilers with high maternal antibody titers to the Moulthrop strain of IBDV (BursaVac). The comparison of a plaque-purified isolate of s977 with other IBDV serotype 1 and serotype 2 strains using virus neutralization indicates that s977 is a subtype of serotype 1 vaccine viruses and the MD variant strain of IBDV and has no relatedness to the Delaware Variant A (VarA) virus. In vivo cross-protection studies in specific-pathogen-free white leghorn chickens showed that an inactivated vaccine using s977 antigen was 2.5 times more protective against challenge with s977 than was an inactivated IBDV Variant E (VarE) vaccine. The vaccination of maternally immune broiler chicks with live s977 did not provide protection against subsequent challenge, indicating that s977 does not have enough antigenic difference to break through maternal immunity. Analysis of denatured viral polypeptides using polyacrylamide gel electrophoresis showed that s977 and two reported variant strains, 51 and VarE, share three protein bands, 90 kD (VP1), 40 kD (VP2), and 31 kD (VP3), that were not observed in BursaVac. BursaVac and s977 shared a 74 kD precursor band that was absent or very faint in the VarE and 51 strains. The most unique characteristic of s977 was the relative abundance of a wide, 56-63 kD band that contained two distinct immunoreactive bands when blotted with antiserum to s977. BursaVac contained a 56 kD band that failed to react with s977 antiserum. Analysis of polypeptide bands using laser densitometry indicated the presence of a number of bands between 20 kD and 25 kD in the s977, 51, and VarE preparations but only a 25 kD band in BursaVac. The number of bands decreased with the degree of relatedness to standard vaccine strains. It appears that, antigenically, S977 may hold an intermediate position between the classic virus strains and the more recently reported serotype 1 variants.