Isolation and characterization of attenuated plaque variants of infectious bursal disease virus

Attenuated plaque variants were obtained from infectious bursal disease virus adapted to chick embryo cell cultures. The large plaque (Lp) clone and the small plaque (Sp) clone formed homogeneous plaques about 5 and 1 mm in diameter, respectively. Neutralization tests indicated that these clones differed little from their parent strain in antigenicity. Sp clones showed a retarded growth rate in chick embryo cell cultures as compared with Lp clones. The clones were significantly less pathogenic for chick embryos than the parent strain, although Lp clones were more pathogenic than Sp clones, and they were much less pathogenic for 1-day-old chicks and 28-day-old chickens. Both clones had immunizing potency in 28-day-old chickens, although the Lp clone had a somewhat higher potency than the Sp clone. These findings suggest the Lp and Sp clones, in particular the Lp clones, to be useful as live virus vaccine strains.