Formation, characterization and interfering capacity of a small plaque mutant and of incomplete virus particles of infectious bursal disease virus.

Serial undiluted passages of infectious bursal disease virus in chick embryo cells were accompanied by a von Magnus type fluctuation of infectivity in viral harvests and a gradual decrease of plaque size. From the 9th undiluted passage on, the whole virus population consisted of small plaque-forming virus. The small plaque size remained constant when subsequent infections were carried out at low multiplicities. Small plaque virus interfered with the replication of large plaque standard virus. The small plaque/low yield mutation favoured the generation of defective particles which could be separated from complete particles by their lower densities in CsCl-gradients, where six fractions became visible and could be analyzed separately. Most of the defective virus particles had lost the larger of their two dsRNA segments and showed an aberrant protein composition. They had a very low residual infectivity and were also able to interfere with the replication of complete virus.