Pathogenicity studies of an Arkansas variant infectious bursal disease virus

A variant infectious bursal disease virus (IBDV), IBDV-s977, was blind passaged in cell culture, plaque purified, and attenuated by serial passage at a high multiplicity of infection (MOI) in chick embryo fibroblasts (CEF). Cell culture passages of virus caused less bursal atrophy and splenomegaly than did the original isolate and retained immunogenicity; however, virus tended to persist for a longer time in the bursa and spleen of birds infected with the highest CEF passages. Antibody to both low MOI and high MOI passages of IBDV-s977 poorly neutralized virus that was isolated from bursal tissue 28 days postinfection (PI). The spleens of chickens infected with the eighteenth CEF passage were negative for virus at 3 and 7 days PI but had high titers of virus at 14 and 28 days PI. There was also more virus in the bursa of birds infected with the fifteenth and eighteenth CEF passages at 28 days PI than at 7 or 14 days PI. Defective interference (DI) was demonstrated when cell cultures were coinfected with a constant amount of low MOI virus and serial dilutions of high MOI virus. There was an increase in interference score with increased passage number in CEF, and there was more interference in virus passaged at a high MOI. There was an inverse relationship between interference score and bursal lesion score and splenomegaly at 7 days PI, indicating that DI particles may be involved in virus attenuation. There was a positive relationship between interference and viral persistence in the bursa and spleen at 28 days PI. Antiserum to s977 was shown to enhance the nonlytic replication of s977 in CEF, presumably within macrophages, providing a possible mechanism for the pathotypic variation seen in emerging strains of IBDV.