Growth of serotypes I and II and variant strains of infectious bursal disease virus in Vero cells.

The growth of five strains of infectious bursal disease virus--three strains of serotype I (SAL, D-78, 2512), one of serotype II (OH), and one variant strain (Variant-A)--were compared in Vero and chicken embryo fibroblast (CEF) cell cultures in order to characterize the replication of different strains of IBDV in Vero cells. For all five virus strains, the latent period in Vero cells ranged from 12 to 18 hr, which was longer than the 4-to-6-hr latent period observed in CEF cultures for strains SAL, D-78, and OH. Virus strains SAL, D-78, and OH, which were examined in both Vero and CEF cultures, also had a more extensive maturation phase and higher yields of virus in Vero than in CEF cultures. Total titers of these viruses of 5.35 to 6.10 log10 TCID50/ml in CEFs occurred 24 to 30 hr postinoculation (PI), although the cytopathic effect (CPE) was not seen until 72 hr PI. By comparison, their total infectious virus titers of 6.85 to 8.35 log10 TCID50/ml in Vero cells occurred from 48 hr PI, coinciding with the appearance of CPE. The growth curve of Variant-A in Vero cells differed from the other viruses by showing steadily rising extracellular and cell-associated virus titers throughout the 72-hr observation period. Only very low titers of Variant-A were obtained in CEF cultures, and thus no growth curve in CEFs was performed.