Pathogenicity of cell culture-derived and bursa-derived infectious bursal disease viruses in specific-pathogen-free chickens

That passage of infectious bursa] disease virus (IBDV) 30 and 40 times in an established cell line (BGM-70) resulted in loss of pathogenicity has been reported; however, both viruses maintained antigenicity and immunogenicity. That the passaged virus might have lost some ability to replicate in the natural host, resulting in lack of antigenic simulation and a poor immune response, was speculated. In this study, the pathogenicity and the replication of the serotype 1 variant IN strain were investigated in specific-pathogen-free (SPF) chickens. The original bursa-derived virus was passaged 10 times (low passage [LP]) and 47 times (high passage [HP]) in BGM-70 cells. Two concentrations of the LP virus were used for inoculation of different groups of birds, high titer 1.5 X 10(5) 50% mean embryo infective dose (EID50)and low titer 1.5 X 10(2) EID50. Birds inoculated with the bursa-derived virus had significantly (P < 0.05) small bursas with severe inflammation and necrosis throughout the 21-day experimental period. Bursa and spleen/body weight ratios and bursal lesion scores of the birds inoculated with the LP and HP viruses were not significantly different from those of the uninoculated control group at any time postinoculation (PI). Virus replication in the bursa of Fabricius was investigated by virus isolation in SPF chicken embryos, immune electron microscopy (IEM), immunofluorescence (IF), and antigen-capture enzyme-linked immunosorbent assay (AC-ELISA). The virus was isolated from bursal tissues from birds inoculated with the bursa-derived virus from day 3 (first isolation attempt) through day 10 PI, and the highest virus concentration was detected at 3 days Pl. Low virus titer was detected only at 3 days PI in chicken embryos inoculated with the bursal homogenates from birds inoculated with the LP virus at a high dose or the HP virus, No virus was isolated from birds inoculated with the LP virus at a low dose or the uninoculated control group. The virus antigen was also detected in bursal tissues collected from birds inoculated with the bursa-derived virus at 3, 5, and 7 days PI by IEM and AC-ELISA and until day 10 PI by IF. No virus antigen was detected in bursal tissues collected from birds inoculated with the cell culture-adapted viruses by embryo inoculation, IEM, IF, or AC-ELISA, The virus or its RNA was detected in the bursal homogenates up to 21 days PI by, reverse transcriptase/ polymerase chain reaction. These results indicated that adaptation of IBDV to BGNI-70 cell culture resulted in a significant reduction in the ability of the virus to replicate in the bursa of Fabricius, and, consequently, no lesions were detected.