Attenuation of Infectious bursal disease virus X-28 Strain and its Pathogenicity

IBDV X-28 strain was attenuated virus strain, which was adapted in chicken's embryo fibroblasts (CEF) and had stabile pathogenicity. It was adapted through 17 times in Vero and 27 times in CEF. Cytopathic effect (CPE) of IBDV X-28IBDV X-43 were observed from 72 hour to 96 hours. TCID_(50)/0.1mL was between 10~(-8.47) and 10~(-9.26 )and EID_(50)/0.2mL was between 10~(-4.71) and 10~(-5.80). The results of pathogenicity tests showed that the virus of each generation could lead the embryo infection or death and the ratio of infection or mortality was from 40% to 50%. It has no obvious pathogenicity to 4-week-old SPF chickens and bursa were all no the visual pathologic change. Bursa and body ratio(b/B) was 0.350.040 in average. IBDV X-28 could not return to a virulent strain because it propagated 6 generation in SPF chickens,but did no harm to bursa and caused bursa-body ratio changes.