Epidemiological Studies On Pathogenicity Of Infectious Bursal Disease Virus Field Strains In China

Pathogenicity of 31 field strains of infections bursal disease viruses(IBDV) isolated during 1997-2001 in China was compared in SPF chickens at different ages. The results indicated that the mortality induced by different strains varied very much. The mortality was also influenced by the age of chickens when infected. Chickens at 4 weeks of age were the most sensitive. Among 31 strains, GX-8/99 was the most virulent, which could cause as high mortality as 70%-94% in the relatively wide ranges of ages. In contract to GX-8/99, another strain YN2/99 caused no mortality. The left strains induced mortality in the ranges of 6%-77%. However, all strains could cause severe atrophy of the Bursa and thymuses, no matter they caused high or low mortality. The results also indicated that the virulence of the most IBDV field collected in recent years was between the typical very virulent and standard virulent strains.A very virulent strain GX8/99 of IBDV was studied for its mortality in SPF chickens and commercial egg-type chickens at different age inoculated with bursal suspension of 20-2000 ELD50 in repeated experiments. It demonstrated that GX8/99 could induce high mortality between 55%-92% at the age of 4-13 weeks. It still caused some mortality of 10%-15% at the age of 120 days, but did not induce any mortality after 128 days of age. In commercial egg-type chickens of 4-5 weeks, GX8/99 of 2000 or 200 ELD50 caused mortality between 35%-94% from frock to frock. In 2 week-old commercial egg-type chickens with certain some level of maternal antibody to IBDV, the mortality was only 10%. On the other side, the replication ability of GX8/99 in embryoo increased when it was passed in embryoo for 2 passages. Its ELD50 would be 10 95/0.1ml. But the Pathogenicity in chickens is becoming lower.From field strains of IBDV isolated in China during 1997-1999 were compared for their pathogenity and sequences in high viriable (hv) region of VP2 genes. It was indicated that GX8/99 was a real very vilulant IBDV strain(vvIBDV), pathogenicity of other 3 strains was not as high as the typical wIBDV but still muck higher than reguler standard vIBDV strains. The hv region of VP2 of the 4 strains have a very high homology with wIBDV reference strain HK46, 96.8-99.5% at DNA level and 96.6%-100% at amino acid level, but a lower homology with vaccine strain D78, as only 91.7%-93.6% or 91.8-93.2% at DNA level or amino acid level respectively. It seems like that there was some relationship between hv region and puthogenicity. Relatively, the new wIBDV strains GX8/99 had less homology to wIBDV reference strain HK46 and other 3 field strains as 96.8%-97.2% at DNA or aa levels, than the homology among HK46 and 3 strains, strains GX8/99 more than 98.4%-98.6% at DNA or aa levels. There was a 100% homology which was among SD-3/98, JS30/99 and HK46 for the hv region of VP2. It was demoastrated that the new wIBDV strains GX8/99 had changed in both pathogenicity and VP2 gene.To understand of the current commercial vaccine could also protect chickens from wIBDV challenges, two vaccines A and B were tested and compared for their protective immune efficacy in SPF chickens against wIBDV strain GX8/99. The results indicated that both commercial vaccines protected mothernal antibody-free chickens very effectively against GX8/99 challenge. But both vaccine theselves also incuce atrophy of the Bursa and thymuses.Many reports has been demonstrated that the sub-clinic infections of immunosuppressive viruses in chickens are very common. And immunosuppression and other pathogenic changes could become very clear and severe only when dual or multiple viral infections and their interactions happened. In this study work, we tested the roles of multipleimmunosuppressive viral infection(as MDV. REV and CAV) in pathogenesisin SPF chickens. But it was only a primary tries, the results were not conclusive.