Studies On Pathogenicity Of Very Vilulent Of Infections Bursal Disease Virus In SPF Chickens At Different Ages

Since 1987, vvIBDV which can cause high mortality was first reported in Europe, many kinds of vvIBDV were isolated all over the world. But there isn't an unified definition about vvIBDV. In common view, the IBDV strain which can cause sensitive chickens as high mortality as 50% or over 50% is considered as vvIBDV. During the age of 3-6 weeks, birds are most sensitive to vvIBDV. The mortality reduced significantly beyond this ages.This studies have provided reliable basic parameters for experiment on animals. 1. 《The study of different doses of vvIBDV challenged to different ages of SPF chickens》 ,A very virulent strain GX8/99 of IBDV were used to challenged different ages of SPF chickens with different doses. There are 26 experimental groups, at least 20 SPF chickens per group, age range from 1 to 240 days old. There are several repeated experiment groups at critical ages. Birds which alive after challenged were inoculated ND-Lasota for the study of immunosuppression by infectious bursal disease virus. 2.《Protective efficiency against vvIBDV GX8/99 after B87 vaccination》 ,to understand of the classical vaccine could also protect SPF chickens from vvIBDV ,14 days post IBD-B87 vaccination, SPF chicken were challenged to vvIBDV GX8/99.Following are results of study:1.Mortality rate and antibody positive rate of different ages of SPF chicken challenged by GX8/99.1-21 days old group: 500ELD₅₀/bird,number of died birds/number of challenged birds.1day old group, 13/20(65); 7 days old group, 4/20(20); 14 days old group, 13/20(65); 21 days old group, 16/30(53).The result indicated that vvIBDV also cause SPF chickens high incidence of disease and mortality atvery young ages.28-79 days old group: 500ELD50/bird, mortality:28 days old group,113/205(55.1);37 days old group,211/295(71.5);42 days old group,20/30(73);49 days old group,26/50(52.0);54 days old group, 129/200(64.5);66 days old group 39/58(67.24);72 days old group,20/20(100) ; 79 days old group,17/20(85).Obviously, ages make little to mortality during this period.128 days old group : 500ELD5o/bird,0/20,no illness,no death,; 100ELD50/bird,95% antibody positive; 50ELD50/bird,70% antibody positive.196 days old group : 5OOELD5O/bird, 0/20,no illness,no death,95% antibody positive; 25ELDso/bird,30% antibody positive.234 days old group : 5OOELD5o/bird, 15% antibody positive; 2000ELD50/bird,25% antibody positive.2.Immune organs/body weight ratios within 24-168-288h after challenged by GX8/99 strain in SPF chickens at 30 days old.The results show that vvIBDV could cause severe atrophy of bursa and thymuses of SPF chickens. The spleen could recovered gradually after 12 hours.3. Average time of death after challenged with 8/99strain;Challenged by 5OOELDso/bird,the average time of death of age 28,54 and 72 days old are 88.6h,77.0h and 101.Oh respectively. Challenged by 5OELD5o/bird,the average time of death of age 28,54 and 72 days old are 90.4h,110.0h and 137.0h respectively.4. Pathological organ changes rate of challenged died bird and body weight changes.vvIBDV could cause Immune organs pathological changes, the pathological change rate of bursa is more than 75%. Although there is no death when challenged after 128 days old, chickens lost weight.5. birds which alive after challenged inoculate ND-Lasota,HI were testedafter 14 days after inoculation. The result showed immunosuppression at the first inoculation. The ability of immune respond boosted at the second inoculation.6. Protective efficiency against wIBDV GX8/99 after B87 vaccinationTo make sure the classical IBDV vaccine B87 could also protect chicken from wIBDV challenged, 60 SPF chickens were vaccinated at age of 30 days old, and challenged with 500ELD50 GX8/99 14 days after vaccination. The result indicated that B87 vaccine protected SPF chicken very effectively against GX8/99 challenge.