Very virulent infectious bursal disease virus (vvIBDV) in vaccinated broiler flock: Course of the disease, identification and characterisation of isolated strain

In a broiler farm with about 180.000 birds kept in 6 houses, birds were vaccinated against IBD via drinking water at day 17 of age, according to the advice of the hatchery, using TAD ® Gumboro Vac Forte (Lohmann Animal Health). At day 36 only in two houses (1 and 2) with 56.900 birds a sudden onset of depression, vent pecking, whitish diarrhoea accompanied with increase in mortality were observed. On necropsy, haemorrhages were found in the thigh and pectoral muscles. The BF appeared oedematous, hyperaemic, and had a gelatinous yellowish transudate covering the serosal surface. Trials of direct IBDV antigen detection using polyclonal antibodies in agar gel precipitation test revealed positive results. In chicken embryo fibroblasts IBDV was isolated in the first passage. The isolation in embryonated SPF-eggs was successful after the second passage. Further investigations using RT-PCR in combination with restriction enzyme analysis and sequences of variable region VP2 was carried out. The result revealed that isolated virus in tissue culture was classic virulent IBDV (Vaccinal strain). Examination of the bursa homogenate as well as the isolated virus in chicken embryo resulted in the detection of vvIBDV as well as the classic strain (Vaccinal strain). The total mortality rate at the time of slaughter (day 39) was 6.1% and 5.1% in house 1 and 2, respectively. The total mortality in the non-infected flock (house 3) was 1.9%. The IBD related mortality rates were 4.5% (house 1) and 2.9% (house 2). In house 1, where the highest mortality rate was observed, a slight reduction in daily weight gain as well as in production number was detected. The results described here indicate either a failure in the vaccination procedure in these houses or that the birds of these houses might originated from other breeder flocks with different maternal derived antibody titres.