Abstract

Three field strains of infectious bursal disease virus (IBDV) were isolated in 2015 from the bursa of Fabricius of affected broiler chickens originating from Moroccan broiler farms and were characterized as very virulent IBDV. Thus, the aim of this study was to investigate the pathogenicity of the above-identified IBD strains (accession number), MN241434.1, MN241436.1, and MN241439.1. In this trial, 75 three-week-old specific-pathogen-free chickens were randomly divided into four groups. Three groups of 1, 2, and 3 (n = 20 chicks/group) were inoculated via intra oculonasal route with 103.5 EID50 of MN241434.1, MN241436.1, and MN241439.1, respectively. Group 4 (n = 15) was kept as control. Chickens from all groups were monitored during 14 days post-challenge (dpc). Dead and euthanized birds were necropsied and their lymphoid organs (bursa, thymus, and spleen) were subjected to histopathological examination and lesion scoring. Serological response and presence of IBDV in the bursa of Fabricius were investigated using ELISA and rRT-PCR, respectively. Chickens in all challenged groups showed characteristic signs of IBD, including prostration, dehydration, aqueous diarrhea, and ruffled feathers, starting mostly at 2 dpc. The total mortality rate was 100% at 4 dpc in groups 1 and 2 and 76.47% in group 3. Bursa of Fabricius from affected birds revealed severe lymphoid depletion and necrosis with mean bursal lesion score between 3 and 5 (4 for MN241434.1 and MN241436.1, and 4.125 for MN241439.1). Similar severe lesions were observed in the spleen of all infected broiler chickens. Histological lesions in the thymus were detected from 2 dpc with individual cell necrosis. Serological results revealed that survived chickens in group 3 developed Ab anti-IBDV at 7, 9, and 14 dpc. RT-PCR confirmed the presence of the IBDV in BF of all infected chickens. These results indicated that the investigated local field IBDV strains genetically characterized as vvIBDV were phenotypically hypervirulent pathotypes of IBDV in SPF chickens hence reflecting the epidemiological Moroccan and regional IBD. This constitutes a potential candidate reference strains to be used in the development of strategies for the prevention and control of IBD in Morocco.