Abstract
Background: Infectious bursal disease (IBD) caused by infectious bursal disease virus (IBDV) is an economically important worldwide disease of commercial poultry. Classic, variant and very virulent IBDVs were identified in different parts of the world. Even though classic and variant viruses can be identified based on nucleic acid sequence variation, very virulent viruses require pathogenicity testing to prove their identity. The present study describes identification and pathogenic effects of an reemergent Giza2008 vvIBDV-like strain, Egypt/IBDV/Behera2011, in specific pathogen free chicks.
Methods: Eighty, one-day old specific pathogen free (SPF) chicks were inoculated with 103 ID50/ml of the virus via oculo-nasal route. At specified intervals, 10 chicks were randomly selected, euthanized and then necropsied. IBDV strain Pathogenicity was evaluated by mortality percent, and bursa, spleen, and kidney /body weight ratio. Bursae were collected for histopathology and transmission electron microscope (TEM). IBD antigens were demonstrated by fluorescent antibody technique (FAT) and Agar gel immune-diffusion (AGID). Anti-IBDV antibodies were measured by ELISA.
Results: Twenty inoculated chicks died (25%), mortalities peaked 5 dpi (7.5%). Greater bursa/ body weight ratio was observed at 3, 7, and 10 dpi then decreased at 14, 21, and 28 dpi. Histopathologically, severe bursal tissue reaction was observed in inoculated chicks, with variable degree of damage. Mean Severity Index (MSI) ranged from 0.5 to 4. IBD viral particles were visualized by TEM. FAT and AGID confirmed the presence of IBDV. ELISA were positive for IBDV antibodies from 7 dpi, peaked 21 dpi then declined 28 dpi.
Conclusion: The local very virulent strain of IBDV (Egypt /IBDV/ Behera 2011) which was prevalent among broiler chickens in 2011 in EGYPT caused 100% morbidity and 25% mortality accompanied with severe tissue reaction suggestive for very virulent identity.

Key words: SPF chickens, immunosuppression, pathogenicity, vvIBDV