Infectious bursal disease  conditions were observed in 15 commercial and 9 backyard broiler flocks in central area of Saudi Arabia during 2007-2008. The age of birds ranged from 2 to 8 weeks. The size of commercial flocks ranged from 5000 to 15000 birds and these flocks were vaccinated with classical strain of infectious bursal disease (IBD) vaccine at 14 days of age through drinking water. Number of birds in backyard flocks ranged from 200 to 300  and the vaccination programme of these birds was not known. High mortalities, respiratory symptoms, stunting and enlargement of bursa were seen in diseased birds of commercial flocks. Infectious bursal disease was suspected based on these clinical symptoms and postmortem findings, although these birds had been vaccinated against IBD virus. In order to confirm our diagnosis and to identify the causative agent, antigen capture-enzyme linked immunosorbent assay (AC-ELISA) was carried out on 142 bursal samples collected from diseased birds using kits containing monoclonal antibodies against variant strains of IBDV and 61.23% samples were found positive. It was observed that traditional vaccinal strains (54.02%) were significantly higher than less pathogenic strains  not used in vaccine preparation (29.89%) and non traditional highly pathogenic strains of IBDV (16.09%).  It was concluded that new variant strains of IBDV were detected in the samples in Saudi Arabia  and to our knowledge  this is the first  report  about the existence of these virus strains in commercial and backyard broiler flocks in this country.

Key words: IBD, IBDV, vvIBDV, antigen capture-ELISA.