Antigenic Characterization and Pathogenicity Study of Recent Field Isolates of Infectious Bursal Disease Viruses
Antigenic Characterization and Pathogenicity Study of Recent Field Isolates of Infectious Bursal Disease Viruses
H.A. SULTANl, H.A. HUSSEIN2, and F.F. EL-KHAYAT3
ABSTRACT
Epidemiological investigation of severe outbreaks of infectious bursal disease (IBD) among vaccinated chicken flocks during the period 2002-2003 revealed that all IBDV outbreaks occurred at 18-33 days of age. 70% (21/30) of these outbreaks characterized by typical clinical signs and gross lesions. The mortality rates in these flocks ranged between 9.4-12.8 % (average 11.1%); 16.4-30 % (average 23.2 %) and 12-23.5 % (average 17.75 %) in commercial broilers. commercial layer replacement pullets and native Baladi varieties, respectively. On the other hand. 30% (9/30) of these outbreaks occurred at 18-21 days of age and the mortality rates ranged between 1.4-4.9% (average 3.15%) in commercial broilers and native Baladi varieties. IBDV was detected in 100% of the tested bursal homogenates from acutely affected birds using agar gel precipitation test (AGPT) and antigen capture- enzyme linked immunosorbent assay (AC-ELISA). Antigenic typing of IBDV from selected outbreaks were carried out by monoclonal antibodies-based AC-ELISA and the pathogenicity of some representative infectious bursal disease virus (IBDV) field isolates were studied. Of the selected 22 IBDV -positive bursal samples, 59.1% (13/22) were typed as classic IBDVs and 40.9% (9/22) were of variant IBDVs. The majority of IBDV variant antigens detected (89% Of IBDV variants) were related to IBDV Del/E variant strain and one sample (11% of IBDV variants) was related to RS593 strain. The pathogenicity study of representative IBDV field isolates in 49-day-old egg-type chickens revealed that the highest mortalities were 53.30% and 36.6% in birds infected with the two field isolates which typed as classic IBDVs. Whereas. in birds infected with three variants IBDVs related to Del/E (two) and RS593 (one). the mortalities 10%, 13.3% and 10%, respectively. Moreover, the pathogenicity of such IBDV field isolates indicated that IBDV related to variants Del/E or RS593 caused severe bursal atrophy (BF) as judged by bursal index (BI) in comparison to the classic vvIBDV in susceptible infected chickens. However, the high incidence of variant IBDV antigens detected in tested samples indicates the existence of circulating variant IBDVs despite of vaccination of flocks with classical IBDV vaccines. Regardless of the antigenic differences detected by AC-EI.ISA and pathogenicity test in the representative isolates, they share antigenic sites with other serotype-1 IBDV strains. as they cross-reacted in AC-ELISA and AGPT. Although, the epidemiological investigation and antigenic typing by AC-CLISA test as well as pathogenicity study suggested that IBD field isolates are in the majority of highly virulent pathotype producing acute disease with severe clinical picture. The current study presents evidence of two variant isolates existing in commercial broiler and native Baladi farms with history of problems associated with IBDV.
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