Molecular Typing of Field Isolates from two outbreaks of Infectious Bursal Disease Virus from Pakistan

A reverse transcriptase polymerase chain reaction restriction fragment length polymorphism (RT-PCR/RFLP) technique was used for the identification and characterization of Pakistani field isolates of infectious bursal disease virus (IBDV). A total of 8 bursa samples were collected from two outbreaks during September and October 2003 from Tehsil Sumandri, Dist. Faisalabad with 40-50% mortality in commercially reared broiler chicken flocks experiencing signs typical of infectious bursal disease (IBD). Four samples were found to contain IBDV genome by One Step RTPCR using VP2 gene specific primers. The assay amplified a 743 bp fragment from 701-1444 nucleotides. RT-PCR product was further subjected to restriction digestion using MboI and MvaI restriction enzymes. A third enzyme SspI was used to identify the very virulent phenotype. The RFLP profile was found similar for all four isolates with MvaI enzyme but different for one isolate when digested with MboI. All three MvaI-positive viruses were further found positive for SspI digestion and yielded RFLP profile similar to vvIBDV in Europe whereas one isolate was SspI negative and had a RFLP profile similar to classic IBDV strains. The clinical history of high mortality and SspI restriction enzyme positivity revealed that vvIBDV strains exist in Pakistan.