Determination of virulence of JS strain of infectious bursal disease virus

The virulence of JS strain of infectious bursal disease virus(IBDV) was determined by measuring the virus median lethal dose(LD50) and median embryo lethal dose(ELD50) in 35-day-old SPF chickens and 10-day-old SPF chicken embryos respectively.The results showed that LD50 was about 10-3.6 0.1 mL per chicken and ELD50 was estimated at 10-5.2 0.1 mL per embryo.The pathogenicity was then studied in 1-to 6-week-old SPF and commercial chickens.The results showed that the pathogenicity rate was 100% in SPF and commercial chickens older than 3 weeks of age.The highest mortality rates were 93.75% and 75% in SPF and commercial chickens respectively.The VP2 gene of IBDV JS strain was amplified by RT-PCR technique and the sequence analysis showed that VP2 gene of IBDV JS strain had more than 99% homology to that of the very virulent strains HK46,OKYM and UK661.These results suggest that IBVD JS strain should be a very virulent strain.