A highly sensitive, broad-spectrum infectivity assay for infectious bursal disease virus

In order to develop an infectivity assay for infectious bursal disease virus (IBDV), an enzyme-linked immunosorbent assay (ELISA) for detecting IBDV antigens was developed using a rabbit antiserum to IBDV. The ELISA detected both serotypes 1 and 2 of IBDV, purified virus proteins at a concentration of about 0.1 ng/well and about 10(4) to 10(5) infectious units/well, and was about 10(3) times more sensitive than an agar gel precipitin test. By using the ELISA for detecting IBDV antigens in the cultured fluids, infectivity titration could be determined within 5 days of cultivation in seven of the 10 virus-cell combinations when chicken embryo fibroblasts (CEFs), BGM-70 cells, and LSCC-BK3 cells were used. IBDV antigens were not detected in three combinations remaining after 7 days of cultivation. When 18 IBDV strains, including highly virulent, classically virulent, and attenuated strains, were tested for growth in four types of cells using the ELISA, eight strains in CEFs, seven strains in chicken kidney cells, five strains in BGM-70 cells, and 17 strains in LSCC-BK3 cells were detected. These results indicated that LSCC-BK3 cells had the broadest spectrum for IBDV. When 26 field bursal homogenates were tested for infectious IBDV using LSCC-BK3 cells, all 19 field IBDV isolates that were detected by immunostaining of the cells were also detected by the ELISA. These results indicate that this infectivity assay for IBDV using LSCC-BK3 cells and the ELISA has a high sensitivity and a broad spectrum for IBDV and is especially useful for large-scale applications.