Infectious bursal disease virus: further characterization with evidence for a single-stranded RNA virus.

Infectious bursal disease virus (IBDV) that had been adapted to grow and was then cloned in chick embryo fibroblast (CEF) cell culture was examined for its physicochemical properties, the cellular site of virus replication, and the nature of its viral RNA. The IBDV was an RNA virus, acid-stable, absolutely resistant to chloroform, and moderately thermolabile. It appeared to replicate only in the cytoplasm, as shown by virus-specific antigens restricted to the cytoplasm of infected cells. The viral RNA was composed of single-stranded RNA, as evidenced by flame-red fluorescence on acridine-orange staining and an absence of specific fluorescence in infected cells on immunofluorescent staining with antiserum specific for double-stranded RNA. The IBDV virion had a hexagonal outline with an average diameter of 62 nm and possessed a single layer of capsid composed of hollow capsomeres without envelope. The buoyant density as determined in a continuous sucrose gradient was 1.178 g/cm3. The IBDV was found to possess morphologial and physicochemical properties different from those of any established RNA virus group.