Homologous interference by a foot-and-mouth disease virus strain attenuated for cattle.

An attenuated strain of foot-and-mouth disease virus (FMDV) of the A24 Cruzeiro subtype grew less well than wild-type virus in primary bovine fetal kidney (PBK) cells resulting in a 4-log lowered efficiency of plaque formation. Both wild-type and attenuated virus grew equally well in baby hamster kidney (BHK) cells and in suckling mice. Using PBK cells, virus-specific RNA of the wild-type accumulated up to 6 hours after infection. In contrast, PBK cells infected with the attenuated strain made less than 20 per cent of the RNA synthesized by wild-type virus. Infection of the cell with wild-type virus followed by superinfection with attenuated virus led to almost complete inhibition of viral RNA synthesis, an effect which is dependent on the concentration of input attenuated virus. Three subsequent undiluted successive passages of the attenuated virus resulted in a preparation which no longer interfered with wild-type RNA synthesis and which induced more of its own viral RNA synthesis in PBK cells. The basis of this interference was considered. Interference occurred intracellularly and was directed only against viruses within the genus FMDV. A role for interferon was ruled out. Attempts to demonstrate the physical presence of defective interfering (DI) particles of FMDV in the attenuated strain failed; but, cyclic patterns of infectivity were produced during successive undiluted passages.