Persistent infections with "runde" virus in cell cultures and suckling mice

A morphologically intact, persistently infected BHK 21/c13 cell culture was established by serial trypsinization and reseeding of cells infected with approximately one baby mouse LD50/cell. The extracellular virus infectivity dropped more than 3 log10 units from the 1st to the 4th passage, and was then unaltered up to passage 20. In 2-week-old mice but not in baby mice, a chronic disease with virus persistence up to 150 days was produced after intracerebral infection with moderate virus doses. The humoral immune response of the animals seemed to be adequate by the methods employed. Development of defective interfering virus particles was indicated by the von Magnus phenomenon in BHK 21/c13 cultures infected with serially passed, undiluted virus samples. Intracerebral infection of baby mice with such samples resulted in transient paresis and motion disturbances instead of the usually fatal encephalitis. Specific virus antigens but no infectious virus could be recovered from such animals.