Tissue Culture Studies of the Virus of Progressive Pneumonia, A Slow Infectious Disease of Sheep

SummaryProperties of progressive pneumonia virus (PPV) were studied in sheep lung cell monolayers. Growth studies revealed that, with a multiplicity of infection of 2 TCID50 virus/cell, virus replicated to a titer of 106.6 TCID50 virus/ml in 4 days. Typical polykaryocytes were first seen at 1 day post inoculation (PI) and became larger and more numerous as the virus titer increased. Inoculation of high concentrations of virus produced cell fusion within 3 hr PI. Indirect immunofluorescence revealed homogeneous cytoplasmic fluorescence in infected cells as early as 1 day PI. The number of cells showing fluorescence and the intensity of the fluorescence increased steadily with increase of virus titer. The genetic material for PPV is probably single-stranded RNA. The various properties indicate that PPV is closely related to maedi, zwoegerziekte, and visna viruses.