A plaque assay for malignant catarrhal fever virus and virus neutralizing activity.

A cell-free strain of malignant catarrhal fever virus which produced a readily recognizable cytopathic effect was obtained by serial passage of the virus in a rabbit kidney cell line. Plaque assay of the virus was more rapid and gave higher titres 11 days postinoculation than tube titration, but the latter advantage decreased with a longer incubation period. Plaques were clear with sharp edges and measured 0.5 to 2 mm in diameter after 15 days. A plaque neutralization test was developed and successfully employed for the titration of malignant catarrhal fever virus neutralizing activity in the sera and nasal secretions of blue wildebeest.