Viral encephalomyelitis of pigeons. V. Comparative studies of virulent and cell-culture-adapted virus.

Two strains of pigeon herpes encephalomyelitis virus BVC 78, a virulent field strain, and BVC 78 T7, a cell-culture-adapted strain, were assayed in embryonated chicken eggs and in chicken-embryo-fibroblast (CEF) cell culture to investigate their cytopathogenicity, growth kinetics, plaque characters, and virulence. In CEF the BVC 78 induced syncytia 24-48 hr postinoculation (PI), while the BVC 78 T7 induced only cell rounding 72 hr PI. The strains differed in growth patterns in CEF. The BVC 78 and BVC 78 T7 strains had respective logarithmic phases at 24 and 72 hr PI and maximum virus yields at 36 and 120 hr PI. The BVC 78 T7 was more cell-associated than the BVC 78, the ratio of cell-bound to cell-free virus was about 2 for the former and near unity for the latter. In CEF the BVC 78 induced plaques 1-2 mm in diameter by the third day PI, while the BVC 78 T7 produced small plaques 0.3-0.5 mm in diameter by the 5th day PI. The BVC T7 strain was of low killing capacity for chicken embryos, in contrast to the high killing capacity of the BVC 78. Pigeons inoculated subcutaneously with the former strain and challenged 21 days later with the virulent strain proved resistant.