Appearance of Persistently Cytopathic Eastern and Western Encephalitis Viruses after “Blind” Passage in Cultures of L Mouse Fibroblasts

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Abstract
Summary: Eastern and Western encephalitis viruses (EE and WE) were successfully passed serially in cultures of L mouse fibroblasts in 11 of 16 and 7 of 8 attempts, respectively. “Blind” passage of virus was often required for development of a persistently cytopathic effect since although mouse brain suspensions usually destroyed cells initially, cytonecrosis was commonly not evident during second to fourth serial passages even though mouse infectious virus was produced. Thereafter, each virus became persistently cytopathic and readily propagated through 7–20 (EE) and 7–37 (WE) serial passages. These findings emphasize the potential value of blind passage of specimens in cell cultures during attempts to isolate viruses from natural sources. Selection of cytopathic virus from the original mouse brain suspension is considered the most probable explanation of these observations, although other possibilities are discussed. The occasional failure of EE virus to propagate and the common disappearance of cytopathic virus effect during second to fourth serial passages of either virus remain unexplained; they seemed unrelated to the serum (rabbit, human, chicken or sheep) used to maintain cells, to duration of the interval between virus inoculation and harvest, and to autointerference. The latter phenomenon did not explain adequately the absence of definitive cytonecrosis during blind passages of virus, since interference could not be regularly produced upon simultaneous inoculation of L cell cultures with infectious and heat- or ultra-violet-inactivated WE virus.