A Focus Assay Method for Japanese Encephalitis Virus Using Complement and Anti‐Virus Serum

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Abstract
A sensitive, quantitative, short‐time, and reproducible focus assay for Japanese encephalitis (JE) virus is described. After 2 or 3 days of incubation, the infected cells were treated with anti‐JE virus serum and complement, and subsequently stained with trypan blue; then clear foci were produced. This method made it easy to titrate the infectivities not only of all seven JE virus strains tested but also of West Nile (WN), Murray Valley encephalitis (MVE), and St. Louis encephalitis (SLE) viruses using hyperimmune anti‐JE virus serum for the latter. Moreover, even cell lines which hardly formed plaques by the agar overlay method easily produced foci within 2 or 3 days by this method.