Raengsakulrach B, Nisalak A, Gettayacamin M, Thirawuth V, Young GD,
           Myint KS, Ferguson LM, Hoke CH Jr, Innis BL, Vaughn DW
           Department of Virology, Armed Forces Research Institute of Medical
           Sciences, Bangkok, Thailand.
           Am J Trop Med Hyg 1999 Mar;60(3):329-37

        Placebo-controlled field efficacy trials of new Japanese encephalitis
        (JE) vaccines may be impractical. Therefore, an animal model to evaluate
        efficacy of candidate JE vaccines is sought. Previous work has shown
        that exposure of monkeys to JE virus (JEV) via the intranasal route
        results in encephalitis. Here we report the further development of this
        model and the availability of titered virus stocks to assess the
        protective efficacy of JE vaccines. To determine the effective dose of
        our JE challenge virus, dilutions of a stock JEV (KE-93 isolate) were
        inoculated into four groups of three rhesus monkeys. A dose-dependent
        response was observed and the 50% effective dose (ED50) was determined
        to be 6.0 x 10(7) plaque forming units (pfu). Among animals that
        developed encephalitis, clinical signs occurred 9-14 days
        postinoculation. Infection with JEV was confirmed by detection of JEV in
        nervous tissues and IgM to JEV in the cerebrospinal fluid. Viremia with
        JEV was also detected intermittently throughout infection. Validation of
        the model was performed using a known effective JE vaccine and saline
        control. One ED90 of virus (2.0 x 10(9) pfu) was used as a challenge
        dose. Four of four animals that received saline control developed
        encephalitis while one of four monkeys administered the JE vaccine did
        so. This study demonstrates that the virus strain, route of inoculation,
        dose, and the outcome measure (encephalitis) are suitable for assessment
        of protective efficacy of candidate JE vaccines.