AN INTRANASAL CHALLENGE MODEL FOR TESTING JAPANESE ENCEPHALITIS VACCINES IN RHESUS MONKEYS

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 (ED 50 ) was determined to be 6.0 3 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 3 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. Japanese encephalitis (JE) is the leading cause of mos- quito-borne viral encephalitis in the world, accounting for approximately 35,000 cases and 10,000 deaths each year in Asia. The JE virus (JEV) is transmitted by culicine mosqui- toes through an epizootic cycle involving primarily pigs and birds. Most infections in humans are asymptomatic. It is es- timated that only one in 20 to one in 1,000 infections result in encephalitis. However, once encephalitis occurs, it is fatal in 25% of the cases with 50% of the survivors experiencing neurologic sequelae. 1-3