The Effect of Host Age,Virus Dose and Route of Inoculation on Inapparent Infection in Mice with Japanese Encephalitis Virus. ∗

Summary

The occurrence of inapparent infection or death due to Japanese encephalitis virus has been related to the dose and route of inoculation of virus in young and old mice. The virus strain employed, which had been isolated from naturally infected mosquitoes and used in low passage, predictably produced death on intracranial inoculation at high dilutions with only a rare mouse surviving to develop antibody. In contrast, virus inoculated intraperitoneally, subcutaneously, intravenously, or intradermally produced only occasional deaths and usually at low virus dilutions; yet very small inocula provoked hem-agglutination-inhibiting antibody responses which correlated well with neutralizing and complement-fixing antibodies. Intravenous administration of 10⁸ weanling mouse IC LD₅₀ of virus consistently produced about 75% mortality; intranasal injection of virus killed mice, whereas duodenal inoculation (during laparotomy) or pharyngeal administration did not. A single previous inapparent infection of mice inoculated into the duodenum conferred immunity to intravenous virus challenge 46-54 days later. Six-month-old mice were more resistant than young mice to peripheral as well as intracranial inoculation although infectivity was about the same in both groups. The mechanisms involved in the determination of apparency of infection which can be correlated with host age, virus dose, and route of inoculation have yet to be learned.