Experimental studies on vertical infection of mice with Japanese encephalitis virus III. Effect of gestation days at the time of inoculation on placental and fetal infection.

An experiment was carried out on the effect of gestation days at the time of inoculation on the establishment of experimental vertical infection with Japanese encephalitis virus in mice. In it, mice of the CFW strain in a closed colony were inoculated intravenously with a field strain at different times over a period from 3 to 12 days of gestation. After that, an attempt was made to recover the virus from the placenta and fetus to estimate the rate of infection. As a result, placental infection was established not when the virus was inoculated at 3 days of gestation, but when it was inoculated at 4 days of gestation or later. The rate of infection was relatively high when the virus was inoculated at 6 days of gestation or later. Fetal infection was established relatively frequently when the virus was inoculated some time between 7 and 10 days of gestation, but quite infrequently when the virus was inoculated at any other time than this. There was a difference in rate between placental and fetal infection at a given time of inoculation in days of gestation. This difference seemed to have been induced not by a difference in intensity of viremia appearing after inoculation, but by a difference in degree of development between placental and fetal tissues at the time of inoculation.