Stillbirth and abortion caused by Japanese encephalitis virus in swine inflict a great damage upon animal husbandry. The present experiment was carried out to induce disorders similar to them in small laboratory animals. Pregnant mice and hamsters were inoculated with the AT31 strain of the virus, which has a strong peripheral infectivity. The following results were obtained.
1) Stillbirth and abortion were induced in hamsters at a high frequency, but not in any mouse, at all.
2) The rate of occurrence of these disorders was the highest in animals inoculated at 5 days of pregnancy. The later the virus was inoculated into animals, for example, at 7 or 10 days of pregnancy, the lower became this rate.
3) Abnormal infants were classified into stillborn ones, week ones, and infants which became ill soon after birth.
4) Fetal infection was clearly recognized when animals were subjected to laparotomy immediately before parturition and examined for the numbers of dead embryos and dead fetuses in relation to the total number of implanted eggs.
5) The virus was demonstrated in those abnormal infants at a high frequency. It showed a high titer. It was detected not only from the brain but also from the rest of the body. The virus titer of the brain was at the same level as that of the rest of the body.
6) The typical histological changes of encephalitis were observed in the brains of infected fetuses and newborn infants.
These results indicate that a pregnant hamster can be used as a model system for studies on stillbirth and abortion caused by Japanese encephalitis virus in swine. Accordingly, they paved the way to evaluation of the preventive effect of vaccine.
