Abstract
Stillbirth and abortion can be induced in hamsters at a high frequency by peripheral intracardiac inoculation with Japanese encephalitis virus. In the present experiment hamsters were conferred with immunity of varying degrees beforehand to examine the relationship between the degree of immunity and the frequency of occurrence of stillbirth and abortion. The results obtained are summarized as follows.
1. Hamsters immunized with live virus. (1) When inoculated with live virus, all the animals produced such a high titer of hemagglutination-inhibition antibody as 1:160-1:320. (2) In the immunized group, no animals (0/9) were involved in stillbirth or abortion when challenged by virus inoculation. The average number of young per litter was 8.6 In the unimmunized control group, stillborn and aborted young were seen in all the animals (9/9). The average number of young, including stillborn and aborted ones, per litter was 2.8.
2. Hamsters immunized with inactivated virus. (1) When three different concentrations of vaccine, high, moderate, and low, were used for immunization, antibody was produced positively in 12/12, 11/16, and 1/14 animals of the three groups, respectively. (2) The rate of occurrence of fetal infection by challenge inoculation was 0% (0/8), 55.6 (5/9), 100% (6/6), and 100% (6/6) in these three immunized groups and the control group, respectively.
3. Relationship between antibody titer and obstetrical disorders. Throughout the present experiment the rate of occurrence of stillbirth and abortion or fetal infection was 0% (0/11) in animals in which the neutralizing antibody titer (the rate of reduction of plaques formed in 1:20 dilution of serum) before challenge inoculation was more than 77, 50% (1/2) in animals in which the titer was 55, and 100% (10/10) in animals in which the titer was less than 47.
These results made it clear that no fetal infection could be established in an animal which had antibody in the blood to some extent.
