Complement-fixing antibody in sera of animals experimentally infected with R. sennetsu and of patients with sennetsu rickettsiosis found in Miyazaki district were detected by using soluble (S) and particulate (P) antigens, and following results were obtained.
1. Although R. sennetsu infected mice could not survive even after treatment with tetracycline, antibody titers were recognized 7 days after infection, reached the maximum in 3-4 weeks, and remained at high level after that. Titers against P antigen were somewhat higher than those against S antigen, but both patterns of rise and fall of the titers were almost same. In guinea pigs, antibodies were found 14 days after infection. The maximum titer was observed after 3-4 weeks. The titer declined gradually after that. In hamsters, titers against P antigen rose after 17 days and high titers lasted after that. However, those against the S antigen were temporarily recognized from 17 to 21 days after infection.
2. Sera from four patients had relatively high titers against P antigen. In three of 4 cases, antibody titers were demonstrated on the 12th day of illness when R. sennetsu was isolated from their venous blood. In three cases, antibody against S antigen was detected. In general, titers against P antigen were higher than those against S antigen.
