Detection of Rickettsia tsutsugamushi in experimentally infected mice by PCR

We developed a rapid procedure for the detection of Rickettsia tsutsugamushi DNA by the PCR technique. The primer pair used for the PCR was designed from the DNA sequence of the gene encoding a 120-kDa antigen, which was proven to be group specific by immunoblot analysis with mouse hyperimmune sera against various rickettsial strains. This PCR method was able to detect up to 10 ag of plasmid DNA (pKT12). Specific PCR products were obtained with DNAs from R. tsutsugamushi Kato, Karp, Gilliam, TA716, TA1817, and Boryong, but not with DNAs from other rickettsiae, such as R. prowazekii, R. typhi, R. akari, and strain TT118. In a study with experimentally infected mice, the PCR method could detect rickettsial DNA from 2 days after inoculation (DAI), whereas serum antibody against R. tsutsugamushi could be detected from 6 to 8 DAI by an immunofluorescence test. Although clinical manifestations subsided after 14 DAI, rickettsial DNA in blood samples could be detected by PCR for up to 64 DAI. These results suggest that this PCR method can be applied to the early diagnosis of scrub typhus and can also be used to detect the residual rickettsiae after clinical symptoms subside.