The use of polymerase chain reaction method for the detection of Rickettsia tsutsugamushi in wild rodents

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We studied the applicability of polymerase chain reaction (PCR) method for the detection of R. tsutsugamushi in wild rodents. The PCR method which amplified the gene coding for the group-specific antigen of R. tsutsugamushi was used in this study. Specific PCR products (88 bp) were obtained with the DNAs from three reference strains (Gilliam, Karp, and Kato) and two cell culture adapted field isolates (KN-1 and GJ-1). The minimum number detectable by the PCR method was estimated to be 1.3 copies of rickettsial genome. In a study with experimentally infected mice, the PCR method could detect rickettsial DNA in one of two infected mice at four months after inoculation. Thereafter, fifty five wild rodents were captured in five areas of Okayama Prefecture, and R. tsutsugamushi DNA was detected, by the PCR method, by amplifying DNA from the spleen of each rodent. The rickettsia was also isolated from the same rodents by the mouse inoculation method. By the PCR method, rickettsia DNAs could be detected in 12 of 13 rodents from which the rickettsiae were isolated, and in 10 of 42 rodents from which no rickettsiae were isolated. These findings indicate that the PCR method is a simple and specific procedure to detect R. tsutsugamushi in wild rodents. On the other hand, the results of the PCR method demonstrated that the middle area of Okayama Prefecture was highly (44-81%) contaminated with R. tsutsugamushi.