Polymerase chain reaction (PCR) amplification of DNA was used to detect the etiologic agent of murine typhus, Rickettsia typhi, in experimentally infected adult fleas. A primer pair derived from the 17-kilodalton antigen sequence of typhus and spotted fever group rickettsiae was used to amplify a 434-base-pair (bp) fragment of the genome of the murine typhus rickettsiae. The amplified 17-kilodalton protein antigen-specific sequence was detected in ethidium bromide-stained agarose gels in individual fleas as early as 2 days after exposure to rickettsemic rats (two of six tested). The 434-bp sequence was not detected in uninfected control fleas. A dot hybridization assay used to detect the 434-bp fragment was also specific and about 100-fold more sensitive than the agarose gel PCR assay. Since the PCR assay employed a boiled extract of triturated fleas, both PCR and an antigen capture enzyme-linked immunosorbent assay (ELISA) could be performed on the same individual flea homogenate. The ELISA identified 12 infected fleas out of 29 randomly selected fleas, compared with 14 specimens which were positive by PCR. The PCR assay detected rickettsiae in samples in which no viable rickettsiae were detected by plaque assay. Like the ELISA, the PCR assay sensitivity was due in part to its suitability for detecting small numbers of both live and dead R. typhi in fleas.
