Identification of Ctenocephalides felis fleas as a host of Rickettsia felis, the agent of a spotted fever rickettsiosis in Yucatan, Mexico.

In search for the vector of the recently recognized spotted fever rickettsiosis of the Yucatán, ticks, fleas, and lice were collected from vegetation and dogs in localities where seropositive persons had been found. The arthropods were examined by polymerase chain reaction (PCR) using primers for the genus-specific 17-kDa protein gene followed by restriction fragment length polymorphism (RFLP) and DNA sequencing. Eleven (20%) of 54 pools of Ctenocephalides felis fleas contained DNA of Rickettsia felis. None of 219 Amblyomma cajennense, 474 Rhiphicephalus sanguineus, 258 Boophilus sp. ticks, and 33 Poliplax species lice contained DNA of Rickettsia. The identity of the rickettsial DNA was confirmed as R. felis by PCR/RFLP for the citrate synthase and outer membrane protein A genes and by DNA sequencing. The results indicate that the host of R. felis in Yucatán is C. felis and suggest that the spotted fever rickettsiosis that has infected >5% of the population of the Yucatán and can present as a dengue-like illness is likely to be caused by R. felis.

Identification of the target cells of Orientia tsutsugamushi in human cases of scrub typhus.

Orientia tsutsugamushi is the etiologic agent of scrub typhus, a chigger-borne zoonosis that is a highly prevalent, life-threatening illness of greatest public health importance in tropical Asia and the islands of the western Pacific Ocean. The target cell of this bacterium is poorly defined in humans. In this study, O. tsutsugamushi were identified by immunohistochemistry using a rabbit polyclonal antibody raised against O. tsutsugamushi Karp strain in paraffin-embedded archived autopsy tissues of three patients with clinical suspicion of scrub typhus who died during World War II and the Vietnam War. Rickettsiae were located in endothelial cells in all of the organs evaluated, namely heart, lung, brain, kidney, pancreas, and skin, and within cardiac muscle cells and in macrophages located in liver and spleen. Electron microscopy confirmed the location of rickettsiae in endothelium and cardiac myocytes.

Rickettsia felis: molecular characterization of a new member of the spotted fever group.

In this report, placement of Rickettsia felis in the spotted fever group (SFG) rather than the typhus group (TG) of Rickettsia is proposed. The organism, which was first observed in cat fleas (Ctenocephalides felis) by electron microscopy, has not yet been reported to have been cultivated reproducibly, thereby limiting the standard rickettsial typing by serological means. To overcome this challenge, several genes were selected as targets to be utilized for the classification of R. felis. DNA from cat fleas naturally infected with R. felis was amplified by PCR utilizing primer sets specific for the 190 kDa surface antigen (rOmpA) and 17 kDa antigen genes. The entire 5,513 bp rompA gene was sequenced, characterized and found to have several unique features when compared to the rompA genes of other Rickettsia. Phylogenetic analysis of the partial sequence of the 17 kDa antigen gene indicated that R. felis is less divergent from the SFG rickettsiae than from the TG rickettsiae. The data corroborate results from previous reports that analysed the citrate synthase, 16S rRNA, rompB (135 kDa surface antigen), metK, ftsY, polA and dnaE genes that placed R. felis as a member of the SFG. The organism is passed trans-stadially and transovarially, and infection in the cat flea has been observed in the midgut, tracheal matrix, muscle, hypodermis, ovaries and testes.

Phylogenetic analysis of the rompB genes of Rickettsia felis and Rickettsia prowazekii European-human and North American flying-squirrel strains.

The rickettsial outer membrane protein B (rompB) gene encodes the major surface antigens of Rickettsia species. We undertook sequencing and molecular analysis of the rompB gene of Rickettsia felis and a comparison with its homologs in spotted fever group (SFG) and typhus group (TG) rickettsiae, including the complete sequences of two North American flying squirrel strains and two European human strains of Rickettsia prowazekii. We sequenced 5,226 base pairs (bp) of the R. felis rompB, encoding a protein of 1,654 amino acids. We also sequenced 5,015 bp of rompB of the flying squirrel strains, encoding a protein of 1,643 amino acids. Analysis of the R. felis rompB gene sequence showed 10-13% divergence from SFG rickettsiae and 18% divergence from the TG rickettsiae. The rompB of all sequenced strains of R. prowazekii showed an overall similarity of 99.7-99.9%.