Rickettsia and Orientia

Abstract

The family Rickettsiaceae comprises two genera of small, obligately intracellular bacteria that reside free within the host cell’s cytosol, namely, Rickettsia and Orientia. The genus is divided by the phylogenetic clustering of species into the typhus group (TG) and spotted fever group (SFG), defined originally by their distinctive lipopolysaccharide antigens, and the transitional and other basal groups that are widely distributed in arthropods. Among SFG and TG rickettsiae the genomes have remarkable synteny. Orientia resides free in the cytosol and is maintained in nature by transovarian transmission in trombiculid mites, which transmit the infection to humans during feeding at the larval stage. For immunohistologic detection of rickettsiae, the specimen can be snap-frozen for frozen sectioning or fixed in formaldehyde for the preparation of paraffin-embedded sections. Autopsy tissues can also be examined for rickettsiae by immunohistochemistry or polymerase chain reaction (PCR). The technique of in situ hybridization has been developed but has not been reported for the detection of rickettsiae in clinical samples. Antimicrobial susceptibility studies of rickettsiae are not routinely performed clinical laboratory tests. Detection of three or more rickettsiae in vascular endothelium in biopsy specimens or four or more rickettsiae in captured circulating endothelial cells is diagnostic of rickettsial infection.