Rickettsia

Rickettsiae are small obligate, intracellular bacteria with small genomes owing to reductive evolution except for Coxiella burnetii, which has been cultured axenically. Most rickettsiae reside in an arthropod host during their ecologic cycle and are transmitted by tick or mite feeding or in louse or flea feces. Rickettsia, Orientia, Ehrlichia, Neoehrlichia, Anaplasma, Neorickettsia, Wolbachia, and Coxiella vary phenotypically and cause diseases such as Rocky Mountain spotted fever, epidemic typhus, murine typhus, scrub typhus, human monocytotropic ehrlichiosis, human granulocytotropic anaplasmosis, and Q fever. Because of high infectivity by stable aerosols, Rickettsia and Coxiella organisms are bioterror threats. Although treatable with doxycycline, clinical diagnoses are difficult, and there are no vaccines.