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The human pathogens in the family Rickettsiaceae are small bacteria of the genera Rickettsia and Orientia. They are closely related to members of the family Anaplasmataceae that includes the genera Ehrlichia and Anaplasma. These organisms are obligate intracellular parasites that are transmitted to humans by arthropods. Many rickettsiae are transmitted transovarially in the arthropod, which serves as both vector and reservoir. Rickettsial infections, but not the ehrlichioses, typically are manifested by fever, rashes, and vasculitis. They are grouped on the basis of their clinical features, epidemiologic aspects, and immunologic characteristics (Table 26-1). Coxiella burnetii resides in the family Coxiellaceae and is more closely related to the genus Legionella; for convenience, it is discussed at the end of this chapter.

TABLE 26-1Rickettsial, Ehrlichial Diseases, and Q Fever

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