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The rickettsiae are a heterogeneous group of small, obligately intracellular, gram-negative coccobacilli and short bacilli, most of which are transmitted by a tick, mite, flea, or louse vector. Except in the case of louse-borne typhus, humans are incidental hosts. Among rickettsiae, Coxiella burnetii, Rickettsia prowazekii, and R. typhi have the well-documented ability to survive for an extended period outside the reservoir or vector and to be extremely infectious: inhalation of a single Coxiella microorganism can cause pneumonia. High-level infectivity and severe illness after inhalation make R. prowazekii, R. rickettsii, R. typhi, R. conorii, and C. burnetii bioterrorism threats.

Clinical infections with rickettsiae can be classified according to (1) the taxonomy and diverse microbial characteristics of the agents, which belong to seven genera (Rickettsia, Orientia, Ehrlichia, Anaplasma, Neorickettsia, Candidatus Neoehrlichia, and Coxiella); (2) epidemiology; or (3) clinical manifestations. The clinical manifestations of all the acute presentations are similar during the first 5 days: fever, headache, and myalgias with or without nausea, vomiting, and cough. As the course progresses, clinical manifestations—including occurrence of a macular, maculopapular, or vesicular rash; eschar; pneumonitis; and meningoencephalitis—vary from one disease to another. Given the 15 etiologic agents with varied mechanisms of transmission, geographic distributions, and associated disease manifestations, the consideration of rickettsial diseases as a single entity poses complex challenges (Table 83-1).


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