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 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 six genera (Rickettsia, Orientia, Ehrlichia, Anaplasma, Neorickettsia, 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 14 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 174-1).
Table 174-1 Features of Selected Rickettsial Infections |Favorite Table|Download (.pdf)
Table 174-1 Features of Selected Rickettsial Infections
|Disease||Organism||Transmission||Geographic Range||Incubation Period, Days||Duration, Days||Rash, %||Eschar, %||Lymphadenopathya|
|Rocky Mountain spotted fever||Rickettsia rickettsii||Tick bite: Dermacentor andersoni, D. variabilis||United States||2–14||10–20||90||<1||+|
|Amblyomma cajennense, A. aureolatum||Central/South America|
|Rhipicephalus sanguineus||Mexico, Brazil, United States|
|Mediterranean spotted fever||R. conorii||Tick bite: R. sanguineus, R. pumilio||Southern Europe, Africa, Middle East, Central Asia||5–7||7–14||97||50||+|
|African tick-bite fever||R. africae||Tick bite: A. hebraeum, A. variegatum||Sub-Saharan Africa, West Indies||4–10||?||50||90||++++|
|Maculatum disease||R. parkeri||A. maculatum||United States, South America||2–10||?||88||94||++|
|Rickettsialpox||R. akari||Mite bite: Liponyssoides sanguineus||United States, Ukraine, Turkey, Mexico, Croatia||10–17||3–11||100||90||+++|
|Tick-borne lymphadenopathy||R. slovaca||Tick bite: Dermacentor marginatus, D. reticularis||Europe||7–9||17–180||5||100||++++|
|Flea-borne spotted fever||R. felis||Flea (mechanism undetermined): Ctenocephalides felis||Worldwide||8–16||8–16||80||15||—|
|Epidemic typhus||R. prowazekii||Louse feces: Pediculus humanus corporis, fleas and lice of flying squirrels, or recrudescence||Worldwide||7–14||10–18||80||None||—|
|Murine typhus||R. typhi||Flea feces: Xenopsylla cheopis, C. felis, others||Worldwide||8–16||9–18||80||None||—|
|Human monocytotropic ehrlichiosis||Ehrlichia chaffeensis||Tick bite: Amblyomma americanum, D. variabilis||United States||1–21||3–21||26||None||++|
|Ewingii ehrlichiosis||E. ewingii||Tick bite: A. americanum||United States||None|
|Human granulocytotropic anaplasmosis||Anaplasma phagocytophilum||Tick bite: Ixodes scapularis, I. ricinus, I. pacificus, I. persulcatus||United States, Europe, Asia||4–8||3–14||Rare||None||—...|
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