## Rickettsial Organisms and Their Vectors **Key Point:** Rickettsia rickettsii is the causative agent of Rocky Mountain spotted fever (RMSF), transmitted by hard ticks (Dermacentor, Amblyomma, Ixodes species). ### Classification of Rickettsial Diseases | Disease | Organism | Vector | Geographic Distribution | Eschar | |---------|----------|--------|------------------------|--------| | **Rocky Mountain Spotted Fever** | *Rickettsia rickettsii* | Hard ticks (Dermacentor, Amblyomma, Ixodes) | North & South America | Absent | | **Scrub Typhus** | *Orientia tsutsugamushi* | Trombiculid mites (chiggers) | Asia-Pacific region | Present (eschar) | | **Epidemic Typhus** | *Rickettsia prowazekii* | Human body louse | Worldwide (overcrowded conditions) | Absent | | **Endemic Typhus** | *Rickettsia typhi* | Rat flea | Worldwide (urban/port areas) | Absent | **High-Yield:** RMSF is the most common rickettsial infection in North America. The classic triad is fever, rash (petechial, non-blanching, involves palms/soles), and history of tick exposure. **Mnemonic:** **RMSF = Rickettsia + Rocky + ticks** — Remember that *R. rickettsii* causes RMSF and is tick-borne; *O. tsutsugamushi* causes scrub typhus and is mite-borne. ### Clinical Features of RMSF 1. **Incubation period:** 2–14 days (usually 3–5 days) 2. **Rash:** Appears 2–6 days after fever onset; petechial, non-blanching, involves palms/soles (centripetal spread) 3. **Eschar:** NOT present (unlike scrub typhus) 4. **Complications:** Vasculitis → meningitis, myocarditis, acute kidney injury, DIC 5. **Mortality:** 1–5% if untreated; <1% if treated early **Clinical Pearl:** The absence of an eschar in RMSF is a key differentiating feature from scrub typhus. RMSF is more severe and has higher mortality if diagnosis is delayed. [cite:Harrison 21e Ch 175] 
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