Rickettsia rickettsii
One-Sentence Definition
Rickettsia rickettsii is an obligate intracellular alphaproteobacterium transmitted by tick bite, causing Rocky Mountain spotted fever — a vasculitis-prone illness requiring prompt doxycycline.
Taxonomy & Morphology
- Rickettsiales; small coccobacillus; grows in host cells only
- Related: R. typhi (murine typhus), Orientia tsutsugamushi (scrub typhus)
Virulence Highlights
- Endothelial infection → vasculitis, edema, hemorrhage
- Spread via tick saliva during feeding
Clinical Syndromes
- Fever, headache, rash (often starts peripherally; can be absent early)
- Severe: DIC, encephalitis, multiorgan failure
- History of tick exposure or travel — critical
Diagnosis
- Clinical suspicion + serology (paired IgG) — acute serology often negative early
- PCR on skin biopsy/blood in reference labs
- Do not withhold doxycycline waiting for labs
Treatment Notes (conceptual)
- Doxycycline first line (including children — benefit outweighs tooth staining risk in short course per guidelines)
- Chloramphenicol alternative where doxycycline unavailable
Related Notes
- Borrelia burgdorferi (another tick-borne spirochete) · MOC - Public Health & Epidemiology
- Serology in Clinical Microbiology · Adaptive Immunity
Active Recall
- Why treat before serology confirms?
- Pathologic target cell/tissue?
- Vector?