Rocky Mountain Spotted Fever
Infectious Disease
Illness script · Infectious Disease
Rocky Mountain Spotted Fever
Tick-borne illness caused by Rickettsia rickettsii presenting with fever, headache, and centripetal rash involving palms and soles.
This illness script for Rocky Mountain Spotted Fever covers predisposing factors, classic presentation, mechanism, workup, management, and the clinical pivots that separate it from look-alikes—written for USMLE Step 1 and clerkship reasoning.
01
Predisposing factors
- Most common in South Atlantic/Southeast US (despite name); peak in spring–summer
- Children <15 and adults >40 most affected
- Exposure to Dermacentor variabilis (dog tick) or D. andersoni (wood tick)
- Outdoor activities, dog ownership increase tick exposure
- Tick attachment ≥6 hours typically required for transmission
02
Presentation
- Classic triad: high fever, severe headache, centripetal rash
- Rash begins day 2–5 as macular → petechial; starts wrists/ankles, spreads to trunk
- Palms and soles involvement is classic (seen in ~50–80%)
- Rash may be absent early ('spotless fever') — do not wait for rash to treat
- Nausea, myalgias, periorbital edema common; altered mental status in severe cases
- Tick bite history absent in up to 40% of patients
03
Pathophysiology
- R. rickettsii is an obligate intracellular bacterium that invades vascular endothelial cells
- Endothelial damage → increased vascular permeability, vasculitis, microvascular thrombosis
- Widespread small-vessel injury causes rash, edema, hyponatremia, and multi-organ failure
- Thrombocytopenia results from platelet consumption at sites of vascular injury
04
Diagnostics
- Clinical diagnosis drives treatment — never delay therapy awaiting confirmation
- Serology (IFA for anti-Rickettsia IgG) is gold standard but titers rise only in week 2–3
- PCR of skin biopsy or blood offers earlier confirmation but low sensitivity
- Labs: thrombocytopenia, elevated LFTs, hyponatremia (classic triad of labs)
- Skin biopsy immunohistochemistry confirms diagnosis but rarely changes acute management
05
Management
- Doxycycline 100 mg BID is first-line for ALL ages, including children — do not substitute
- Start empirically on clinical suspicion; delay increases mortality dramatically
- Treatment duration: 3 days after fever resolves (minimum 5–7 days total)
- Chloramphenicol is an alternative in true doxycycline allergy (less effective, more toxic)
- ICU admission for severe disease; case fatality rate ~20–25% if untreated vs <5% if treated early
06
Clinical pivots
How to separate this script from the look-alikes that show up on exams and on the wards.
Meningococcemia
Meningococcemia rash is purpuric/non-blanching from onset with septic shock; RMSF rash begins macular at wrists/ankles and evolves over days.
Ehrlichiosis/Anaplasmosis
Ehrlichiosis rarely causes rash ('spotless disease'); RMSF characteristically involves palms/soles with prominent rash.
Viral exanthem (e.g., enterovirus)
Viral exanthems lack the centripetal spread from extremities to trunk and do not involve palms/soles with thrombocytopenia and hyponatremia.
Immune thrombocytopenic purpura (ITP)
ITP is isolated thrombocytopenia without fever, headache, or tick exposure; RMSF has systemic vasculitis and constitutional symptoms.
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Educational use only. This illness script is a study framework, not medical advice. Confirm decisions with current guidelines and your clinical supervisors.