A 42-year-old woman from Virginia, USA, presents with a 4-day history of fever (38.8°C), severe headache, and myalgia. She reports a tick bite on her leg 1 week ago. On examination, she has petechial rash on her wrists and ankles that has now spread to involve the trunk and palms. Platelet count is 95,000/µL, and AST/ALT are mildly elevated. There is no eschar. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Rocky Mountain Spotted Fever (RMSF) ### Key Diagnostic Features **Key Point:** RMSF is characterized by a centrifugal rash (starting on wrists/ankles, spreading to trunk and palms) appearing 2–4 days after fever onset. The absence of an eschar distinguishes it from scrub typhus. ### Comparative Rickettsial Diseases | Feature | RMSF | Scrub Typhus | Ehrlichiosis | |---------|------|--------------|---------------| | **Causative agent** | *Rickettsia rickettsii* | *Orientia tsutsugamushi* | *Ehrlichia chaffeensis* | | **Vector** | Dermacentor tick | Trombiculid mite | Lone star tick | | **Eschar** | Absent | Present (50–80%) | Absent | | **Rash onset** | Day 2–4 | Day 4–6 | Day 4–8 (or absent) | | **Rash pattern** | Centrifugal (wrists/ankles → trunk) | Centripetal (trunk → extremities) | Maculopapular, trunk-sparing | | **Rash on palms/soles** | Yes (characteristic) | No | No | | **Lymphadenopathy** | Absent/mild | Marked | Mild | | **Thrombocytopenia** | Common | Common | Common | | **Geography** | USA, Americas | SE Asia, India | USA, Mexico | ### Clinical Pearl **Clinical Pearl:** The involvement of palms and soles with petechiae is highly specific for RMSF. This centrifugal (peripheral-to-central) spread is the opposite of scrub typhus. ### High-Yield Features in This Case **High-Yield:** - Tick bite history (Dermacentor tick in Virginia) - Fever + headache + myalgia (classic triad) - Petechial rash starting on wrists/ankles (centrifugal) - Rash involving palms (highly specific for RMSF) - **Absence of eschar** (rules out scrub typhus) - Thrombocytopenia and transaminitis - Rash appearance on day 4 (typical timing) ### Pathophysiology RMSF is caused by *Rickettsia rickettsii*, transmitted by Dermacentor ticks. The organism causes vasculitis of small and medium vessels, leading to the characteristic centrifugal rash and multi-organ involvement. ### Treatment Doxycycline 100 mg BD for 7 days. Early treatment (within 5 days of onset) significantly reduces mortality from ~20% to <5%. **Warning:** Do NOT wait for rash confirmation before starting doxycycline in suspected RMSF — delayed treatment increases mortality risk. [cite:Harrison 21e Ch 297]