A 28-year-old woman from the United States presents with fever, rash (petechial, involving palms and soles), and a history of tick exposure in a wooded area 5 days ago. Rickettsial serology is pending. What is the first-line treatment for suspected Rocky Mountain spotted fever (RMSF)?

Explanation

## Rocky Mountain Spotted Fever (RMSF) — First-Line Therapy ### Clinical Context **Key Point:** RMSF is caused by *Rickettsia rickettsii* and is a medical emergency; treatment must be initiated **empirically on clinical suspicion** — do NOT wait for serological confirmation. ### Why Doxycycline is First-Line | Criterion | Doxycycline | Ceftriaxone | Vancomycin | Chloramphenicol | |-----------|-------------|-------------|-----------|----------| | **Rickettsia coverage** | Excellent | None | None | Good | | **CNS penetration** | Moderate | Good | Excellent | Excellent | | **Oral bioavailability** | Excellent | N/A (IV only) | N/A (IV only) | Good | | **First-line status** | Yes | No | No | No (2nd-line) | | **Mortality if delayed** | Low | High | High | Moderate | | **Cost & availability** | Low | Moderate | High | Moderate | ### Treatment Regimen - **Doxycycline 100 mg orally/IV twice daily for 7–14 days** - **Defervescence:** 24–48 hours - **Efficacy:** >95% when started early - **Pediatric dosing (≥8 years):** 2.2 mg/kg twice daily (max 100 mg) **High-Yield:** RMSF is one of the few rickettsial infections where **early empiric doxycycline is life-saving**. Delayed treatment increases mortality from 20% to >70%. ### Why Other Agents Fail **Warning:** Beta-lactams (ceftriaxone, cephalosporins) are **NOT effective** against *Rickettsia* because rickettsiae are obligate intracellular pathogens; beta-lactams cannot penetrate the cell membrane. **Clinical Pearl:** Vancomycin and other gram-positive agents do not cover rickettsiae and are inappropriate monotherapy. If RMSF is suspected, doxycycline must be started immediately, even before test results. ### Exceptions & Alternatives - **Pregnancy:** Chloramphenicol 50 mg/kg/day (doxycycline is teratogenic) - **Doxycycline allergy:** Chloramphenicol (slower response, higher mortality) - **Severe CNS involvement:** Consider chloramphenicol for superior CNS penetration, but doxycycline is still preferred **Mnemonic:** **RMSF = Rickettsia = Rapid Doxycycline** — the faster you treat, the better the outcome. **Tip:** On NEET PG, if you see "tick exposure + petechial rash + palms/soles," immediately select **doxycycline**. Do not be distracted by beta-lactams or vancomycin — they will not work. [cite:Harrison 21e Ch 187]