## Drug of Choice for Leptospirosis **Key Point:** Doxycycline is the first-line antibiotic for leptospirosis in the immune-competent host, particularly in the leptospiremic (first) phase of the disease. ### Mechanism of Action Doxycycline is a tetracycline antibiotic that inhibits bacterial protein synthesis by binding to the 30S ribosomal subunit. It achieves excellent tissue penetration and intracellular levels, making it effective against Leptospira species. ### Treatment Regimen - **Dose:** 100 mg twice daily for 7 days (leptospiremic phase) - **Timing:** Most effective when started within the first 5–7 days of illness - **Efficacy:** Reduces fever duration and prevents progression to Weil's disease ### Comparative Drug Efficacy | Drug | Phase of Use | Indication | Notes | |------|-------------|-----------|-------| | **Doxycycline** | Leptospiremic (early) | First-line, immune-competent | Oral, excellent bioavailability | | **Penicillin G** | Immune phase (late) | Severe/Weil's disease | IV only, used in hospitalized patients | | **Ceftriaxone** | Immune phase (late) | Severe leptospirosis | IV, alternative to penicillin | | **Chloramphenicol** | Immune phase (late) | Rare; historical use | Bone marrow toxicity limits use | **High-Yield:** Doxycycline prophylaxis (200 mg once weekly) is also recommended for high-risk individuals (farmers, sewer workers, military) in endemic areas. **Clinical Pearl:** The biphasic nature of leptospirosis is crucial: early leptospiremic phase (days 1–7) responds to doxycycline; late immune phase (after day 7) requires penicillin G or cephalosporins for meningitis/renal involvement. **Warning:** Doxycycline is contraindicated in pregnancy and children <8 years; use penicillin G or ceftriaxone instead in these populations.
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