A 28-year-old man with acute bacillary dysentery caused by Shigella flexneri presents with severe bloody diarrhea and fever. He has a history of anaphylaxis to penicillins. Which is the preferred antibiotic?

Explanation

## First-Line Therapy for Shigella in Penicillin-Allergic Patients **Key Point:** Ciprofloxacin remains the drug of choice for Shigella infection even in penicillin-allergic patients, as fluoroquinolones have no cross-reactivity with β-lactams. ### Antibiotic Selection in Penicillin Allergy **High-Yield:** The presence of penicillin allergy does NOT change the first-line agent for Shigella. Fluoroquinolones are safe alternatives because they belong to a completely different drug class with no cross-reactivity with β-lactams. | Drug | Cross-reactivity with PCN | Efficacy vs Shigella | Recommendation | |---|---|---|---| | Ciprofloxacin | None | Excellent | **First-line even in PCN allergy** | | Ceftriaxone | ~1–3% (minimal) | Good | Alternative if FQ resistance; caution in severe PCN allergy | | Chloramphenicol | None | Moderate | Rarely used; toxicity concerns | | Tetracycline | None | Poor (resistance) | Not recommended | **Clinical Pearl:** Fluoroquinolones are the safest choice in penicillin-allergic patients because they inhibit bacterial DNA gyrase—a mechanism completely independent of β-lactam action. There is no structural similarity, and cross-reactivity is virtually nonexistent. ### Why Ceftriaxone Is Not Preferred Here **Warning:** Although cephalosporins have low cross-reactivity with penicillins (~1–3%), they should be avoided in patients with a documented history of **anaphylaxis** to penicillins, as severe reactions are possible. Ceftriaxone is reserved for situations where fluoroquinolones are contraindicated or resistant. ### Why Chloramphenicol and Tetracycline Are Suboptimal - **Chloramphenicol:** No longer first-line due to rare but serious aplastic anemia risk; reserved for life-threatening infections when no alternatives exist - **Tetracycline:** Poor efficacy against modern Shigella strains due to widespread resistance; also contraindicated in children <8 years ### Dosing Ciprofloxacin in Adults - **Oral:** 500 mg twice daily for 3–5 days - **IV:** 400 mg twice daily (if unable to tolerate oral) - Achieves excellent concentrations in stool and intestinal mucosa **Mnemonic: FQ-SAFE** — Fluoroquinolones are Safe, Avoid β-lactams, Effective against resistant strains, and Excellent intracellular penetration. ### Allergy Documentation Matters **High-Yield:** The type of penicillin allergy is critical: - **Anaphylaxis, Stevens-Johnson syndrome, or severe rash:** Avoid all β-lactams; use fluoroquinolones - **Mild rash or GI upset:** Cephalosporins may be used with caution (low cross-reactivity) - **Delayed reaction:** Usually safe to use cephalosporins In this case, **anaphylaxis** mandates avoidance of all β-lactams, making ciprofloxacin the unambiguous choice. [cite:KD Tripathi 8e Ch 12]