What antibiotics are effective against Shigella infections?

Antibiotics Effective Against Shigella Infections

Ciprofloxacin is the preferred first-line antibiotic for treating Shigella infections in adults, dosed at 500mg twice daily for 3-7 days, based on CDC recommendations and FDA approval for this indication. 1, 2

First-Line Treatment Options

For Adults

• Ciprofloxacin remains the gold standard, with FDA approval specifically listing Shigella boydii, Shigella dysenteriae, Shigella flexneri, and Shigella sonnei as covered organisms 2
• Ciprofloxacin demonstrates superior clinical outcomes compared to older agents, with 95% clinical success rates and near-universal bacteriologic clearance 3
• TMP-SMX (trimethoprim-sulfamethoxazole) is an alternative if local susceptibility testing confirms the strain is susceptible, dosed at 160mg TMP/800mg SMX (one double-strength tablet) twice daily for 5 days 1, 4
• Azithromycin serves as an alternative for resistant strains, dosed at 500mg on day 1, then 250mg daily for 4 days, with 82% clinical success rates even against multidrug-resistant strains 1, 5

For Children (1-17 years)

• TMP-SMX at 10mg/kg/day TMP and 50mg/kg/day SMX in two divided doses for 5 days, if susceptibility is confirmed 1, 4
• Cefixime, ceftriaxone, or azithromycin are recommended by the American Academy of Pediatrics as alternatives 6
• Fluoroquinolones are not FDA-approved for children due to arthropathy concerns in weight-bearing joints, though they remain highly effective 2, 6

Critical Treatment Algorithm

Step 1: Initial antibiotic selection

• Base choice on local antibiotic susceptibility patterns when available 1
• For travel-acquired cases, assume high TMP-SMX resistance and start with ciprofloxacin 1
• When resistance patterns are unknown, fluoroquinolones are preferred 1

Step 2: Assess clinical response at 48 hours

• If no improvement in fever, stool frequency, or bloody diarrhea occurs within 2 days, switch to an alternative antibiotic 7, 1

Step 3: Reassess at 96 hours (4 days total)

• If still no improvement after the second antibiotic, refer for stool microscopy to rule out amebiasis 7
• At this stage, resistant shigellosis remains more likely than amebiasis 7

Historical Context and Resistance Patterns

• Ampicillin (100mg/kg/day in children, 500mg four times daily in adults for 5 days) was historically first-line but now has approximately 37% resistance rates in the US 7, 6
• Nalidixic acid (55mg/kg/day in four divided doses for 5 days) and tetracycline (50mg/kg/day in four divided doses for 5 days) are alternatives for strains resistant to ampicillin and TMP-SMX 7
• Multiresistant Shigella strains are now widespread globally, making empiric fluoroquinolone use increasingly necessary 1

Special Clinical Situations

Shigella dysenteriae type 1 infections:

• Single-dose ciprofloxacin therapy fails in 40% of cases; use at least 2 doses or standard 5-day therapy 8
• For non-type 1 Shigella species, single 1-gram doses of ciprofloxacin achieve 100% cure rates 8

HIV-infected patients:

• All Shigella infections require treatment due to higher complication risks 1
• Higher rates of adverse effects occur with TMP-SMX in this population 1
• Consider extending treatment duration to 14 days for bacteremia 1

Pregnant and lactating women:

• Fluoroquinolones are not approved for use 7
• TMP-SMX or azithromycin are safer alternatives if susceptibility permits 1

Common Pitfalls to Avoid

• Do not use mass prophylaxis or treat family contacts prophylactically—WHO explicitly recommends against this practice 7, 1
• Amebic dysentery is frequently misdiagnosed as shigellosis—if two different antibiotics fail, strongly consider amebiasis and perform stool microscopy for Entamoeba histolytica trophozoites 7, 1
• Do not rely on follow-up stool cultures unless the patient is a food service or healthcare worker 1
• Periodic susceptibility testing is essential in endemic areas, as Shigella rapidly acquires resistance 7, 1

Monitoring Treatment Response

• Clinical improvement should include resolution of fever, decreased stool frequency, and absence of blood in stools 1
• Bacteriologic clearance typically occurs by study day 2-3 with effective therapy 5, 3
• Ciprofloxacin-treated patients average 29 stools during treatment versus 46 with ampicillin, demonstrating faster symptom resolution 3