What is the drug of choice for treating bacillary dysentery (Shigella infection) in a healthy adult?

Drug of Choice for Bacillary Dysentery

The choice of first-line antibiotic for bacillary dysentery (shigellosis) must be based on local antibiotic susceptibility patterns, but when susceptibility is unknown or strains are sensitive, ampicillin (500 mg four times daily for 5 days) or TMP-SMX (160 mg TMP/800 mg SMX twice daily for 5 days) are the traditional first-line agents for adults. However, given widespread multidrug resistance, azithromycin or ciprofloxacin are now preferred empiric choices in most settings 1, 2.

Treatment Algorithm

First-Line Options (in order of preference based on current resistance patterns):

For healthy adults with suspected shigellosis:

• Azithromycin: Single dose 1,000 mg for dysentery 3, 2

  • Preferred in areas with high fluoroquinolone resistance
  • Effective against most resistant strains
  • Better safety profile than fluoroquinolones for cardiac risk

• Ciprofloxacin: 500 mg twice daily for 3 days 2, 3

  • Critical caveat: Avoid if ciprofloxacin MIC ≥0.12 μg/mL, even if reported as "susceptible" 2
  • Increasingly ineffective due to rising resistance, particularly against S. dysenteriae type 1

• Traditional agents (if susceptible): 1

  • Ampicillin: 500 mg four times daily for 5 days
  • TMP-SMX: 160 mg TMP/800 mg SMX twice daily for 5 days

Second-Line Options (for resistant strains):

• Ceftriaxone 2, 4
• Nalidixic acid: 55 mg/kg/day in four divided doses for 5 days 1
• Tetracycline: 50 mg/kg/day in four divided doses for 5 days 1

Clinical Decision Points

Assess response at 48 hours: If no clinical improvement after 2 days, switch to an alternative antibiotic from the recommended list 1. The guidelines emphasize that resistant shigellosis is still more likely than amebiasis at this stage.

If still no improvement after additional 2 days: Refer for stool microscopy to rule out amebiasis (look for Entamoeba histolytica trophozoites, not just white blood cells) 1.

Critical Considerations

Resistance patterns are the key determinant: The evidence consistently emphasizes that multiresistant Shigella strains have become widespread globally 1, 5, 6. Periodic antibiotic susceptibility testing by regional reference laboratories is essential 1.

Geographic variation matters: Fluoroquinolone resistance is particularly problematic in certain regions, and S. dysenteriae type 1 tends to be multidrug-resistant and causes more severe disease with complications like hemolytic-uremic syndrome 5, 6.

Avoid mass prophylaxis: WHO explicitly does not recommend prophylaxis of family members or mass prophylaxis as control measures 1.

Pitfalls to Avoid

1. Do not use fluoroquinolones blindly: Even laboratory-reported "susceptible" isolates with ciprofloxacin MIC ≥0.12 μg/mL should not be treated with fluoroquinolones 2

2. Do not confuse white blood cells with amebic trophozoites: Amebic dysentery is frequently misdiagnosed; large white cells are nonspecific indicators of dysentery, not proof of amebiasis 1

3. Do not use antimotility agents: These are contraindicated in shigellosis 6

4. Do not withhold antibiotics: Unlike many diarrheal illnesses, antimicrobial therapy is the mainstay of treatment for all cases of shigellosis, as it decreases severity, duration, and pathogen excretion 1, 7

Supportive Care

Concurrent oral rehydration therapy should be given, though dehydration is typically not severe in shigellosis 8. Early refeeding is emphasized 1, 8.