What are the symptoms and treatment options for a Shigella infection?

Shigella Causes Bacillary Dysentery with Bloody Diarrhea, Fever, and Abdominal Cramps

Shigella infection causes an illness of variable severity characterized by diarrhea, fever, nausea, cramps, and tenesmus, with the hallmark presentation being bacillary dysentery (frequent scant bloody stools, fever, abdominal cramps, and tenesmus). 1

Clinical Manifestations

Primary Symptoms

• Bloody diarrhea (dysentery) is the classic presentation, though not all Shigella infections present with visible blood 1
• Fever is common and often documented in medical settings 1
• Abdominal pain and cramps are prominent features 1
• Tenesmus (painful straining during bowel movements) is characteristic 1
• Nausea frequently accompanies the gastrointestinal symptoms 1

Important Clinical Caveat

• Asymptomatic infections may occur, meaning not all infected individuals develop symptoms 1
• Dysentery identifies only 1.9-85.9% of confirmed Shigella infections, with sensitivity decreasing over time, meaning many Shigella cases present without bloody stool 2
• Shigella infection is associated with significant mortality (OR 2.8,95% CI 1.6-4.8), whereas dysentery presentation alone is not associated with mortality 2

Treatment Approach

When to Treat Empirically

Empiric antimicrobial therapy should be initiated for:

• Ill immunocompetent patients with fever documented in a medical setting, abdominal pain, bloody diarrhea, and bacillary dysentery presumptively due to Shigella 1
• Infants <3 months of age with suspicion of bacterial etiology 1
• Recent international travelers with body temperatures ≥38.5°C and/or signs of sepsis 1
• Immunocompromised patients with severe illness and bloody diarrhea 1

First-Line Antibiotic Therapy

For Adults:

• Fluoroquinolone (ciprofloxacin 500 mg PO twice daily or 400 mg IV twice daily) OR azithromycin 500 mg daily, depending on local susceptibility patterns and travel history 1
• The 2013 German guidelines recommend ciprofloxacin 400 mg bid IV or 500 mg bid PO, with azithromycin 500 mg daily as an alternative 1

For Children:

• Azithromycin, depending on local susceptibility patterns and travel history 1
• Third-generation cephalosporin for infants <3 months of age and those with neurologic involvement 1

Critical Resistance Considerations

• Fluoroquinolone resistance is 61.9% in some regions (particularly Bangladesh), making azithromycin increasingly important 3
• Trimethoprim-sulfamethoxazole resistance is 60.8%, ampicillin resistance is 34.5%, and ciprofloxacin resistance is 31.1% in endemic areas 3
• Multi-drug resistant Shigella exhibits a prevalence of 33.4% 3
• The choice of first-line drug should be based on knowledge of local susceptibility patterns 1

Older Treatment Regimens (Historical Context)

The 1992 CDC guidelines listed these options, though resistance has since increased 1:

• Ampicillin: Children 100 mg/kg/day in four divided doses for 5 days; Adults 500 mg four times daily for 5 days
• TMP-SMX: Children 10 mg/kg/day TMP and 50 mg/kg/day SMX in two divided doses for 5 days; Adults 160 mg TMP and 800 mg SMX twice daily for 5 days
• For resistant strains: Nalidixic acid 55 mg/kg/day in four divided doses for 5 days, or Tetracycline 50 mg/kg/day in four divided doses for 5 days

Treatment Monitoring

• If no clinical response occurs within 2 days, the antibiotic should be changed to another agent 1
• If no improvement occurs after an additional 2 days of treatment, refer for stool microscopy to rule out amebiasis 1
• At this stage, resistant shigellosis is still more likely than amebiasis 1

Diagnostic Approach

Laboratory Confirmation

• Isolation of Shigella from a clinical specimen is required for confirmed diagnosis 1
• Persons with bloody diarrhea should be treated initially for shigellosis if microscopy is unavailable or if definite Entamoeba histolytica trophozoites are not seen 1

Differential Diagnosis

• Stool specimens should be examined by microscopy to distinguish Shigella from Entamoeba histolytica when possible 1
• Care must be taken to distinguish large white cells (nonspecific indicator of dysentery) from amebic trophozoites 1

Transmission and Prevention

Key Transmission Facts

• Shigella requires very few organisms (10-100 particles) to establish infection, making it highly contagious 1
• Transmission occurs through contaminated food, water, or direct contact with infected individuals 1
• Food workers experiencing diarrhea should be prohibited from working 1

Prophylaxis

• Mass chemoprophylaxis is NOT recommended 1
• Asymptomatic contacts should NOT be offered empiric treatment but should follow infection prevention measures 1
• WHO does not recommend prophylaxis of family members as a control measure 1

Clinical Impact

The mortality associated with Shigella infection (OR 2.8) underscores the importance of appropriate antimicrobial therapy, particularly since many cases present without the classic dysentery picture. 2 This creates a clinical challenge: relying solely on bloody stool to identify and treat Shigella may miss opportunities to reduce mortality among children and vulnerable populations who present with non-dysenteric Shigella infection. 2