Shigella

Background

• Gram-negative bacillus within the family Enterobacteriaceae and order Enterobacterales
  • Most closely related to Escherichia coli
• Species include Shigella dysenteriae, Shigella sonnei, Shigella flexneri, and Shigella boydii
  • At least 40 serotypes
• Strains that encode Shiga toxin include Shigella dysenteriae serotype 1 (most common), Shigella flexneri type 2a, Shigella dysenteriae type 4, and Shigella sonnei
• Automated methods frequently misidentify as Escherichia coli, so needs phenotypic testing with serology
• Needs azithromycin susceptibility to be requested specifically, if it is resistant to others
  • However, there are no standardized breakpoints, so you have to use the epidemiologic cutoff values
• Extensively drug resistant (XDR) Shigella is resistant to ampicillin, fluoroquinolones, third-generation cephalosporins, azithromycin, and co-trimoxazole

Clinical Manifestations

• Most commonly causes self-limited diarrheal illness with watery or loose stools
• Disease may be more severe, including high fever, abdominal cramps, abdominal tenderness, tenesmus, mucoid stools, and hematochezia
• Complications of severe disease include bacteremia, pseudomembranous colitis, toxic megacolon, intestinal perforation, hemolysis, and hemolytic-uremic syndrome
  • Bacteremia more common in neonates and patients with malnutrition
• Serotype 1 is more likely to cause severe illness

Post-Infectious Complications

• Hemolytic-uremic syndrome
• Reactive arthritis
• Seizures

Diagnosis

• Molecular testing with PCR of stool is the most common method of diagnosing diarrheal illness
• Culture of stool or rectal swab
  • Mass spectrometry cannot reliably differentiate from Escherichia coli, so needs biochemical identification

Management

• Supportive care and oral rehydration
• Avoid antidiarrheal or antimotility agents, which can prolong the clinical course and increase risk of severe disease
• If severe disease or immunocompromised, should be treated empirically with antimicrobials
  • Ideally should be directed by susceptibility data
  • Azithromycin for 3 days or ceftriaxone for 2 to 5 days are good empiric options, though resistance is increasing
  • Other options include ciprofloxacin for 3 days, ampicillin for 5 days, or trimethoprim-sulfamethoxazole for 5 days
    • Avoid fluoroquinolones if MIC ≥0.12 μg/mL
    • Avoid amoxicillin, which may not be well-absorbed