What is the significance of Shigella sonnei in urine?

Significance of Shigella sonnei in Urine

Finding Shigella sonnei in urine is clinically significant and represents a true urinary tract infection requiring appropriate antimicrobial therapy based on susceptibility testing. While Shigella species are primarily known as gastrointestinal pathogens, their presence in urine indicates a pathological process that requires treatment.

Pathophysiological Significance

• Unusual but documented pathogen: Shigella sonnei is not a common urinary pathogen but has been documented in several case reports as causing symptomatic urinary tract infections 1, 2, 3.
• Association with predisposing factors: Often associated with underlying urological abnormalities such as vesicoureteric reflux in pediatric patients 1, 3.
• Differentiation from contamination: Must be distinguished from asymptomatic bacteriuria or contamination by confirming the presence of pyuria and significant bacterial counts.

Diagnostic Considerations

• Quantitative culture threshold: Similar to other urinary pathogens, significant bacteriuria is defined as ≥50,000 CFUs/mL of a single pathogen 4.
• Pyuria requirement: The presence of pyuria (white blood cells in urine) is essential to distinguish true infection from asymptomatic bacteriuria 4.
• Proper specimen collection: Specimens should be processed promptly or refrigerated to prevent overgrowth of organisms at room temperature 4.

Clinical Implications

• Requires treatment: Unlike asymptomatic bacteriuria where treatment may do more harm than good, symptomatic Shigella sonnei UTI requires appropriate antimicrobial therapy 1, 3.
• Antimicrobial resistance concerns: S. sonnei has increasingly developed multidrug resistance, including resistance to ampicillin, cotrimoxazole, ciprofloxacin, and fluoroquinolones 3, 5, 6.
• Potential for complications: If left untreated, may lead to ascending infection and further complications, particularly in patients with anatomical abnormalities.

Treatment Approach

• Antimicrobial selection: Based on local susceptibility patterns and adjusted according to sensitivity testing of the isolated organism 4.
• Common effective agents:
  • Ampicillin (if susceptible): 500 mg four times daily for 5 days in adults 4
  • TMP-SMX (if susceptible): 160 mg TMP and 800 mg SMX twice daily for 5 days in adults 4
  • For resistant strains: Consider nalidixic acid, tetracycline, or fluoroquinolones based on susceptibility 4
• Treatment duration: 7-14 days of antimicrobial therapy is recommended for urinary tract infections 4.

Special Considerations

• Potential gastrointestinal source: Consider evaluating for concurrent or preceding gastrointestinal infection, as this may be the source of the urinary infection 3.
• Anatomical evaluation: In patients with Shigella sonnei UTI, consider evaluation for anatomical abnormalities such as vesicoureteric reflux, particularly in pediatric patients 1, 3.
• Antimicrobial stewardship: Due to increasing resistance patterns, susceptibility testing is crucial for guiding appropriate therapy 5, 6.

Prevention of Recurrence

• Address underlying conditions: Manage any anatomical abnormalities that may predispose to infection.
• Follow-up cultures: Consider follow-up urine cultures after completion of treatment to ensure eradication.
• Hygiene education: Provide education on proper hygiene practices to prevent fecal-urinary transmission.

Shigella sonnei in urine represents a true infection requiring appropriate diagnosis and treatment, particularly in patients with predisposing factors such as anatomical abnormalities of the urinary tract.