High Bacteremia and Fluoroquinolone Resistance in 2023 Philadelphia Shigellosis Outbreak

Overview

During a 2023 outbreak of Shigella flexneri in Philadelphia, 67 hospitalized patients were studied, revealing a 31.3% bacteremia rate and widespread resistance to fluoroquinolones and β-lactams. Trimethoprim-sulfamethoxazole remained the only reliably effective oral antibiotic. Most affected individuals experienced homelessness and opioid use disorder.

Background

Shigellosis, caused by Shigella species, is a fecal-oral transmitted infection leading to severe dysentery and potential complications such as organ failure and shock. In the U.S., S. flexneri and S. sonnei predominate, with outbreaks historically linked to contaminated sources, homelessness, and sexual transmission. Recent outbreaks have shown increased multidrug resistance and higher bacteremia rates, particularly among vulnerable populations such as people experiencing homelessness and those with opioid use disorder.

Data Highlights

Key Findings

• 31.3% of hospitalized patients with shigellosis had bacteremia, a rate much higher than historical norms.
• Nearly all cultured Shigella isolates were multidrug-resistant, with universal resistance to ampicillin and high resistance to fluoroquinolones and ceftriaxone.
• Trimethoprim-sulfamethoxazole was the only oral antibiotic with reliable susceptibility, showing only 10.4% resistance.
• The outbreak predominantly affected individuals experiencing homelessness (67%) and opioid use disorder (68.7%), with 76.1% of OUD patients reporting injection behavior.
• One death occurred, indicating a case fatality rate of 1.49% during this outbreak.
• Shigella flexneri type III was the predominant serotype identified (88% of isolates).

Clinical Implications

Clinicians should be aware of the high risk of invasive shigellosis with bacteremia among patients experiencing homelessness and opioid use disorder. Given the extensive resistance to fluoroquinolones and β-lactams, trimethoprim-sulfamethoxazole should be considered the preferred oral treatment option. Early recognition and appropriate antimicrobial therapy are critical to reduce morbidity and mortality in this vulnerable population.

Conclusion

This Philadelphia outbreak of multidrug-resistant Shigella flexneri type III demonstrated unprecedented bacteremia rates and significant antibiotic resistance, particularly among patients with homelessness and opioid use disorder. Trimethoprim-sulfamethoxazole remains the most effective oral treatment option in this setting.

References

1. Stedman et al. 2025 -- Elevated Incidence of Bacteremia and Resistance to Fluoroquinolones During a Shigellosis Outbreak in Philadelphia