Rise in Possible Fluoroquinolone-Resistant Shigella Isolates, CDC Warns

Megan Brooks

June 07, 2018

The US Centers for Disease Control and Prevention (CDC) continues to see an increasing number of Shigella isolates that test within the susceptible range for the fluoroquinolone antibiotic ciprofloxacin (minimum inhibitory concentration [MIC] values of 0.12-1 μg/mL) but harbor one or more resistance mechanisms, the agency said in a Health Alert Network (HAN) advisory today.

The advisory provides recommendations on managing and reporting of Shigella infections that have been treated with ciprofloxacin or azithromycin and resulted in possible clinical treatment failure.

It is a follow-up to a HAN advisory issued in April 2017 that spelled out recommendations for diagnosing and managing Shigella strains with possible reduced susceptibility to ciprofloxacin.  

The CDC says it "remains concerned about potential clinical failures with fluoroquinolone treatment."

The agency advises healthcare providers to carefully monitor patients with Shigella infections who require fluoroquinolone treatment and report any possible treatment failures. When a treatment failure is suspected, clinicians should submit a stool specimen for antimicrobial susceptibility testing and consider consulting an infectious disease specialist to identify best treatment options, the agency says.

The CDC is also seeing an increasing number of Shigella isolates with azithromycin MICs that exceed the epidemiologic cutoff value (ECV) and is asking for reports of any possible treatment failures occurring among patients with Shigella infections treated with azithromycin.

The CDC recommends that clinicians who identify or receive a report of a patient with Shigella infection and possible fluoroquinolone or azithromycin treatment failure do the following:

  • Consult an infectious disease specialist to identify other possible treatment; some Shigella isolates with susceptible ciprofloxacin MICs harbor one or more quinolone resistance mechanisms;

  • Contact the local health department to coordinate reporting treatment failure information to the CDC at EntericBacteria@cdc.gov;

  • Obtain a stool specimen for culture and submit for additional antimicrobial susceptibility testing; and

  • Request that the laboratory expedite submission of the Shigella isolate to the state public health laboratory, which should notify CDC at EntericBacteria@cdc.gov to coordinate additional laboratory testing and/or shipment of the isolate to CDC.

In April 2017, the CDC identified an increase in the proportion of Shigella isolates with MIC values of 0.12 to 1 μg/mL for ciprofloxacin. "This percentage continues to rise," the agency says.

Preliminary surveillance data from 2016 show that 8.2% of Shigella isolates tested by the National Antimicrobial Resistance Monitoring System laboratory had a ciprofloxacin MIC in the 0.12 to 1 μg/mL range, and 9.5% had an azithromycin MIC greater than the ECV (ie, non–wild-type; reduced susceptibility).

Testing of 2017 surveillance isolates is ongoing. Among those tested, 16.5% have a ciprofloxacin MIC in the 0.12 to 1 μg/mL range, and 22.1% have reduced susceptibility to azithromycin, the CDC reports.

The agency says it is particularly concerned about people who are at high risk for multidrug-resistant Shigella infections and are more likely to require antibiotic treatment, such as homosexual men, the homeless, and immunocompromised patients. These patients often have more severe disease, prolonged shedding, and recurrent infections, the CDC notes.

The CDC notes that, to date, they have not received any reports of clinical treatment failures in patients with Shigella infections. Therefore, it's unclear whether fluoroquinolone treatment of a Shigella infection with a ciprofloxacin MIC of 0.12 to 1 μg/mL is associated with a worse clinical outcome or whether such treatment increases the risk for transmission.

For now, current Clinical and Laboratory Standards Institute ciprofloxacin MIC breakpoints for the Enterobacteriaceae family (excluding Salmonella) will continue to apply to Shigella isolates, the CDC says.

"Shigellosis is a nationally notifiable condition; all cases should be reported to local health departments," the agency reminds providers in the advisory.

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