Azithromycin Not Effective Against All Shigella Infections

Ricki Lewis, PhD

February 13, 2014

A team from the Centers for Disease Control and Prevention has tracked Shigella isolates that show decreased susceptibility to azithromycin (DSA-Shigella), according to a note published in the February 14 issue of the Morbidity and Mortality Weekly Report.

Antibiotic therapy is recommended for Shigella infection that is severe, causes bloody diarrhea, or occurs in immunocompromised individuals. Azithromycin is used to treat both children and adults because the bacterium is resistant to older antibiotics, but even azithromycin is not effective against all Shigella infections.

Because there are no clinical laboratory guidelines to identify susceptible strains of the bacteria, healthcare providers cannot identify those patients whose Shigella infections will likely not respond to azithromycin. To quantify the situation, the CDC's National Antimicrobial Resistance Monitoring Systems (NARMS) has measured DSA-Shigella using the azithromycin minimum inhibitory concentration in every twentieth isolate provided by public health laboratories and samples from outbreaks.

Katherine E. Heiman, MPH, from the Division of Foodborne, Waterborne, and Environmental Diseases, National Center for Emerging and Zoonotic Infectious Diseases, CDC, and colleagues have monitored DSA-Shigellasince 2011. They define decreased susceptibility to azithromycin as a minimum inhibitory concentration greater than 16 μg/mL.

NARMS testing provided 29 DSA-Shigella isolates, plus 3 from a past NARMS study from 2002-2013 and 2 sent to public health officials. An additional 21 samples came from retrospective testing of isolates.

These 55 patients harbored any of 3 Shigella species (flexneri, sonnei, or boydii) and ranged in age from 1 to 89 years; 7 (13%) were younger than 18 years, and 44 (80%) were men. Median duration of illness was 11 days. Not all information was available for all patients, but 46% (12/26) of the patients had bloody diarrhea, 50% (16/32) had fever, and 45% (19/42) were hospitalized.

Men who have sex with men (MSM) were overrepresented in the samples of DSA-Shigella. Thirteen (81%) of 16 men were HIV-positive, and 79% (11/14) identified as MSM. All their bacterial isolates harbored various antibiotic resistance genes carried on plasmids, which can horizontally transfer the resistance. Seven of 19 patients did not respond to the antibiotics prescribed, 3 of which were azithromycin.

The researchers conclude that MSM, especially those infected with HIV, are at increased risk for severe infection with DSA-Shigella. They encourage clinicians to culture stool specimens from MSM and HIV-infected men with symptoms of Shigella infection to assess azithromycin susceptibility and prescribe other antibiotics if necessary, although the report does not indicate which ones.

Limitations of the study include underestimating the scope of the problem by sampling only 1 in 20 submitted isolates and testing using a method (pulsed-field gel electrophoresis) that might not detect all resistance mutations. The data are too recent to reveal trends. The investigators conclude, "To better track illnesses and guide patient management, clinical laboratory guidelines for azithromycin susceptibility testing among Enterobacteriaceae are urgently needed."

The authors have disclosed no relevant financial relationships.

Morb Mortal Wkly Rep. 2014;63(6):132-133. Full text

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