Research conducted by the U.S. Centers for Disease Control and Prevention (CDC) suggests a greater proportion of Shigella infections in the United States are now resistant to a very important antibiotic.

Shigella causes an estimated 500,000 cases of diarrhea in the United States annually and is transmitted easily from person to person and through contaminated food and recreational water.

Outbreaks of shigellosis frequently are large and drawn-out. Although diarrhea caused by Shigella sonnei typically resolves without treatment, patients with mild illness often are treated with antimicrobial medications because they can reduce the duration of symptoms and the shedding of shigellae in feces.

After PulseNet, the national molecular subtyping network for foodborne disease, detected a multistate cluster of S. sonneiinfections in December 2014, CDC’s National Antimicrobial Resistance Monitoring System (NARMS) laboratory determined that isolates from the cluster were resistant to ciprofloxacin.

Between May 2014 and February 2015, CDC identified 157 cases of ciprofloxacin-resistant S. sonnei infections.

Ciprofloxacin is the first-line treatment for adults with shigellosis because the bug’s resistance to other antimicrobials is either common or increasing. Of 126 total isolates with antimicrobial susceptibility information, 87 percent were not susceptible to ciprofloxacin.

There were 95 cases of ciprofloxacin-resistant Shigella infections in residents of, or travelers to, San Francisco in late 2014, but only nine of them made it into PulseNet.

About half of the 157 ciprofloxacin-resistant