According to a report by the Centers for Disease Control and Prevention (CDC), international travelers are bringing a multidrug-resistant intestinal illness to the U.S. and spreading it to others. Between May 2014 and February 2015, 243 people in 32 states and Puerto Rico were sickened by Shigella sonnei bacteria, which is resistant to the antibiotic ciprofloxacin. The drug-resistant bacteria were being brought in repeatedly by ill travelers which were then passed to other people, causing outbreaks across the country.

First-line treatment for adults with shigellosis in the U.S. is typically ciprofloxacin; however, upon investigating the recent clusters, the CDC and public health officials discovered that nearly 90% of the cases tested were resistant to the antibiotic. Until recently, fluoroquinolone resistance had occurred in just 2% of Shigella infections tested in the U.S.

Back in December 2014, CDC’s PulseNet lab network identified an increase in Shigella sonnei infections with an uncommon genetic fingerprint. Further testing found that the bacteria were resistant to ciprofloxacin. PulseNet detected several large clusters: 45 cases in Massachusetts, 25 cases in California, and 18 cases in Pennsylvania. About half of the PulseNet cases with patient information were associated with international travel, mostly to the Dominican Republic and India. The San Francisco Department of Public Health reported another 95 cases, with almost half occurring among the homeless or people living in single-room occupancy hotels.