The Centers for Disease Control and Prevention (CDC) said that international travelers are also bringing back a multidrug-resistant intestinal illness - Shigellosis - that is spreading throughout the country.

According to CDC, Shigella sonnei bacteria resistant to the antibiotic ciprofloxacin were introduced repeatedly as ill travelers returned from overseas, causing a series of outbreaks of shigellosis between May 2014 and February 2015. The disease has infected 243 individuals in 32 states and Puerto Rico so far. Ciprofloxacin is the first choice to treat shigellosis among US adults.

Until recently, ciprofloxacin resistance has occurred in just 2% of Shigella infections tested in the United States, but it was found in 90% of samples tested in the recent clusters that occurred in Massachusetts, California and Pennsylvania. Shigellosis can spread in groups such as children in child care facilities, homeless people and gay and bisexual men.

"These outbreaks show a troubling trend in Shigella infections in the United States," said CDC Director Tom Frieden, MD, MPH. "Drug-resistant infections are harder to treat and because Shigella spreads so easily between people, the potential for more - and larger - outbreaks is a real concern. We're moving quickly to implement a national strategy to curb antibiotic resistance because we can't take for granted that we'll always have the drugs we need to fight common infections."

Most Shigella in the US is already resistant to the antibiotics ampicillin and trimethoprim/sulfamethoxazole. The resistance of shigella to ciprofloxacin is increasing globally.

"The increase in drug-resistant Shigella makes it even more critical to prevent shigellosis from spreading," said Anna Bowen, MD, MPH, a medical officer in CDC's Waterborne Diseases Prevention Branch and the lead author of the study.