International travelers who return home and infect others are spreading the intestinal illness shigellosis in the United States.

A drug-resistant intestinal illness is reportedly spreading across the United States.

Officials at the Centers for Disease Control and Prevention (CDC) say shigellosis is being brought into the country by international travelers.

The illness made 243 people sick in 32 states and Puerto Rico between May 2014 and February 2015. Clusters of the illness have been reported in Massachusetts, California, and Pennsylvania, according to a CDC press statement.

CDC officials say the illness is being spread by people who travel outside the United States, then return home and infected others who have not traveled.

Get the Facts: What Is Shigellosis? »

Scientists say this Shigella sonnei bacteria is resistant to the antibiotic ciprofloxacin, the drug most often used to treat it. In the United States, most Shigella strains are already resistant to the antibiotics ampicillin and trimethoprim/sulfamethoxazole.

Antibiotic resistance is a growing threat in the United States, and President Obama recently unveiled an action plan to combat it.

Researchers said the Shigella bacteria spread quickly in places with less than perfect sanitation, such as childcare facilities and homeless shelters.

“These outbreaks show a troubling trend in Shigella infections in the United States,” said CDC Director Dr. Tom Frieden in a press release. “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.”

Get the Facts: What Is Ciprofloxacin? »

Ciprofloxacin is sometimes prescribed to international travelers in case they develop diarrhea while outside the United States.

CDC officials said they are investigating whether the routine use of ciprofloxacin is causing the drug-resistant Shigellosis strain to spread.

“The increase in drug-resistant Shigella makes it even more critical to prevent shigellosis from spreading,” said Dr. Anna Bowen, a medical officer in the CDC’s Waterborne Diseases Prevention Branch. “Washing your hands with soap and water is important for everyone. Also, international travelers can protect themselves by choosing hot foods and drinking only from sealed containers.”

Shigellosis is a bacterial infection that attacks the digestive system. Its primary symptom is diarrhea. 

Some of the more severe cases require medical treatment. The illness usually lasts two to seven days.

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