The Centers for Disease Control and Prevention raising a warning to doctors and the public about an increase in extensively drug-resistant (XDR) cases of Shigella, a highly transmissible bacteria that causes an infection called shigellosis, an inflammatory diarrhea.

On Tuesday, the CDC held a call to inform clinicians about this emerging public health threat. The Colorado Department of Public Health and Environment and the U.K. Health Security Agency also presented on the call due to their experience dealing with XDR Shigella.

The new forms of the bug are resistant to all five of the antimicrobial treatments that are typically used, which was never seen before 2016 and have been increasing in prevalence ever since, says Dr. Louise Francois Watkins, a medical officer at the CDC. Most Shigella strains are resistant to one or some of the drugs, but not all five, she told NPR.

Because of limited data, the CDC doesn't yet have official alternative recommendations, but there are promising potential treatments.

Francois Watkins says the new strains remain relatively uncommon and the risk to the general public is low, but the CDC wants to make sure that people know about it so that people who are infected can get proper medical care. Treatment of shigellosis normally includes bed rest and plenty of fluids, and antibiotics for non-XDR strains.

The CDC's health alert said that of the approximately 450,000 annual Shigella infections, none had been caused by extensively drug-resistant strains in 2015. In 2022, about 5% were.

Shigella spreads when infected fecal matter enters another person's mouth or nose, directly or indirectly. That can be through sexual activity, or because of poor handwashing or contaminated food or water.

While shigellosis is typically seen in young children, the XDR form of the stomach bug is more prevalent among adults. The CDC said it's finding most cases of XDR Shigella among men who have sex with men, people experiencing homelessness, international travelers and people living with HIV.

"XDR Shigella has a real, alarming capacity to spread globally, especially among these vulnerable populations," says Dr. Naeemah Logan, a CDC medical officer.

Other strains of Shigella are increasingly drug-resistant, and there are concerns that the bacteria's the drug-resistant genes mutations could jump to other bacteria, such as E. coli, Francois Watkins says.

"The problem of antimicrobial resistance is actually bigger than Shigella," Francois Watkins says, and "one of the major drivers of antibiotic resistance is inappropriate antibiotic use."

People should only take them as prescribed, she advises.

Handwashing and sanitizing are the most important measures people can take to protect themselves from Shigella, Logan tells NPR, and people who are sexually active should wash sex toys — and themselves — with soap and water before and after sex.

People who have diarrhea should avoid swimming pools and water parks, and they should avoid having sex for two weeks after symptoms resolve, she says.

Normally shigellosis goes away without antibiotic treatment, but it can cause prolonged illness — about 6,400 patients in the U.S. need to be hospitalized every year. Doctors prescribe antibiotics to speed up recovery, prevent transmission or avoid complications in vulnerable patients, such as those who are immunocompromised, but that option isn't available for the XDR strains.

The European Centre for Disease Prevention and Control issued an alert a week before the CDC's, saying there had been 221 confirmed and 37 possible cases among travelers who had visited Cabo Verde off of West Africa since September 2022. The alert said the most likely transmission route was through food, and many of the cases were linked to all-inclusive hotels. Affected guests returned home to the U.K., the U.S., and nations across the European Union.