Extensively drug-resistant Shigella climbs in U.S., CDC report finds

Extensively drug-resistant strains of Shigella, a gut bacterium long associated with day care outbreaks and travelers’ illness, made up 8.5% of U.S. samples with resistance data in 2023, up from none during 2011 through 2015, according to a Centers for Disease Control and Prevention report.

Doctors and researchers in the U.S. and Britain say sexual contact among gay and bisexual men is helping drive the spread, though the CDC’s own analysis did not measure sexual behavior.

The report, published in April in the agency’s Morbidity and Mortality Weekly Report, examined 16,788 Shigella samples with resistance data that were submitted to the CDC’s PulseNet surveillance network from 2011 through October 2023. The figures describe those samples, not all U.S. infections. Of those samples, 510, or 3%, were considered extensively drug-resistant, meaning resistant to five commonly used antibiotics: ampicillin, azithromycin, ceftriaxone, ciprofloxacin and trimethoprim-sulfamethoxazole. More than 84% of those cases occurred in 2022 and 2023.

Among the 492 patients with age and sex data, 86% were adult men, and the median age of all patients was 41. Of the 258 patients with hospitalization data, about 38% were hospitalized; no deaths were reported. Of 116 patients whose HIV status was known, 54, or nearly 47%, also had HIV. Among 205 patients with international travel information, 82% reported no recent trips abroad.

Shigella causes an estimated 450,000 U.S. infections a year, the CDC says. Most cases clear up without antibiotics, but severe cases can require them. For the extensively drug-resistant strains, the CDC report says no oral antibiotic is approved by the Food and Drug Administration. Pivmecillinam, fosfomycin and oral carbapenems might be effective, the report says, but as of 2025 none was approved by the FDA for shigellosis.

The CDC said data on sexual exposure were incomplete and not analyzed, and it pointed to earlier studies identifying sex between men as an important route of transmission. Ferric Fang, a University of Washington School of Medicine professor who directs the clinical microbiology laboratory at Harborview Medical Center in Seattle, said sexual history often isn’t recorded for hospitalized patients and that many men may not realize the infection can be sexually transmitted, according to CIDRAP News.

At UCLA Health, clinicians identified one case of extensively drug-resistant Shigella in 2022 and have seen roughly one a month since, said Shangxin Yang, who directs the hospital’s molecular microbiology and pathogen genomics laboratory. “It’s almost like an explosion of this pathogen in the community,” Yang told CIDRAP News.

The pattern extends beyond the United States. The UK Health Security Agency reported 2,560 diagnoses of likely sexually transmitted Shigella in England in 2025, up from 2,052 in 2023. In a subset of samples tested for resistance, 86% of Shigella sonnei and 94% of Shigella flexneri showed resistance to antibiotics, and more than half of S. sonnei cases were extensively drug-resistant. Because routine lab data don’t record sexual orientation, the agency uses adult men with no recent travel history as a proxy for sexually acquired cases.

A Cambridge-led study in The Lancet Infectious Diseases analyzed 3,514 S. sonnei samples collected in Britain from 2004 through 2020. It sorted cases into presumed sexually transmitted (men ages 16 to 60 without recent high-risk travel), other domestic cases and travel-related cases. Using genetic modeling, the researchers found that related samples from presumed sexually transmitted cases were an average of 117 kilometers (about 73 miles) apart within about 2½ years of evolutionary time, compared with 46 kilometers (about 29 miles) for other domestic cases. The measure reflects modeled geographic spread, not tracking of individual infections. By the end of the study period, roughly 70% of the sexually transmitted strains were resistant to at least one clinically relevant antibiotic, compared with 40% of non-sexually transmitted strains and 49% of travel cases, the university said.

Kate Baker, the study’s senior author, said in a university press release that the disease has gone from relatively treatable to “close to untreatable” over about a decade. The researchers believe the resistance partly stems from antibiotics given for other sexually transmitted infections.

That phenomenon, known as bystander resistance, occurs when an antibiotic used against one bacterium selects for resistance in another, said Alicia Calvo-Villamanan, a postdoctoral researcher at the Gulbenkian Institute for Molecular Medicine in Lisbon, who commented on the study for the Science Media Centre Spain. Azithromycin resistance in Shigella slowed after UK guidelines stopped recommending the drug for gonorrhea, Ms. Calvo-Villamanan noted, while also cautioning that the study has the usual limitations of such work, including assumptions where patient data were missing.

The Cambridge researchers found no strong evidence that sexually transmitted strains were spilling over into other communities. But in January, UCLA researchers reported a novel extensively drug-resistant strain in two patients with no apparent link to each other, one of them a cancer patient who fit none of the known risk factors, CIDRAP reported.

The California Department of Public Health says the best way to prevent infection through sex is to avoid sex with anyone who has had diarrhea in the past two weeks, noting the bacteria can remain in stool for weeks after symptoms end. It also recommends condoms or dental dams during oral sex and washing hands, genitals and the anal area before and after. There is no vaccine.

Daniel Richardson, a sexual health consultant at University Hospitals Sussex, said in the Cambridge release that doctors should ask patients with dysentery about sexual transmission, and that those patients may need testing for other sexually transmitted infections.