Shigella, the bacterium responsible for dysentery, is best known as a disease transmitted through contaminated food and poor hand hygiene. It is one of the leading causes of diarrheal illness among children in developing countries and is responsible for more than two hundred thousand deaths worldwide each year.

However, certain strains of the bacterium, including Shigella sonnei and Shigella flexneri, are also transmitted through sexual contact, primarily via oral-anal contact. Over the past two decades, these strains have spread steadily across Western countries. Today, the disease is considered endemic within certain sexual networks of men who have sex with men.

New data from the UK Health Security Agency show that confirmed cases of potentially sexually transmitted Shigella rose to two thousand five hundred and sixty in twenty twenty-five, compared with two thousand and fifty-two in twenty twenty-three.

Researchers from the University of Cambridge, whose findings were published in The Lancet Infectious Diseases, analyzed three thousand five hundred and fourteen bacterial samples collected across the United Kingdom between two thousand four and two thousand twenty. They divided the cases into three groups: men aged sixteen to sixty who had not recently traveled to regions where Shigella is endemic—Africa, Asia, or Central and Latin America—and who were likely men who have sex with men, representing about thirty-four percent of all samples; other locally acquired infections, accounting for approximately thirty-six percent; and travel-associated cases, which made up the remainder.

The study's central finding was that sexually transmitted strains spread geographically more than twice as fast as other local strains. Over approximately two and a half years of bacterial evolution, the average distance between closely related strains transmitted among men who have sex with men reached one hundred and seventeen kilometers, compared with only forty-six kilometers among other locally circulating strains.

Researchers also identified a significantly larger number of separate transmission chains within this group—about one hundred more than in other populations occupying the same geographical area—even though men who have sex with men represent a minority of the male population in Britain. These strains also remained genetically distinct from other circulating strains for approximately fifteen years of evolution, compared with about two and a half years in other populations, indicating a prolonged and largely independent transmission network. The highest concentrations of cases were recorded in major urban centers, particularly London and Manchester.

Antibiotic resistance was considerably more common among sexually transmitted strains. Seventy percent were resistant to at least one clinically relevant antibiotic, compared with forty percent of other local strains and forty-nine percent of travel-associated strains.

Between twenty fifteen and twenty twenty, the annual growth rate of sexually transmitted strains increased by fifteen percent compared with strains transmitted through more traditional routes. A separate analysis of four hundred and sixty-eight samples collected in England between twenty sixteen and twenty twenty-one identified a strain that spread rapidly shortly after the COVID-19 pandemic while simultaneously developing resistance to three major antibiotics: ciprofloxacin, azithromycin and ceftriaxone.

Professor Kate Baker, one of the study's lead investigators from the Department of Genetics at the University of Cambridge, said many men who have sex with men remain unaware of the growing risk.

"Sexual transmission has become a permanent part of Shigella transmission patterns in the United Kingdom," Baker said. "It is important that this message reaches the relevant communities in order to help slow its spread."

She added that the problem does not involve a single strain but several overlapping strains evolving drug resistance simultaneously, meaning patients infected through sexual transmission may require treatment that differs substantially from those infected during international travel.

Another significant finding linked rising azithromycin resistance among men who have sex with men to historical treatment guidelines for gonorrhea. Until twenty eighteen, azithromycin formed part of the recommended combination therapy for gonorrhea. Researchers believe repeated exposure to the drug unintentionally provided Shigella carried by the same patients with a selective survival advantage. After clinical guidelines switched to ceftriaxone-only treatment in twenty eighteen, that selective advantage gradually declined and almost disappeared.

Professor Baker said the finding illustrates that antibiotic treatment directed at one disease can influence bacteria throughout the body rather than affecting only the intended pathogen.

Earlier studies by Baker and her team found that up to one-third of patients with sexually transmitted Shigella require hospitalization for an average of four to five days, while roughly two-thirds are simultaneously diagnosed with another sexually transmitted infection, including HIV. Transmission occurs through direct or indirect oral-anal contact. Shigella is exceptionally infectious: as few as ten bacteria are sufficient to cause disease, whereas Salmonella generally requires more than one thousand. An infected person may remain contagious for approximately one month.

Symptoms include watery diarrhea that may contain blood, mucus or pus in severe cases, nausea or vomiting, abdominal pain, fever above thirty-eight degrees Celsius and, in some cases, low mood. Worldwide, deaths from Shigella result from dehydration caused by severe diarrhea, bowel or stomach perforation, or malnutrition.

Mark Tweed of the Terrence Higgins Trust, an organization focused on HIV, described the infection as a growing concern among some gay and bisexual men with frequent sexual partnerships.

"Research has linked infection to having multiple sexual partners, drug use in sexual settings, the use of HIV pre-exposure medication, and simultaneous infection with other sexually transmitted diseases," Tweed said. "But these are statistical associations, not proof that any single behavior is responsible for the increase."

He urged anyone who suspects they may have Shigella infection to seek medical assessment.

Dr. Daniel Richardson, a sexual health consultant at the University Hospitals Sussex NHS Foundation Trust, said physicians treating patients with dysentery should routinely ask about sexual history and tailor treatment accordingly, noting that sexually transmitted strains show substantially higher antibiotic resistance than other forms of the disease.

Professor Baker emphasized that traditional public health advice for preventing Shigella, such as handwashing and food hygiene, is not sufficient to prevent sexual transmission.

"If you begin feeling unwell, or if you have recently recovered from diarrheal illness, avoid sexual activity for two weeks after complete recovery," she said. "Tell your doctor about your sexual history and ask for comprehensive screening for sexually transmitted infections."

She also recommended thoroughly washing the hands, pelvis and buttocks after anal contact, changing condoms between anal and oral sex, using latex gloves for finger penetration or fisting, and avoiding the sharing of sex toys or equipment used for rectal douching.