Sexually transmitted diarrhoea spreading rapidly among some networks of gay men in the UK, study finds

Variants of sexually-transmitted Shigella – a bacterial infection that causes bloody or prolonged diarrhoea and can lead to severe dehydration – are causing more disease and becoming increasingly antibiotic-resistant among sections of the UK’s LGBTQ+ community, a new Cambridge study has found. 

The research shows that between 2015 and 2020, sexually transmitting forms of Shigella in the UK grew 15% faster year-on-year than variants contracted through traditional routes of travel abroad, foodborne illness, or nursery outbreaks. 

According to the UK Health Security Agency, Shigella potentially acquired during sexual contact saw a “sharp rise in laboratory diagnoses during 2025”, with 2,560 recorded cases last year. 

Shigella, the bacteria behind the diarrhoeal disease dysentery, is one of the top pathogens affecting children in low- and middle-income countries. In the UK, Shigella was historically acquired during travel overseas and transmitted to close contacts. 

However, sexually transmitted Shigella strains, first identified in the US in the mid-1970s, have been rising steadily in many western countries since the 2000s, and the disease is now “endemic” in some communities of gay, bisexual and other men who have sex with men (GBMSM). 

The study used techniques deployed during the pandemic to track Covid-19 variants through DNA sequencing, only this time for bacteria instead of a virus. It shows sexually transmitted Shigella spreads much faster and is growing drug resistant much quicker than non-sexually transmitted infection (STI) strains. 

Over a short evolutionary timespan of approximately 2.5 years, sexually transmitted Shigella strains were found to spread more than twice as fast in the UK, averaging 117 km distance between related strains, compared with 46 km for non-STI strains.

By the end of the study period, upwards of 70% of sexually transmissible Shigella strains were resistant to at least one clinically relevant antibiotic, compared with 40% among non-sexually transmitted strains and 49% among travel cases.

Cambridge epidemiologists behind the study, published today in the journal The Lancet Infectious Diseases, say it highlights a “critical gap in public health management”, as typical advice for Shigella, such as handwashing and food hygiene, is ineffective for interrupting sexual transmission.

“Many men who have sex with men are unaware of the serious and increasing risk posed by sexually transmitted Shigella,” said Professor Kate Baker, senior author of the study from Cambridge’s Department of Genetics.  

“Sexual infection is now a sustained part of Shigella transmission in the UK. It is vital that this message reaches the communities most affected, so we can help to prevent the spread,” Baker said. 

“Sexually transmissible shigellosis needs to be treated as a distinct public health threat, requiring different surveillance, prevention, and treatment strategies.” 

Previous research by Baker and others suggests that up to a third of patients with sexually transmitted Shigella are hospitalised, and are in hospital for an average of four to five days. It has also been shown that up to two thirds of GBMSM with sexually transmitted Shigella are co-infected with other STIs, including HIV.

The sexually transmitted form of Shigella diarrhoea is primarily contained within networks of GBMSM who engage in high-frequency sexual encounters, and focused on major cities such as London, Brighton, and Manchester.*

While sexually transmitted Shigella may not be limited to GBMSM, the study found no statistically significant rates of increase among non-GBMSM communities. They also found no strong evidence for “spillover” from sexual networks into other communities. 

Spread of STI variants usually involves some form of direct or indirect contact between the mouth and the anus, allowing tiny amounts of faecal material containing Shigella to be transmitted between partners. Shigella is highly contagious, even among bacteria: it only takes ten bacterial organisms to cause disease. Salmonella, for example, needs over a thousand.

In collaboration with UKHSA, this study analysed 3,514 samples of Shigella collected from across the UK between 2004–2020 from people over the age of 16. Around a third (34%) were from men who have sex with men. Another third (36%) were domestic cases not associated with GBMSM transmission. The last third (30%) were travel cases brought into the UK. 

Sexually transmitted infection rates began to rise dramatically around 2010 after decades of little growth, so researchers think it may in part be driven by the emergence of online meet-up platforms such as Grindr, as well as practices such as mouth-to-anus ‘rimming’ and chemsex parties: events based around group sex and use of stimulant drugs. 

“It’s a real cause for concern that sexually transmitted Shigella is becoming a growing problem in some sexual networks of gay and bisexual men, with both rising cases and rapidly increasing antibiotic resistance,” said Marc Tweed, from the Terrence Higgins Trust in Brighton. 

“Studies have linked transmission with dense sexual networks, multiple partners, sexualised drug use, PrEP use and concurrent sexually transmitted infections. But these are associations, not proof that any one behaviour alone is driving the increase,” Tweed said. “If you think you may have Shigella, you should contact your local sexual health clinic to book an appointment.” 

Researchers now estimate that more than half of all Shigella infections in the UK are sexually transmitted, while 30% are linked to travel, and the remainder down to infrequent localised outbreaks among young children, and transmission in the household.

“Some advice is obvious, but needs hammering home. If you start to feel unwell, or are recently recovering from a scary bout of diarrhoea, do not engage in sexual activity until two weeks after you’re fully recovered, mention your sexual history to your doctor if you seek medical care, and ask about having a full sexual health screen,” said Baker. 

STI strains are rapidly becoming resistant to recommended antimicrobial treatment. The latest study suggests that, among sexually transmitted Shigella, antibiotic-resistant strains grew 71% faster than drug-susceptible strains. Among non-STI strains, antibiotic resistance only gives a 42% advantage.

“This isn’t just one form of sexually transmissible diarrhoea. This is multiple overlapping variants emerging that are all quickly becoming resistant to the drugs we use to treat them,” said Baker. “It’s highly likely that if you contracted your Shigella through sex you require different treatment to someone who contracted it through travel.”

Researchers believe this resistance partly results from antibiotic treatment or preventative prescriptions for other sexually transmitted infections.  

“Our evidence suggests that the variants of Shigella transmitting in sexual networks were actually getting resistant against treatments for other STIs, like gonorrhoea, so people need to remember that when they’re taking antibiotics they’re treating their whole body,” said Baker. 

An additional analysis looked at 468 samples collected in England between 2016 and 2021, which revealed rapidly expanding strains of sexually transmitted Shigella that had acquired resistance to three major antibiotics: ceftriaxone, azithromycin and ciprofloxacin. 

“This disease has gone from being relatively treatable with off-the-shelf drugs to being close to untreatable over the course of the last decade or so,” said Baker. 

Marc Tweed from the Terrence Higgins Trust said: “It’s important to pay close attention to what’s causing antibiotic resistant strains of Shigella to persist and spread. This includes the cumulative antibiotic exposure created by repeated treatment for STIs after asymptomatic screening.” 

Dr Daniel Richardson, a Consultant in Sexual Health at University Hospitals Sussex, said: “Clinicians should be asking patients with dysentery about sexual transmission and giving advice accordingly,” 

“They need to know that sexually transmitted Shigella has more antimicrobial resistance than other forms of the disease, and that patients may need checking for other sexually transmissible illnesses.” 

Study first author Julia Marshall, who conducted the work while an MPhil student in Baker’s lab, added: "This work shows the opportunity and public health insights offered by the integration of genomics and advanced modelling methods."

Reference: Marshall, J.E. et al: ‘The spread of sexually transmissible drug-resistant shigellosis in England: a genomic epidemiology study.’ July 2026, The Lancet Infectious Diseases. DOI: 10.1016/ S1473-3099(26)00227-6.

_{* Data from the UKHSA shows that in 2025, most diagnoses of sexually transmitted Shigella were seen in London (54%), followed by Thames Valley (6%) and Surrey and Sussex (6%).}