Sexually Transmitted Bloody Diarrhoea Now 'Endemic' Among Some Gay and Bisexual Men, Scientists Warn

Shigella, a bacterial infection that can cause bloody diarrhoea, is continuing to spread through sexual contact among some networks of gay and bisexual men in England, researchers have warned. UK Health Security Agency (UKHSA) figures recorded 2,560 diagnoses in 2025, a rise of about 10 per cent on the previous year.

The infection is considered endemic in some communities, meaning it is regularly present within particular networks. Scientists are also concerned about antibiotic resistance, with testing in 2025 finding multidrug or extensively drug-resistant bacteria in 86 per cent of tested Shigella sonnei isolates and 94 per cent of tested Shigella flexneri isolates associated with presumed sexual transmission.

A Cambridge-led study published in The Lancet Infectious Diseases found that sexually transmitted forms of Shigella increased by 15 per cent faster year on year between 2015 and 2020 than infections linked to traditional routes, including contaminated food, travel and nursery outbreaks.

How Shigella Spreads

Shigella spreads through the faecal-oral route when tiny amounts of faecal matter reach the mouth. This can happen during sexual activity or through contaminated hands, food or water. As few as 10 bacterial organisms can cause infection.

Symptoms usually appear within one to four days and include diarrhoea, sometimes containing blood or mucus, stomach cramps and fever. Many cases resolve without specific treatment, although severe illness can cause dehydration and require hospital care.

UKHSA surveillance suggests that more than half of Shigella diagnoses in England in 2025 were among adult men classified as potentially having acquired the infection through sexual contact. London accounted for 54 per cent of diagnoses in this group.

However, the figures need careful interpretation. UKHSA uses adult men with no reported recent international travel, or unknown travel history, as a proxy for men who have sex with men who may have acquired the infection sexually. The data does not confirm each person's sexual orientation or how they contracted the bacteria.

Geneticist Kate Baker, a lead author of the Cambridge study, said many men who have sex with men remained unaware of the risks. She warned that sexually transmitted Shigella posed a 'serious and increasing risk.'

Antibiotic Resistance Is Limiting Treatment Options

The UKHSA resistance figures apply to samples submitted for detailed testing, not every diagnosed infection. More than half of the tested S. sonnei isolates were extensively drug-resistant, with resistance markers affecting all three recommended first-line treatment options.

This can make severe infections more difficult to treat. Researchers are also investigating whether antibiotic use for other infections may contribute to resistance through a process known as 'bystander resistance.' This remains a possible explanation, not a proven cause of the increase.

The concern extends beyond the UK. US surveillance by the Centers for Disease Control and Prevention found that extensively drug-resistant Shigella isolates rose from zero per cent in 2011 to 8.5 per cent in 2023. About one-third of patients with these resistant infections were hospitalised.

How To Reduce the Risk

Health authorities advise washing hands and intimate areas with soap and water before and after sex, cleaning sex toys, and using condoms or dental dams. People should also avoid sexual contact with anyone who has had diarrhoea in the previous two weeks.

Anyone experiencing symptoms should seek medical advice and mention possible sexual exposure. Testing can help identify the infection and guide treatment. Clinicians may also recommend screening for other sexually transmitted infections.