Cambridge study says sexually transmitted Shigella is rising fast among gay and bisexual men in the UK

A new University of Cambridge study says sexually transmitted Shigella is moving through the UK faster than other forms of the infection and is becoming more resistant to antibiotics.

Shigella can cause bloody or prolonged diarrhoea and dysentery. The research, published in The Lancet Infectious Diseases, found that between 2015 and 2020 sexually transmitted forms of the bacteria rose 15 percent faster each year than variants acquired through other routes, including travel or contaminated food.

The UK Health Security Agency recorded 2,560 cases last year that may have been linked to sexual contact. Cambridge researchers said Shigella is now “endemic” in some communities of gay, bisexual and other men who have sex with men, often shortened to GBMSM.

Using DNA-based tracking methods similar to those used during the Covid-19 pandemic, the team analysed 3,514 UK cases from 2004 to 2020 in collaboration with the UKHSA. The sample covered patients over 16 and included cases linked to GBMSM, cases not associated with GBMSM and infections brought into the UK through travel.

The study found sexually transmitted strains spread more than twice as fast over two-and-a-half years as non-sexually transmitted strains. The average distance between related sexually transmitted strains was 117km, compared with 46km for non-sexually transmitted strains.

Antibiotic resistance was also more common in sexually transmitted strains. By the end of the study, more than 70 percent of those strains resisted at least one relevant antibiotic. The comparable figures were 40 percent for non-sexually transmitted strains and 49 percent for travel-related cases.

Researchers said the findings expose a “critical gap in public health management” because standard advice for Shigella, such as handwashing and food hygiene, does little to interrupt sexual transmission.

Professor Kate Baker, senior author of the study from Cambridge’s Department of Genetics, said: “Many men who have sex with men are unaware of the serious and increasing risk posed by sexually transmitted Shigella.” She added that sexual infection is now a sustained part of transmission in the UK and said the condition should be treated as a distinct public health threat.

Previous work by Baker and others suggests that up to one third of patients with sexually transmitted Shigella are hospitalised for an average of four to five days. The research also says up to two thirds of GBMSM with Shigella are found to have other STIs, including HIV.

London, Brighton and Manchester were among the major cities where sexually transmitted forms of Shigella were identified in GBMSM networks with a high number of sexual encounters. The infection does not only affect GBMSM, but the article said no increased rate has been found among other groups.

Shigella is usually spread through direct or indirect mouth-to-anus contact and tiny amounts of faecal matter. It is highly contagious: Salmonella needs more than 1,000 bacterial organisms to spread, while Shigella needs just 10.

Researchers estimate that more than half of UK Shigella infections are now spread through sexual contact, 30 percent are linked to travel, and the remainder come from localised outbreaks among young children.

In the study period, the data showed 34 percent of cases were among men who have sex with other men, 36 percent were not associated with GBMSM and 30 percent were brought into the UK from overseas travel.

The study noted that sexually transmitted infections rose sharply around 2010. Researchers said online platforms such as Grindr, a rise in mouth-to-anus activity and chemsex parties may have contributed, though those links are not proof of causation.

Marc Tweed of the Terrence Higgins Trust in Brighton said transmission has been linked with dense sexual networks, multiple partners, sexualised drug use, PrEP use and concurrent sexually transmitted infections, but those are associations rather than proof that any one behaviour is driving the increase. He advised anyone who thinks they may have Shigella to contact a local sexual health clinic.

Baker also urged people not to have sex until two weeks after they are fully recovered from diarrhoea, to tell doctors about their sexual history when seeking care and to ask for a full sexual health screen.

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The study says antibiotic-resistant strains of Shigella grew 71 percent faster than strains that can still be treated with medication, while 42 percent of strains not spread through sexual contact became antibiotic resistant.

Baker said sexually transmissible shigellosis is becoming difficult to treat and said some sexually transmitted variants were also developing resistance to drugs used for other STIs, including gonorrhoea. She said people should remember that antibiotics affect the whole body, not just one infection.

She said the disease has become “close to untreatable” over the past decade or so.