A sexually transmitted disease that triggers severe diarrhoea is a 'growing and urgent public health threat', experts have warned - as cases continue to rise in the UK.

Researchers have found that sexually transmitted shigellosis - a sometimes deadly bacterial infection which has risen in prevalence by a quarter in England since 2023 - is becoming 'virtually untreatable'.

Shigellosis, a gut infection, is caused by shigella bacteria - a group of four germs that cause diarrhoea - and is mainly transmitted through consuming food prepared by an infected person or touching surfaces contaminated with faeces.

However scientists have today revealed it is increasingly being spread via sexual acts, particularly between gay and bisexual men.

This is because the bacteria can also be contracted if an individual comes into contact with faeces, which can happen during anal sex. 

The disease brings on intense bouts of diarrhoea, often accompanied by significant amounts of blood in the stool, as well as stomach cramps, fever, and sickness. 

It also kills more than 200,000 people worldwide per year. Some causes of death are dehydration as a result of severe diarrhoea, as well as intestinal perforation - when a hole forms in the walls of the stomach or in the bowel - and malnutrition.

Worryingly, experts have previously warned that the antibiotics that are usually used to destroy the bug are becoming less effective.

Known as antibiotic resistance, this occurs when bacteria develop the ability to survive powerful drug treatments that used to kill them.

The new study, led by researchers at the University of Cambridge, found that sexually transmitted shigellosis is spreading more rapidly than other forms of the disease and is becoming increasingly resistant to antibiotics.

Data from the UK Health Security Agency (UKHSA) shows diagnoses of suspected sexually transmitted shigella rose from 2,052 to 2,560 in 2025 - around 24.8 per cent - with more than half of the cases reported in London. 

The new research, published in The Lancet Infectious Diseases, analysed 3,514 samples of Shigella sonnei - one of the main bacteria that causes shigellosis - collected across the UK between 2004 and early 2020.

Of the cases analysed, around one third were classified as likely sexually transmitted among men who have sex with men.

Another third were infections acquired in the UK through other routes, while the rest were linked to recent travel to parts of Africa, Asia, Latin America and the Caribbean, areas where shigellosis is more common.  

Researchers found that sexually transmitted strains spread faster and over much greater distances than other strains.

After around two-and-a-half years, these cases were, on average, 117 kilometres apart, compared with 46 kilometres for infections that had been acquired in other ways.

Researchers also found that London and Manchester had a higher proportion of sexually transmitted cases than other parts of England. 

Almost half of cases in these two cities were likely to be sexually transmitted, compared with just over a quarter in less populated parts of England. 

The authors wrote: 'Our study shows the distinct and intensifying sexual transmission of shigellosis, highlighting the urgent need to address sexually transmissible shigellosis as a distinct health threat.'

They also warned that 'in the coming years, it is likely that the spread of sexually transmissible drug-resistant shigellosis will increase'. 

Professor Baker principal investigator in the department of genetics at the University of Cambridge, and the senior author of the study, said sexually transmitted strains are becoming difficult to manage because they are growing more resistant to antibiotics.

Shigellosis is commonly treated with simple remedies like rest and fluid, but in more severe cases - that last longer than a week - antibiotics are usually required.

But the new study found that sexually transmitted cases were far more likely to be resistant to key antibiotics used to treat the infection, including ciprofloxacin, azithromycin and ceftriaxone.

Around seven in 10 sexually transmitted cases were resistant to at least one of these drugs, compared with four in 10 other acquired infections and half of travel-related cases.

Professor Baker said: 'Now we're in a situation where it's virtually untreatable with drugs.'

She added that the infection can cause extremely severe illness, which feels like 'severe abdominal cramping' but can see patients begin 'bleeding into their stool.'

Professor Baker continued: 'It's a really nasty disease. It's not like a winter vomiting bug where you're sick and you feel like death for 24 hours and then you're fine.

'[With shigellosis] you can really be very sick for a week, sometimes longer. There's a small amount of evidence that some people might have longer infections than that, but we don't know.

'The statistics we do have from outbreak reports is that up to a third of cases are hospitalised for four to five days. It's not food poisoning - it's a very serious tummy bug.'

The authors also warned that traditional measures used to prevent ordinary shigellosis - such as good handwashing and food hygiene - are unlikely to stop the sexually transmitted form. 

They wrote: 'The development of alternative interventions to address this public health threat is urgently needed.'

Professor Baker added that people should also be aware that shigellosis can be easily mistaken for other diseases because symptoms are similar.

She said: 'Instead of assuming that you've had food poisoning, think, "actually, this might be something I've contracted through sex". And I guess more importantly, from the point of view of stopping transmission, "something I might pass on by having sex".

'Normal gastrointestinal hygiene [is important], but also realising that this is a transmission pathway that can occur.

'And so just be really careful when you have a tummy bug - make sure you're not going out and engaging in sex shortly after you've recovered, and waiting until you're well clear of your infection before you go out and kind of resume activities. 

'We don't want to stop anyone doing what they want to do - we just want to halt the transmission of this bug.'

Experts not involved in the study also commented on its findings. 

Dr. Hamish Mohammed, consultant epidemiologist at the UKHSA, said: 'Sexually transmitted Shigella predominantly affects gay and bisexual men. This research highlights that this infection is a growing concern, and we know cases rose sharply in 2025, many with extensively drug-resistant strains of the bacteria.

'People can reduce their risk of Shigella by good hygiene during and after sex – helping to protect yourself and your partners. 

'Symptoms include fever, stomach cramps and severe diarrhoea. It is important that gay and bisexual men do not dismiss these symptoms and get tested.

'If you're diagnosed with Shigella, you may also have been exposed to other sexually transmitted infections, including HIV, so a sexual health screen - at a sexual health service or by ordering tests online - is recommended.'