Amid ongoing E. coli outbreak linked to contaminated salad, we ask the experts: why IS lettuce such a food poisoning risk?

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More than 200 Brits have now been struck down with a rare strain of E. Coli linked to contaminated salad leaves.

Food safety chiefs have said they are 'confident' a type of butterhead lettuce, named Apollo, is behind the outbreak of the diarrhoea-causing bug.

Exactly how they became contaminated is yet to be established, however.

Currently, more than 60 sandwiches, wraps and salads sold in 11 major shops have been slapped with 'do not eat' alerts over fears they could contain Shiga toxin-producing E.coli (STEC). 

Yet it is not the first time salad leaves have been blamed for the bug. 

Studies suggest leafy greens are responsible for half of all E. Coli outbreaks. 

Food safety chiefs have said they are 'confident' an Apollo type lettuce has caused the outbreak of the diarrhoea-causing bug. Efforts to confirm the root cause, however, are ongoing

It has seen over 60 sandwiches, wraps and salads sold in 11 major shops slapped with 'do not eat' alerts over fears they could contain Shiga toxin-producing E.coli (STEC)

In December, a 17-year-old girl from Buckinghamshire was left fighting for her life after contracting the deadly E. Coli strain. Antonia Hay is believed to have caught STEC-HUS, caused by E. coli O157, from food at a Christmas market in Great Missenden

Experts believe lettuce's texture can make it more prone to the bacteria and fact it's not cooked, which would usually kill off bugs, increases the risk.

Professor Paul Hunter, a renowned infectious diseases expert from the University of East Anglia told MailOnline: 'During watering during growth allows the potentially contaminated water to rest on the lettuce leaf.

'The rough and waxy surface of the leaf also makes it difficult to wash off E. coli.

'As to whether E. coli can enter the tissue of the lettuce, probably it can, though the evidence is somewhat conflicting.

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'The main risk comes from bacteria attached to the leaf surface. Most other vegetables are either peeled or cooked before eating.

'Things like cucumbers, tomatoes and peppers may not be cooked but grow some way off the ground so less likely to be contaminated.'

A 2019 review of research co-authored by Professor Hunter, also analysed 35 STEC outbreaks linked to lettuce between 1995 and 2018.

Eight were found to have 'poor practice' when processing of the vegetables 'may have contributed to the outbreak', such as insufficient disinfection.

A further six were linked to animal faeces near the growing fields.

'Where the type of lettuce was reported it was usually Cos lettuce, though Iceberg and Mesclun lettuce were each implicated in one', Prof Hunter told MailOnline.

'We also found bagged lettuce already prepared for use to be a common factor.'

Experts say all salad should be washed thoroughly by immersing in cold water.

Storage of vegetables is also key: if salad bags are in the fridge and the temperature is less than four degrees, bacteria will not multiply as rapidly.

Experts also say recent wet, warm weather can creates an 'ideal' breeding ground for E. coli to spread.

Professor Eileen Wall, head of research at Scotland's Rural College (SRUC), a research institution focused on agriculture and life sciences, said: 'Rain can cause contaminated water to splash onto the leaves of the lettuce, while warmth provides a favourable temperature for bacterial proliferation.'

The aspiring actress, spent almost two weeks in intensive care, only coming home temporarily on Christmas Day. She had multiple operations - including one to remove part of her bowel — and a blood transfusion as her kidneys have been failing, forcing her onto dialysis

Antonia's father put out a post on Facebook asking people if they also experienced similar symptoms

Bacteria could spread to lettuce through irrigation water — particularly if the water is contaminated with faecal matter, she added.

Meanwhile, Nicola Holden, a professor of food safety at SRUC, warned humid weather may too have played a part.

She said: 'The rainy weather means that for any field grown crops, humidity will be higher within the leaf canopy. 

'We know that STEC favours high humid conditions.'

The UK Health Security Agency (UKHSA) typically records around 1,500 STEC confirmed infections in a year.

Studies have even shown leafy greens are responsible for up to 58 per cent of E. Coli outbreaks. Experts believe lettuce's texture can make it more prone to the bacteria, with it even getting stuck inside the vegetable

Officials, however, have raised the alarm after 211 Brits were struck down with the strain in just over two weeks — May 25 to June 11.

Of these, 147 were in England, with 27 in Wales and 35 in Scotland.

Just two cases have been recorded in Northern Ireland though officials say this individual likely caught the bug in England.

At least 67 people have also been admitted to hospital.

Victims include children as young as two, though the majority are young adults.

The UKHSA has said it expected the number of people falling ill to rise further as it carried out more research using genome sequencing to check which E. coli cases were linked.

While no bacteria has been found in supermarket products directly, more than 60 sandwiches, wraps and salads have been recalled as a 'precautionary measure'.

STEC is considered to be extremely infectious, and in up to 15 per cent of cases, the bug can cause haemolytic uremic syndrome (HUS), a life-threatening condition that can lead to kidney failure.

Children under the age of five are at the highest risk of HUS.

A small proportion of adults may develop a similar condition called thrombotic thrombocytopaenic purpura (TTP).

In December, a 17-year-old girl from Buckinghamshire was left fighting for her life after contracting the deadly E. Coli strain.

Antonia Hay is believed to have caught STEC-HUS, caused by E. coli O157, from food at a Christmas market in Great Missenden.

The aspiring actress, spent almost two weeks in intensive care, only coming home temporarily on Christmas Day.

She had multiple operations — including one to remove part of her bowel — and a blood transfusion as her kidneys have been failing, forcing her onto dialysis.

It is not known, however, what sort of food led to Antonia contracting the illness.

Her older sister Jemima Hay said she was 'lucky to be alive' and described the situation as 'an absolute nightmare that has left our family broken'.

She wrote on a GoFundMe page: '[Antonia] has continued to show amazing determination and strength throughout this time, despite the immense stress, pain and trauma - all combined with an extreme phobia of injections.

'She is the youngest person to ever be admitted into the ICU ward she is currently in.'

She added: 'She feels as if her dreams have been ripped away from her as her studies on her performing arts course will be severely impacted.'

People have been advised to contact NHS 111 or their GP if they or their children show any symptoms of E.coli infection.

For children under five these can include disinterest in breast or bottle feeding and signs of dehydration such as fewer wet nappies.

Both adults and children are advised to call NHS 111 or their GP if they keep vomiting for two days or have diarrhoea for a week.

Anyone suffering bloody diarrhoea or bleeding from the bottom should call NHS 111 or their GP immediately.