A food scientist has warned against eating supermarket salad leaves, as they're often laced with a bug that triggers extreme vomiting and diarrhoea – and can prove lethal in vulnerable Britons.

The so-called 'explosive diarrhoea' parasite, cyclosporiasis, has already struck down tens of thousands of Americans this summer, and is now sweeping through Europe.  

While the pathogen – spread through contaminated food and water – is not yet epidemic in the UK, experts say our salads are far from risk-free. 

'Salads, lettuce and side salad infections can kill,' Dr Primrose Freestone, a microbiologist at the University of Leicester, told The Times. 

'I wouldn't touch a bagged salad in a million years.'

Dr Freestone's warnings come as health chiefs have announced the debilitating infection – which causes weeks of violent diarrhoea, abdominal cramps and vomiting – has reached the UK.

'Pathogens grow more than 1,000 times better in a salad bag, even if it's kept in the fridge, and they attach themselves to plastic,' Dr Freestone said. 

'The longer they're in there, the worse it becomes. Germs use juices from bagged salad leaves to become more virulent. And that means they are more likely to cause infection.'

So far, cyclosporiasis has hit more than 22,000 people across 45 states, killing two and leaving hundreds hospitalised. At least 67 Britons have fallen ill since May. 

And contaminated lettuce appears to be the culprit, according to reports. But it's not just this bug that clings to bagged lettuce.

Last year, the UK Health Security Agency (UKHSA) reported a 26 per cent rise in a rare strain of E. Coli infections which can result in a life-threatening condition that can lead to kidney failure. 

The sharp rise in cases of Shiga toxin-producing E. Coli (STEC) was linked to lettuce in packaged sandwiches and wraps, which left 86 people hospitalised. 

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

'There’s a natural microflora on lettuce and leaves as they have been growing out in the wild. Heat destroys pathogens, and there’s a risk in eating something from the ground that has not been cooked,' Dr Freestone explains. 

The way salads are grown and processed also increases the risk of infection. 

'Flies, wasps and other critters carry contamination, adding to the issue at this time of year. They can all carry pathogenic bacteria on to fresh produce,' she added. 

While this may sound terrifying, Dr Freestone admitted, there are ways to limit the risk and eat salad leaves safely. 

Firstly, all pre-packed supermarket salad leaves should be washed, even if they're labelled 'washed and ready to eat', she said.

This is because  the rough and waxy surface on the leaf makes it difficult to wash off pathogens, so one wash is rarely enough to kill off any lurking bacteria. 

'Using tap water is fine as it is clean and provides enough of a sheer force to remove debris,' Dr Freestone said. 

'But lettuce has too much of a large surface area to expect to get rid of everything lurking in leaves.' 

However, there is some evidence that bacteria like E. Coli can enter the tissue of lettuce, meaning they can't be simply washed away. 

'You might be able to reduce the risk, but you can't eliminate it.' 

This is even more important when it comes to organic salads. Buying organic will limit exposure to forever chemicals – however, this also means they haven't been doused in the chlorinated water that helps kill off lurking bugs.  

Dr Freestone advises avoiding bagged salads altogether, as the wet and warm environment in a bag of salad provides the perfect conditions for bacteria to thrive. 

Lab trials showed that salmonella – a bacteria that causes food poisoning – increased 2,400-fold in these conditions. 

She added: 'With bagged or pre-prepared salads, one of the most important things you can do is check the use-by date. 

'Dark green and moist leaves means microbial or fungal action on plant cells is leaching out fluids from interiors of cells.

'In other words, your lettuce is starting to rot and you really need to chuck it out.' 

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