An E.coli outbreak that has sickened more than 200 people has been traced to lettuce – so is it safe to eat?
The Food Standards Agency (FSA) told Sky News it was “too early to determine” how the lettuce could have become contaminated – but it was “certain” that was the source.
Three companies have he recalled dozens of sandwiches and wraps sold in supermarkets as a preventive measure.
So how can lettuce end up carrying E.coli — and does that mean a salad-free summer?
How could it have happened?
According to Professor Jim Monaghan, professor of crop science at Harper Adams University in Shropshire, there are three main ways in which lettuce leaves could have been contaminated with E.coli.
E.coli “essentially wants to be in the guts of warm-blooded animals”, he says, but pathogenic strains can be found in 10-15% of dairy herds – meaning a minority of manure is contaminated with it.
This manure—and the E.coli it carries—can end up on lettuce leaves if directly contaminated.
In the case of indirect contamination, the bacteria can enter the soil or water and be transferred to the lettuce.
How do farmers prevent contamination?
If farmers use manure to fertilize a field, they have to wait at least a year before they can plant lettuce there.
They must also test the water they use for irrigation to see if E.coli is present.
“For lettuce growers, if you’re irrigating a crop with water, if you’re not comfortable drinking that water, then you wouldn’t be irrigating the crop,” Professor Monaghan told Sky News.
That doesn’t mean they have to irrigate with chlorinated tap water, he adds, but it does have to meet an “acceptable standard.”
Lettuce growers also assess their sites and avoid growing lettuce in flood-prone areas to avoid possible water contamination.
The FSA did not reveal whether the lettuce came from a UK-based farm or was imported from abroad.
The latest figures show that as of June 11, 211 cases of E-coli have been confirmed – and at least 67 people have required hospital treatment.
Should you be afraid to eat lettuce?
Professor Monaghan says he has not changed his salad buying habits in response to the recent outbreak.
“I’ve got a bag of spinach and two whole lettuces in the fridge at the moment. So knowing what I know about how UK growers manage their risks is fine with me,” he says.
“But obviously something went wrong.
But he notes a few things — beyond the usual rules growers must follow — that convinced him to buy the lettuce.
“One is that the product that caused the problem appears to have left the supply chain,” he says.
The second is that everyone involved in lettuce production—from growers to salad and sandwich makers—will be “sampling like crazy.”
Does washing lettuce leaves get rid of E.coli?
Once the lettuce reaches the factory, it is rinsed to remove surface contamination. Chlorine is then used to reduce bacteria levels before the next rinse.
This is “effective to some extent”, says Professor Monaghan, but it does not eliminate the bacteria completely.
The same goes for washing lettuce at home – it will reduce the amount of bacteria, but not get rid of it.
Read more:
What are the symptoms of an E.coli infection?
Full list of products recalled by sandwich suppliers
What effect does the weather have?
The recent wet weather may have created conditions where E.coli can survive more easily.
On a hot, sunny day, the leaves will dry quickly, reducing the possibility of water contamination.
They will also be irradiated with UV rays that kill bacteria.
But on wet and cloudy days there isn’t the same opportunity to destroy bacteria, says Professor Monaghan.
We’ve had a wet start to the summer – and similar weather has been seen in Spain, Italy and France – all countries that import lettuce to the UK.
Why is it so hard to trace the root cause?
Darren Whitby, head of incidents at the FSA, said identifying the source of the E.coli was a “complex and ongoing investigation”.
One of the reasons they are so difficult to trace is the fact that contaminated products make up a very small proportion of sandwiches on supermarket shelves, Professor Monaghan says.
Then there’s the fact that the products in question have a shelf life of days.
“Two days after a sandwich is made, you can’t find that sandwich,” says Professor Monaghan.
“It was either eaten or pulled off the shelf.
The FSA is investigating the issue together with the UK Health Safety Agency (UKHSA), Food Standards Scotland and mandated public health authorities, businesses and local authorities.
Through epidemiological investigations and whole-genome sequence analysis, they narrowed down the common foods consumed by people who became ill to a small number of lettuce products used in sandwiches, wraps, soups and rolls.
Dozens of products were recalled; although no E.coli was found in them, this was a “precautionary measure” in case they were contaminated.
The fact that E.coli was not found in the products suggests that it has left the supply chain. “There is no smoking gun,” says Professor Monaghan.