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 is “too early to determine” how the lettuce leaves may have become contaminated – but it is “confident” this is the source.
Three companies have done that remembered dozens of sandwiches and wraps sold in supermarkets as a precaution.
So how is it possible that lettuce carries E.coli – and does this mean a summer without salad?
How could it happen?
There are three main ways in which lettuce leaves can be contaminated with E.coli, according to Professor Jim Monaghan, professor of crop sciences at Harper Adams University in Shropshire.
E.coli “essentially wants to reside in the intestines of warm-blooded animals,” he says, but pathogenic strains can be found in 10 to 15% of dairy herds – meaning a minority of manure is contaminated with it.
This manure – and the E.coli contained in it – can end up on lettuce leaves if directly contaminated.
In case of indirect contamination, the bacteria can end up in the soil or water and end up on the lettuce.
How do farmers prevent contamination?
If farmers use manure to fertilize a plot, they must wait at least a year before planting lettuce there.
They should also test the water they use for irrigation to see if E.coli is present.
“As a lettuce grower, if you irrigate a crop with water and you don’t feel comfortable drinking that water, then you wouldn’t irrigate the crop,” Prof Monaghan told Sky News.
That doesn’t mean they have to irrigate with chlorinated tap water, he adds, but it must meet an “acceptable standard.”
Salad growers will also take the risk of assessing their locations and avoiding planting salad in areas prone to flooding to avoid potential water pollution.
The FSA did not reveal whether the lettuce came from a British 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 required hospital treatment.
Should you worry about eating lettuce?
Prof Monaghan says he has not changed his buying habits when buying lettuce in response to the recent outbreak.
“I’ve got a bag of spinach and two whole heads of lettuce in my fridge at the moment. So knowing what I know about how British growers manage their risks, I’m fine with that,” he says.
“But something clearly went wrong.”
However, he notes a few things – in addition to the usual rules growers must follow – that give him peace of mind when buying salad.
“One is that it appears that the product that caused a problem has left the supply chain,” he says.
The second is that everyone involved in salad production – from growers to salad and sandwich manufacturers – will be ‘sampling like crazy’.
Does washing lettuce leaves remove E.coli?
Once the lettuce reaches the factory, it is rinsed to remove surface contamination. Chlorine is then used to reduce bacteria levels before rinsing again.
That is “effective to some extent,” says Prof. Monaghan, but it does not eliminate bacteria completely.
The same goes for washing lettuce at home: it will reduce the amount of bacteria, but not get rid of it.
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What influence does the weather have?
The recent wet weather could have created conditions that make it easier for E.coli to survive.
On a warm, sunny day, the leaves dry quickly, reducing the chance of water contamination.
They are also irradiated with UV rays, which kill bacteria.
But on wet, cloudy days there isn’t the same chance of killing bacteria, says Prof Monaghan.
We’ve had a damp start to the summer – and similar weather has been seen in Spain, Italy and France – all countries that import lettuce to Britain.
Why is it so difficult to determine the cause?
Darren Whitby, incident head at the FSA, said identifying the source of the E.coli was a “complex and ongoing investigation”.
Part of the reason it is so difficult to trace is because the contaminated produce makes up a very small proportion of the sandwiches on supermarket shelves, says Prof Monaghan.
Then there’s the fact that the products in question have a shelf life of days.
“Two days after a sandwich is made, you won’t find that sandwich again,” says Prof. Monaghan.
“It was either eaten or taken out.”
The FSA has been investigating the issue alongside the UK Health Security Agency (UKHSA), Food Standards Scotland and devolved public health authorities, businesses and local authorities.
Through epidemiological research and whole genome sequencing analysis, they narrowed down the common foods consumed by people who became ill to a small number of salad leaf products used in sandwiches, wraps, subs and rolls.
Dozens of products were recalled; although no E.coli was found in them, it was a “precautionary measure” in case they were contaminated.
The fact that E.coli was not found in the products indicates that it has left the supply chain. “There is no smoking gun,” says Prof. Monaghan.