Meat produced from British pigs has been shown to be infected with a livestock strain of MRSA, the Guardian can reveal, raising concerns that the UK is on the brink of another food scandal.

Tests on a sample of 97 UK-produced pork products from supermarkets show that three – sold at Asda and Sainsbury’s – were contaminated with the superbug strain which can cause serious health problems.

The Guardian, working with the Bureau of Investigative Journalism (BIJ), has also established that a loophole in import regulations is leaving an open door for MRSA CC398-infected live pigs from countries such as Denmark, where the disease is rife.

What is the superbug LA-MRSA CC398 and why is it spreading on farms? Read more

The findings have serious implications for human health. MRSA CC398 is a potentially deadly bacteria which can be resistant to even the strongest antibiotics. It is less harmful to humans than the MRSA bug that kills about 300 people in hospitals in England and Wales each year. But it causes unpleasant persistent infections and can seriously harm people with compromised immune systems, such as those already suffering other illness.

It is known to have been responsible for at least six deaths in Denmark, though that is likely to be the tip of the iceberg. People can contract the disease from infected meat, and from infected animals.

The superbug, like other foodborne germs, is killed by thorough cooking - but it can be passed on through lapses in hygiene. Workers on pig farms can also catch the disease from infected animals and pass it on to other people.

Without action to halt it, the spread of the bug could follow the pattern in Denmark, where MRSA CC398 became established over a decade, now afflicts about two-thirds of pig farms and is viewed as a major public health crisis, with 12,000 people believed to have contracted it. Currently, there is no screening programme for MRSA CC398 on British farms.

Prof Tim Lang, of the Centre for Food Policy at City University in London, said: “If we don’t have tight infection control and we don’t try to control the movement of live animals, infection can spread. The British are up in arms about the movement of people, but the EU also has a large movement of animals. We need biosecurity, we need to tighten up this livestock movement. You may get cheap meat, but in the long term it’s going to add to your public health problems.”

Prof Erik Millstone of Sussex University added that the emergence of antibiotic-resistant superbugs from agriculture was “a huge threat to human health” and that there was a clear risk that MRSA CC398 could spread among British livestock herds and to people. He anticipated that the government would downplay this risk but warned: “While the [government] tries its customary tactics of blaming the victim, it won’t work.”

The tests were carried out by Dr Mark Holmes, director of studies in clinical veterinary medicine at Churchill College, Cambridge University, and commissioned by the Alliance to Save Our Antibiotics, founded by the Soil Association, Compassion in World Farming and Sustain. Two samples of minced pork from Asda and one from Sainsbury’s were found positive.

Last year, tests by the Guardian on 100 samples of pork in UK supermarkets, including a mix of imported and UK-bred meat, found nine contained the superbug. However, all but one of the infected products were of Danish origin, the other Irish. That investigation marked the first time MRSA CC398 had been publicly found in products in UK supermarkets.

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Today’s follow-up tests are the first to name supermarkets whose UK-produced pork has been contaminated with the superbug. It is not possible to say whether these products originally came from imported pigs, or whether UK pig herds have been infected through imports. Either way, imports are likely to be the main agent spreading the disease, as the UK’s pork production has remained relatively free from MRSA CC398 until now, and the main method of spreading the disease is from animal-to-animal contact on farms.

At least one regular Danish supplier of imported pigs to the UK was found to be contaminated with the drug-resistant bacteria in 2014, the Bureau of Investigative Journalism has established. The company –Breeding Centre Rønshauge A/S – refused to say how many pigs it had exported to the UK and whether they could have been contaminated. But official export figures show that the company supplied 41 pigs to the UK in July this year, 65 in 2013 and 16 in 2012.

But the UK government does not screen for the infection in imported animals, citing a low risk of serious illness. The main initial effect is a nasty skin infection that is disfiguring, unpleasant and highly infectious, but not fatal.

Emma Rose, from the Alliance to Save Our Antibiotics, said: “It is extremely worrying to find LA [livestock-associated]-MRSA in British-produced pork.”

“Scientists are now warning that the extensive MRSA reservoir in animals could ultimately lead to a pandemic spread in the human population. LA-MRSA is able to cause serious and potentially fatal infections in humans, and as the bacteria is resistant to antibiotics, it is extremely difficult to treat. What’s more, even more dangerous variations are emerging as the superbug evolves.”

The Department for Environment, Food and Rural Affairs (Defra) said: “Livestock-associated MRSA is not the same as MRSA strains that can cause healthcare-associated infections and if meat is handled and prepared properly the risk to people is low. Defra and the National Pig Association recommend that pigs imported to Britain are screened for LA-MRSA.

“The government is reviewing options for surveillance, which will be proportionate to the very low health risk posed by livestock-associated MRSA.”

Asda said: “Our customers can be assured that we are working closely with industry groups and farmers to make sure that antibiotics are used responsibly in farm animals.” Sainsbury’s told the Guardian that MRSA CC398 was “very uncommon” in British pork and that it worked with farmers “to ensure antibiotics are used responsibly and are taking advice from leading industry experts”.

The rise of the CC398 variant has been linked to the overuse of antibiotics in factory farming, where often cramped and dirty conditions allow disease to flourish and farmers become dependent on frequent doses of antibiotics.

The threat from imports of live animals has been overlooked, according to experts. When animals are imported, they have to undergo screening for a variety of other diseases, but screening for MRSA CC398 is purely voluntary. As a result, there is no way of knowing how many infected animals may have been brought to Britain.

There have been at least two confirmed cases of the disease found at UK pig farms, one of them in Northern Ireland and the other in eastern England. However, as no systematic tests are carried out on UK farms, it is impossible to know how many may harbour the bug.

Defra does not collate statistics on the number of farm animals imported, but figures from the Danish government show the UK imported more than 3,000 breeding pigs from the country in the past six years, including 916 in 2013, 598 the following year and 283 last year.

In the last three years, the UK food industry has been rocked with revelations from the Guardian of campylobacter in chicken, which can cause serious illness, stronger versions of salmonella with greater resistance, and of drug-resistant forms of E coli, recently found in one in four supermarket chickens, that thwart all but the strongest antibiotics. Most of the problems are thought to be down to the pressure towards factory farms producing the cheapest possible meat.