Cows Milk contains MRSA, a new superbug with resistance to antibiotics.
Scientists from Cambridge University found the strain by accident and it has already infected humans.
In a handful of cases people have become seriously ill, although it is not known if anyone has died of it.
At the moment the scientists estimate there are probably less than 100 cases, which is a small fraction of the total number of MRSA cases.
However, they appear to be growing rapidly. In 2008 there was a confirmed case in England and one in Scotland. But, by 2010 this had grown to 12 – eight in England and four in Scotland.
It is believed to be the first time the strain has ever been found in cow’s milk.
The scientists emphasised that pasteurised milk and cheese were completely safe to consume, but it was possible to contract the new strain through eating unpasteurised products – although the chance is extremely low.
However, they said dairy farm workers could become carriers and unknowingly infect vulnerable friends and family, in much the same way that nurses can.
Dr Mark Holmes said dairy farmers under “relentless financial pressure” from supermarkets to produce milk as cheaply as possible, which inevitably led to more antibiotic usage in physically stressed herds and hence the emergence of resistant bacteria.
“When you drive your cows hard, you end up with more mastitis,” he explained. “You end up using more antibiotics, and the bacteria that are susceptible to it will die, and the bacteria that are resistant to it will live.”
Helen Browning, director of the Soil Association, commented: “This new evidence confirms our long-held view of the importance of absolutely minimising the use of antibiotics.
“In the relentless drive for increased per animal productivity, and under acute price pressure, dairy systems are becoming ever more antibiotic dependent.
“We need to get farmers off this treadmill, even if that means that milk has to cost a few pennies more. That would be a very small price to pay for maintaining the efficacy of these life-saving drugs.”
But Andrew Opie, food director at the British Retail Consortium, said: “It’s ludicrous to blame supermarkets. Currently, the top 13 best paying milk contracts are all contracts paid by supermarkets.
“Retailers work directly with farmers in their dedicated supply chains and through the group Responsible Use of Medicines in Agriculture to ensure antibiotics are used appropriately.”
The Cambridge University team identified the new variant after being asked to help investigate mastitis in cows.
This is a common udder infection caused by Staphylococcus aureus (SA) bacteria, and is frequently treated with antibiotics.
In 2006 the Veterinary Laboratories Agency, a government body, started analysing milk samples from herds with mastitis to find out what SA bacteria they harboured. Over 18 months they collected samples from 465 herds, identifying 940 SA isolates.
Dr Holmes said: “They reported that they found no MRSA, but they had found 24 bacteria that has unusually high antibiotic resistance.”
The agency was using the current “gold standard” molecular screening test for MRSA, called the PCR test.
However, when Dr Matt Holden of the Wellcome Trust Sanger institute sequenced the bacteria’s entire genomes, he found they were indeed MRSA as they contained a critical gene, called the mecA gene.
They found 13 of the 24 samples were essentially one strain. However, their mecA genes bore only 60 per cent similarity to that of ‘normal’ MRSA in humans. This resulted in the standard PCR tests giving a “false negative”. The study is published today (FRI) in the journal The Lancet Infectious Diseases.
This strain is a bovine type, meaning its typical host is a cow. This makes it highly likely that cows are the source of infection, rather than humans infecting them.
