However, they appear to be growing rapidly. In 2008 there were two confirmed cases in England and Scotland – one each – but by 2010 this had grown to 12, eight in England and four in Scotland.
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.
The 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 freqently 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,