British scientists have found a new strain of the "superbug" MRSA in milk from cows and in swab samples from humans and say it cannot be detected with standard tests.
Researchers said the find was "worrying" but added it was unlikely that the methicillin-resistant Staphylococcus aureus bug, which is resistant to some antibiotics, could cause infections by getting into the food chain via milk.
Mark Holmes and a team of scientists from Cambridge University, found the new MRSA bug while researching S. aureus, a bug known to cause a potentially lethal disease in dairy cows called bovine mastitis. The discovery was published on Friday in The Lancet Infectious Diseases journal.
"To find the same new strain in both humans and cows is certainly worrying. However, pasteurization of milk will prevent any risk of infection via the food chain," said Laura Garcia-Alvarez, who worked on Holmes' team.
MRSA is estimated to kill 19,000 people each year in the United States — far more than HIV and AIDS — and a similar number in Europe.
The overuse and misuse of antibiotics in recent decades have fueled a rise in drug-resistant "superbug" infections such as MRSA and C-difficile.
Last year, scientists warned that a new so-called superbug from India known as New Delhi metallo-beta-lactamase (NDM-1) was rapidly spreading around the world.
During this study, Holmes team found that the new MRSA strain was resistant to some drugs, like normal MRSA, but it was also impossible to identify using standard molecular tests. These tests work by detecting the presence of the gene which makes the bug resistant to methicillin, called the mecA gene.
When another team of researchers decoded all the genes in the bacteria's DNA, they found that the new strain does have a mecA gene, it is only 60 percent similar to the "normal" MRSA mecA gene, meaning it can't be detected with standard tests.
In further research, scientists then found that the new strain was also present in human samples from Scotland, England and Denmark — some from screening tests and others from people with MRSA disease. It has also since been identified in Ireland and Germany, they said in their study.
"From a food safety point of view, we are confident that the discovery of this MRSA in milk does not represent any disease risk," Holmes said in a statement about the findings.
But he said the finding did raise questions about whether cows could be a reservoir for the new strains of MRSA.
"Although there is circumstantial evidence that dairy cows are providing a reservoir of infection, it is still not known for certain if cows are infecting people, or people are infecting cows. This is one of the many things we will be looking into next," he said.
Scientists at Britain's Health Protection Agency (HPA) who also worked on this study said that although the new strain of MRSA cannot be pick up by standard tests, it does not pose a major threat. Newer test currently being trialed in Britain and Europe has proved able to detect it, they said.
"It's important to remember MRSA is still treatable with a range of antibiotics and the risk of becoming infected with this new strain is very low," said Angela Kearns, head of the HPA's Staphylococcus laboratory.