An entirely new strain of the drug-resistant MRSA superbug has been found in cow's milk and people in Britain and Denmark, a study published on Friday said.
The previously unseen variant "potentially poses a public health problem," said lead researcher Mark Holmes, senior lecturer in preventive veterinary medicine at Britain's Cambridge University.
There was no general threat to the safety of pasteurised milk and dairy products, but people working with animals could be at risk, said the study published in The Lancet Infectious Diseases.
Dubbed a "flesh-eating" bacteria in media reports, methicillin-resistant Staphylococcus aureus (MRSA) has emerged as a major threat in hospitals around the world, becoming potentially deadly when it infects wounds.
"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," Holmes told a news conference on Thursday.
"Drinking milk or eating meat is not a health issue, as long as the milk is pasteurized," he said, adding that the process of making cheese also "generally kills most of the bacteria".
Holmes said the main worry was that the new strain would be wrongly identified by traditional genetic screening tests as being drug-susceptible, meaning people could therefore be given the wrong antibiotics.
Colleague Laura Garcia-Alvarez, also from Cambridge University, said it was "certainly worrying" to find the new strain in both cows and humans but said the pasteurisation of milk would keep it out of the food chain.
"Workers on dairy farms may be at higher risk of carrying MRSA, but we do not yet know if this translates into a higher risk of infection," Garcia-Alvarez added.
The team stumbled on the new MRSA bug while investigating mastitis, a serious disease which affects dairy cows.
They found MRSA bacteria with the same mutated gene in 13 of 940 samples from 450 dairy herds in southwest England.
Tests on people treated for MRSA revealed the same new strain in 12 instances in Scotland, 15 from England and 24 from Denmark.
The scientists also spotted a "clustering" of human and cow samples containing exactly the same new strain, suggesting transmission between cattle and humans.
Separately another study released on Friday showed another new form of MRSA in hospitals in Ireland that is closely related to the previously unseen one found in Britain.
Like the British one, it is not detected by current genetic tests and is also found in cows, said the research published in the journal Antimicrobial Agents and Chemotherapy.
"The results of our study and the independent United Kingdom study indicate that new types of MRSA that can colonise and infect humans are currently emerging from animal reservoirs in Ireland and Europe and it is difficult to correctly identify them as MRSA," said David Coleman of Dublin University.
"This knowledge will enable us to rapidly adapt existing genetic MRSA detection tests, but has also provided invaluable insights into the evolution and origins of MRSA," he added.
The announcement of the new types of MRSA comes a day after the World Health Organisation said a lethal E.coli bacteria that has killed 18 people in Europe is "extremely rare" and had never been seen in an outbreak form before.