June 2nd, 2011
06:30 PM ET
Scientists say a new strain of antibiotic-resistant staph has been identified in humans and fresh, unpasteurized cow's milk in Europe, although it's not known how widespread or virulent it is. A bigger concern, according to their study, is that a newer test may miss this strain of methicillin-resistant staphylococcus aureus (MRSA).
"We're missing out on a whole chunk of the bacteria from our understanding of the epidemiology of MRSA," said Dr. Mark A. Holmes of the department of veterinary medicine at the University of Cambridge in England, and a study co-author. "If we're going to continue the successful drive to reduce MRSA in hospitals and in communities we need to understand where it's coming from. We are not seeing the whole picture."
Clusters of the new MRSA strain were found among both humans and dairy cattle in England, Scotland and Denmark, according to the study, published in The Lancet Infectious Diseases.
But the risk of infection in the community-at-large from the new strain of MRSA is remote at best according to one infectious disease expert.
"This isn't something anyone would need to lose sleep over unless you're a farmer in England," said Dr. Gregory Moran, a clinical professor of medicine at the UCLA School of Medicine who is not affiliated with the study. "There is nothing to suggest that this is some new, extra dangerous strain that will spread further and take over from the MRSA that we already have."
The concern is less that the new MRSA strain will spread wildly, and more about the inability of a newer testing method - called polymerase chain reaction (PCR) – to identify it. A PCR test looks for specific DNA sequences in MRSA, identifying markers known to be associated with the bacteria. It is quicker than conventional testing but, according to the study, frequently misses the gene associated with this new MRSA strain.
"Sort of like having an extra source of MRSA that's somehow not being accounted for at the end of the accounting process," said Holmes.
But most hospitals in England– and the United States for that matter - tend to use a more conventional method for identifying MRSA. It involves growing the bacteria in a petri dish impregnated with an antibiotic and observing whether the bacteria grow or not. The concern is that hospitals and medical centers where this conventional method is not used could prescribe the wrong antibiotics.
It is known that animals carry MRSA and sometimes pass it to humans, however it is more likely that resistant bacteria would be spread from person-to-person, according to Moran, an infectious disease expert. Those most affected by cows carrying staph aureus are people who work or live on farms, in close proximity with the animals.
"We live in a sea of bacteria," said Dr. Robert Daum, director of the University of Chicago MRSA Research Center. "There are bacteria in our bodies, in our water, on our elevator buttons. Finding MRSA in some specimens of milk doesn't surprise me and I don't think it necessarily poses a threat to humans."
Holmes added that MRSA migrating from cows to people should not raise concerns about dairy products.
"Drinking milk or eating dairy products is not a public health concern," said Holmes. "The pasteurization of milk kills bacteria, including MRSA, without any problem."
The question lingering for study authors is whether the new MRSA strain will eventually spread from farm workers to the wider community.
"Staph aureus is ubiquitous, it's everywhere," said Moran. "This seems to be one specific strain in one specific geographic area, in one specific species. Unless you're in that area exposed to cows, you should not worry."
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