Watch Out for the Cows. They Might Be Carrying a New Strain of MRSA

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Photo Illustration by Alexander Ho for TIME; Cow: Getty Images

The medical news this week has been dominated by the possible carcinogenicity of cell phones and the virulent E. coli outbreak in Europe. But I hope your closet of fear isn’t full yet — I’ve got one more bit of medical worry to stuff in it.

In a study published Thursday in the Lancet Infectious Diseases journal, a group of British and Danish scientists identified a new strain of methicillin-resistant Staphylococcus aureus (MRSA), the powerful bacteria that has been responsible for more deaths in the U.S. than HIV/AIDS. The new strain’s genetic makeup is different enough from previous MRSA strains that the popular polymerase chain reaction technique (PCR) tests would fail. “That means that the molecular tests most often used to confirm MRSA status will be falsely negative if we don’t take into account the new strain,” Dr. Mark Holmes, a senior lecturer in preventative veterinary medicine and author of the Lancet paper, told reporters at a press conference. “Some MRSA that may be out there may not have been detected.”

The study began with a scientific curiosity. Dr. Laura Garcia-Alvarez, the lead author on the paper, discovered something strange while researching mastitis, a bacterial infection in cows, as a Ph.D. student at Cambridge. The S. aureus bacterium she found during her work was able to grow despite the presence of antibiotics — a pretty clear sign that it was antibiotic-resistant. Yet strangely, when Garcia-Alvarez and her colleagues used the standard molecular tests on the strain, it came back negative for MRSA. The PCR tests were unable to find the mecA gene, which is responsible for methicillin resistance. “We were obviously curious about what was causing the methicillin resistance, if it wasn’t the mecA gene,” said Holmes.

After the Wellcome Trust Sanger Institute sequences the entire genome of the strain, the team found that it did have a gene for methicillin resistance, but one that had only 60% similarity to the original mecA gene. That’s not close enough to show up in standard molecular tests — as Holmes put it, it’s like searching for a word in a computer document and misspelling it by several letters.

Though the first strain was found in cows, subsequent testing found similar new MRSA strains in human beings in Scotland, England and Denmark, along with Ireland and Germany. The new strain has apparently been around for some time — the first Danish sample with it dates back to 1975 — and there’s been an upward trend in the strain over recent years. Not a huge upward trend: Holmes noted that there are likely less than 100 isolations of the new strain each year in Britain. But it’s worrying enough that Holmes and his colleagues were sure to alert public health agencies to be on the lookout for the new strain.

The next question is where the new strain originated. Right now Holmes and his colleagues suspect that cows are a likely reservoir, but they haven’t proved it yet; research is ongoing. (An independent study by Irish and German researchers in the journal Antimicrobial Agents and Chemotherapy came to a similar conclusion.) The concern is that if cows do harbor the MRSA strains, they could transmit them to farm workers — and from them, into hospitals and the community. Holmes emphasized, however, that it’s not possible to contract MRSA from drinking pasteurized milk, even from an infected cow. “Drinking milk or eating dairy is not a public health concern,” Holmes said.

MRSA is, but it’s not something you should be all that worried about. (Even if your possibly carcinogenic cell phone turns out to have MRSA all over it.) Many of us carry MRSA inside our bodies and are none the worse for the wear. Now mutant E. coli — that might be a different story.

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