A new form of drug-resistant Staphylococcus aureus (MRSA) has been found in dairy cows and humans in the U.K. and Denmark, providing more evidence that animals could be passing this superbug on to people—not just the other way around.
The new methicillin-resistant bacterial strain was found in tests of raw milk by a team looking for another infection among the herds. Pasteurization kills off the bacteria, making milk products—even from a cow infected with this antibiotic-resistant strain—safe for consumers, the researchers explain.
But one thing does trouble scientists: this new strain would be missed by the newest types of MRSA testing because it contains a new variant of the known resistance gene. These genetic tests have, until now, been considered the new gold standard of detection and are now widely used in many hospitals in continental Europe.
"Those DNA tests, they all missed this," says Tara Smith, an assistant professor of epidemiology at the University of Iowa. "The gene they were looking for wasn't there."
The research so far shows that about 1 percent of MRSA might have this new gene variant. Although that might sound miniscule, in epidemiological terms, says Smith, who was not involved in the new research, "that's big." And because scientists, health care workers and veterinarians are just learning to look for this strain, the prevalence is probably much higher, she says.
Of milk cows and men
Despite its scary connotations, S. aureus is a common bacterium—about a third of us carry it around without ill effect. And although about 1 percent of that S. aureus is MRSA, most of those carriers do not get sick from it. People in hospitals and those with compromised immune systems, however, are among those less able to fight it off. And when a Staph infection turns out to be resistant to a wide range of antibiotics, it is much trickier to treat.
Even as hospital-acquired MRSA cases have been declining in many countries, including the U.S., cases picked up outside health care settings are on the rise. Some of these cases are likely transmitted from people who have been in contact with a medical setting and then passed MRSA along through a chain of others through a sneeze or skin contact.
And the new findings indicate to Smith "that there's probably more movement of Staph aureus between people and animals than we've realized."
MRSA already comes in many different strains, which hold telltale genetic signatures indicating their host of origin—human, pig or cow. MRSA has also been found in pets and poultry, but these strains are usually found to be human types that had been passed to those animals by close contact. The new strain announced this week, including the cases found in human samples, is a bovine type.
Although the researchers note that they cannot prove that the strain has been moving between cows and people, the evidence for this transfer is pretty good. Geographical clusters of the same isolates were found in both cows and humans throughout different parts of the U.K.
"It changes our view of where the MRSA is coming from," Mark Holmes, of the Department of Veterinary Medicine at the University of Cambridge, and a senior scientist on the team that described the new variety, said in a press briefing. A paper describing the analysis published in the June 3 issue of The Lancet Infectious Diseases.
Smith, who is chairing an upcoming American Society of Microbiology meeting on MRSA in animals, calls the new research "pretty solid—and definitely interesting." It was published at the same time as a paper in Antimicrobial Agents and Chemotherapy that describes the dodgy new genetic variant in more detail.