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.

Dodging detection
The old-fashioned way of testing for MRSA is to plop a sample taken from a person or animal into a dish filled with antibiotics. If the bacteria survives and grows, it has a gene that renders it immune to the drugs. If it dies, then it is a susceptible breed of S. aureus and can be treated with standard antibiotics.

The newer, genetic tests for MRSA are quicker and largely automated, and with their costs quickly coming down, they have been poised to take the place of the culture tests. "Generally, these are incredibly accurate," Holmes said of the genetic tests.

But the new strain indicates that it might be time for a revision of the genetic screening tests—and to not be so hasty to move away from the culture-based tests. A patient with this strain of MRSA tested with the genetic test would be wrongly diagnosed as having a more treatable S. aureus infection and likely given drugs that would prove ineffective.

The gene that evaded the state-of-the-art genetic tests is a remodeled version of the known resistance gene mecA. After sequencing the new strain's genome, however, the researchers found that its mecA gene was only about 60 percent the same as the known version. With so many different base pairs than the target gene, the tests were not catching it—a miss that Holmes equates to searching a digitized text document for a term but spelling it wrong in the search window, thus returning a null result.

In the investigation, 11 of the samples that turned out to be antibiotic-resistant did not have either the old or the new mecA gene, which has Holmes and his team wondering what it is that is making these strains resistant. They are currently sequencing the genomes of these types to try to find out.

Drugged out
The appearance of drug-resistant bacteria of many kinds in humans in recent decades has largely been attributed to the over- and misuse of antibiotics in people. When drugs are not taken to the end of their course or are used for non-bacterial infections such as viral infections, they can kill off weaker bacteria, prompting the stronger ones to survive and propagate. And some can eventually become immune to the drugs entirely.

The situation in the livestock industry has been described in similar terms. In many confined feeding operations, animals are given prophylactic antimicrobials as a matter of course to stave off any infections before they start.

Dairy farmers in the U.K. use antimicrobials to treat the common udder infection mastitis. Holmes explains that farmers are under great pressure to keep their herds healthy despite high production demands. "The majority of farmers are under tremendous financial pressure," he said. "When you drive your cows hard, you get more mastitis." And when their cows get this infection a farmer will naturally want to "use the best tool that they have in their armory to treat those cows."

That the newly described MRSA strain was also found in people in Denmark raised questions for some, given that the country has been at the forefront of reducing antibiotic use in livestock. But because the first isolate from a human in Denmark was from 1975, the strain has been in existence for some time—through some of the high years of antibiotic use in farm animals.

Is antibiotic use in livestock directly responsible for this new strain? "I don't think that can be resolved with this study," Smith says. And the study authors note that with their data they cannot prove that the use of antimicrobials on farms led directly to the development of this new resistant strain. But the correlation has been established in other studies. "It is clear that use of antibiotics in adult dairy cows and other food-producing animals does contribute to increased antimicrobial resistance," concluded the authors of a March review paper published in Foodborne Pathogens and Diseases.

By looking at the limited data they have from their first study, Holmes said the prevalence of this strain "appears to be rising at the moment." He and his team will spend part of the summer sampling more farms and hospitals throughout the U.K. to get a better picture of the strain's prevalence.

Could this strain also be lurking in the U.S.? "I would guess that it is," Smith says. "If we turn over some rocks, I would guess that we would find it." In Denmark, samples from each case of MRSA are collected, providing researchers with a comprehensive view of the disease landscape, but in the U.S. testing is more sporadic.

The new find highlights the need for more widespread testing and a detective hunt to find out where and when this strain emerged, Smith says. And the revelation that dairy herds can carry MRSA also underscores the importance of proper hygiene and animal care, Holmes noted.

The researchers emphasized that the threat to dairy consumers is minimal. Only those eating unpasteurized products with wounds in their mouths or esophagi would be likely to become carriers of MRSA, which could then theoretically spread to someone with a compromised immune system. And while resistant to some of the most common antibiotics, the new variant was still treatable by some drugs. But Holmes does call attention to the hazards for farm workers, who, he notes, "are no different than nurses and doctors who work in hospitals" in their likelihood of picking up MRSA on the job.

But is Holmes, himself, worried? "I'm not one of life's worriers," he said in a light tone at the press briefing. But it is "a legitimate concern," he noted more seriously. "We're lucky we caught it early."