A strain of drug-resistant staph identified in pigs in the Netherlands five years ago, which accounts for nearly one third of all staph in humans there, has been found in the U.S. for the first time, according to a new study.

Seventy percent of 209 pigs and nine of 14 workers on seven linked farms in Iowa and Illinois were found to be carrying the ST398 strain of methicillin-resistant Staphylococcus aureus (MRSA).

The study marks the first time researchers have tested for the strain in the U.S., so there's no way yet to tell when or how it arrived or how widespread it may be, says Tara Smith,  an assistant professor of epidemiology at the University of Iowa in Iowa City and lead author of the study published today in the online Public Library of Science journal, PLoS ONE.

The infection "could be due to movement of animals from farm to farm, or it could be de novo acquisition of [resistance] on this farm," she says. "It is such a small sample that we don’t know whether it has larger significance or not."

The Iowa group undertook the study on two sets of farms, called "production pyramids," while doing surveillance for an unrelated disease. Animals and humans on the second set of farms, also in Iowa, harbored no MRSA. U.S. researchers have been on the lookout for ST398 in the U.S. since it was identified in 2007 in pigs and farm workers in Ontario. The new strain was first found in 2004 in a baby about to undergo heart surgery in the Netherlands, which is nearly MRSA-free because of nationally mandated infection control in its hospitals; it was traced to a pig on her parents' farm.

The strain has spread so rapidly since then that it now accounts for 30 percent of the staph isolates now sent to the Netherlands's national laboratory, compared with 1 percent in 2000, and has started to spread within hospitals. The risk of acquiring ST398 is highest among pig farmers, says Jan Kluytmans, a professor of microbiology and infection control at the Vrije University Amsterdam, who found that 32 percent of patients in one hospital in a pig-farming community were exposed to livestock that carry the bug. "People who live some distance from farms," he says, "have a much lower...rate" of carrying the bacterium.

If it turns out to cause disease in humans in the U.S., ST398 could  further complicate the general struggle against MRSA, which is already being fought on two fronts: against a hospital-acquired strain that began attacking U.S. patients in the late 1960s, and a community strain that began sickening healthy people (who had not been hospitalized) in the 1990s. The staph strains are related, but have different genetic profiles and different resistance patterns. The hospital strain contaminates wounds and causes overwhelming bacterial infections, whereas the community strain causes a range of symptoms from mild infections to rapidly fatal pneumonias. Both can be deadly: In 2007 the U.S. Centers for Disease Control and Prevention (CDC) estimated that in 2005 94,360 Americans contracted invasive infections and 18,650 of them died; 85 percent of the deaths, it said, were caused by the health care strain.

It is not clear whether ST398 has the potential of spreading fast or causing symptoms in the U.S., but researchers say its arrival raises important questions about broad antibiotic use in animals: The Midwestern pig strain is resistant to tetracycline, a common veterinary drug, but U.S. human strains are not. There's also a question of whether the bacterium poses a risk to the food supply. Dutch researchers are about to publish evidence of ST398 in 12 percent of retail pork and other meat samples.

MRSA experts say the biggest concern is the unpredictability created by the addition of a novel strain to the rapidly evolving U.S. epidemic. "Strains change. They pick up new virulence, new resistance factors," says Andreas Voss, a professor of clinical microbiology and infection control at Radboud University Nijmegen in the Netherlands, who uncovered that country's first case in 2004. "I believe it is a potential bombshell that it is there."