It’s been just about seven years since an alert epidemiologist in the Dutch town of Nijmegen identified an aberrant strain of MRSA, drug-resistant staph, in a toddler who was going in for surgery to fix a hole in her heart. The strain was odd because it didn’t behave normally on the standard identifying tests, and because it had an unusual resistance factor — to tetracycline, a drug that it should not have been resistant to, because the Netherlands had such low rates of MRSA that tetracycline wasn’t being used against the bacterium there.
Pursuing the source of the strain, researchers at Radboud University found it in the toddler’s parents and sister, and in the family’s friends. Not knowing where else to look, they asked what the parents and their friends did for a living; discovered they were all pig farmers; and went to their farms, and checked the pigs, and found it being carried by them, too. Suddenly, that strange resistance pattern made sense: The Netherlands uses more antibiotics in pig agriculture than any other country in the European Union, and the drug that it uses the most is tetracycline. Clearly, the aberrant strain — known as MRSA ST398 for its performance on a particular identifying test — at some point had wandered into pigs, become resistant to the drugs being given to the pigs, and then crossed back to humans, carrying that new resistance factor as it went.
From that first discovery unrolled the microbiological equivalent of a car-chase scene, complete with unpredictable turns, skids around corners, and unexpected dead ends. Researchers have identified ST398 in animals, people and retail meat in most of the EU; in pigs, farmers and hospital patients in Canada, and in pigs and a few farm workers, and most recently supermarket meat, in the United States. (You’ll find a long archive of posts on ST398, and more here.)
One of the persistent mysteries about ST398 is how far it has spread in the U.S.. That’s a very difficult question to answer, because no one is consistently doing the tests that would provide data. The national meat-testing scheme that looks for resistant organisms doesn’t test for any staph, let alone this particular strain; and physicians who find resistant staph in patients seldom go as far as checking which strain of the bug is causing the infection, because that information doesn’t affect treatment and so wouldn’t be reimbursed by an insurance company.
So it falls to individual research teams to assemble the picture, data point by data point, out of whatever work they can beg funding for. The team who made that first identification of ST398 on Iowa farms and farmers has added a new piece of information to the growing picture — and it’s an important one, because it suggests that this resistant strain may be moving off farms.
The group, headed by Tara Smith at University of Iowa, report in the April issue of Emerging Infectious Diseases that they got permission to conduct MRSA surveillance in a daycare facility in Iowa. (For obvious reasons, they don’t say where.) Only a few parents gave permission (eight kids out of 168) and only a few staff agreed to cooperate (24 out of 60). In the nose of one of the staff-members, a 24-year-old woman, they found ST 398. She did not have a visible staph infection; as with that first farm family seven years ago, the bacterium was hanging out in her system, but not making her sick.
The woman lives alone, had not been hospitalized in the past year, and has no daily-life contact with farm animals; of her close family members, none of them work on farms or in meat-processing plants. In other words, there are no easy explanations for how she picked up this animal-associated strain. And because so few staff and kids participated in the screening, it also isn’t possible to say whether anyone else in the daycare center was also carrying the bug.
(Staph screening is very non-invasive, by the way; it effectively involves twirling a long-handled Q-tip inside the front of your nostrils. Kinda makes you wonder why families would not have wanted to participate. On the other hand, since Iowa is the pig-growing capital of the U.S., they may have been motivated not to want to know. UPDATE: Tara Smith explains her thoughts on this at her own blog Aetiology; she thinks the underlying issue was different than what I assumed it to be.)
There’s an important detail in the Iowa team’s results. Technically, this drug-resistant ST398 was not MRSA. That is, it was not resistant to beta-lactam antibiotics: semi-synthetic penicillins such as methicillin (the “M” in MRSA) and cephalosporins. It was resistant to other drug classes, including erythromycin.
This is not unusual. Remember the big news from a couple of weeks back, about one in four samples of supermarket meat being contaminated with drug-resistant staph. Most of that staph — some of which was ST398 — also was not MRSA, that is, not resistant to the beta-lactam class. In people, the biggest single finding has probably been the discovery a few years ago that people from a few villages in the Dominican Republic were carrying ST398 with them as them moved back and forth between the Caribbean and New York City. The single most poignant finding is probably the discovery last year that an ST398 with a similar pattern to this Iowa one killed a French child.
Just because this livestock-associated staph is not hugely resistant doesn’t mean we shouldn’t worry about it. Here’s what it does mean: First, because staph promiscuously exchanges resistance DNA, it’s a signal that an unusual strain of staph has spread faster than we have been able to understand it, and could become highly resistant with a small genetic change. Second, it’s a warning — once again — that farm antibiotic use is stimulating the development of resistant organisms in ways that we don’t understand and don’t do a good job of tracking, and that those organisms are spreading beyond where we would normally think to look.
Cite: Moritz ED, Smith TC. Livestock-associated Staphylococcus aureus in childcare worker [letter]. Emerg Infect Dis. 2011 Apr. http://www.cdc.gov/EID/content/17/4/742.htm. DOI: 10.3201/eid1704.101852
- News break: “Pig MRSA” ST398 involved in the death of a child …
- Farm worker infections with MRSA — the first numbers
- Pigs, antibiotics, and staph where it shouldn’t be
- Multi-Drug Resistant Staph in 1 in 4 Meat Samples
- Alarm over “pig MRSA” — but not in the US
Image of PFGE results of staph daycare sampling, courtesy T. Smith.