Via Farmworkers, Superbugs Find a Route Away from Drug-Using Farms

Pig farms from the air. Image: Maryn McKenna

Pig farms from the air. Image: Maryn McKenna

One of the persistent questions regarding antibiotic use in meat production, and its effect on the health of humans who live far away from production farms, is: How do the resistant bacteria that result get from one place to another? That is: Most people accept by now that using antibiotics in livestock-raising causes drug resistance to emerge in the systems of those animals, in their guts or on their skin. But whether those newly resistant bacteria leave the farm, and how they make the trip, is both fought over and—despite much investigation—still under-researched.

Some studies have shown that bacteria can move off farms in groundwater, on the feet of flies, and via dust on the wind. What is insufficiently explored—because it is difficult to get large meat-production facilities to cooperate—is whether farm workers themselves are serving as a transport vehicle.

A new study just published (and open-access, so anyone can read it) helps to answer that question. It looks at the possibility that workers on large hog farms are carrying away drug-resistant staph or MRSA, and especially a type of resistant staph — known familiarly as “pig MRSA” and more technically as “livestock-associated MRSA” — that emerged on hog farms a decade ago and is directly linked to farm-drug use.

(If you’ve been visiting Superbug the blog for a while, you might remember pig MRSA; the story of its discovery in a Dutch farmer’s daughter 10 years ago also was told for the first time in Superbug the book. If it’s a new concept to you, you might be interested in this archive here.)

The new study finds that hog farmers are carrying multi-drug resistant livestock-associated MRSA away from the farm and — this is the crucial bit — that their bodies are hanging onto those bacteria, in a way that might allow them to spread, for up to 14 days.

A little more detail about the study: It was conducted by 13 researchers with overlapping affiliations: the University of North Carolina at Chapel Hill;  the Rural Empowerment Association for Community Help (REACH), a North Carolina nonprofit; the Center for a Livable Future at Johns Hopkins University, and several departments in Hopkins’ Bloomberg School of Public Health; and the Statens Serum Institut in Denmark. It recruited 22 farm workers who work on very large-scale intensive hog farms in North Carolina (site of the biggest concentration of such hog farms in the country), and asked them to swab the inside of their noses — MRSA’s favorite hiding place — over 14 days.

Their finding: Seven of the 22 workers were carrying livestock-associated MRSA, of a variety of resistance patterns, persistently; that is, it was always in their noses no matter when they were checked during the 2-week study period. Another three were carrying a livestock-associated resistant strain at least some of the time. That is very much higher than you would see in the general population. The rate of appearance of any type of MRSA seldom tops 4 percent. The occurrence of livestock MRSA isn’t measured in the United States, so there is no good data about how frequently it occurs.

It’s important to note that this state of carrying the germ, called colonization, was invisible: These workers had no visible staph infections at the time they were sampled. But all staph, including drug-resistant staph, has the ability to hang out quiescently on the body for variable amounts of time, in the nostrils, on the skin, or in other warm salty places such as the armpit or groin. Colonization increases the chances that the person carrying the germ will develop a staph infection, and it also alows staph to spread into a home’s environment, making it available for others to pick up.

This isn’t the first study in the US to link intensive hog raising with “pig MRSA” strains. (There have been many such studies in Europe, where much more attention gets paid to the link between antibiotic use, farms and resistant infections.) Last year, one study found pig MRSA in North Carolina farm workers and an unrelated study found it in Illinois and Indiana farm workers.

There were two other studies in the past 12 months that add to the debate: In November, a research group found higher rates of carriage of any MRSA in people who lived near fields where swine manure was applied as fertilizer, and in January, veterans who live near swine farms in Iowa were shown to be carrying MRSA at 2.76 times the rate of those living further away. That chimes with other data in this new study, which shows hog-farm workers have higher rates of staph strains that are associated with pigs but not yet resistant, and MRSA strains that have not yet passed through hogs and picked up the pig strains’ distinctive genetic signature. They also found a swarm of diversity within the pig-associated strains; not just the strain ST- or CC-398, the most common, but six others as well.

Now: This is still a small study; more studies and larger ones are needed, as the authors themselves say. But it is yet another study that finds the presence of antibiotic-resistant strains of bacteria with distinctive “farm” signatures both on farms, and with links to farms. It also finds not just a single type of resistant bacteria, but a stew of farm-related resistance, other resistance, and bacteria that might become resistant but have not yet. To me, that certainly looks like a big red arrow, blinking SOMETHING HARMFUL HAPPENING HERE, pointing toward farms that use lots of antibiotics. And if we want to be serious about reducing antibiotic resistance, we ought to follow that arrow where it leads.

Cite: Nadimpalli M, Rinsky JL, Wing S et al. Persistence of livestock-associated antibiotic-resistant Staphylococcus aureus among industrial hog operation workers in North Carolina over 14 days. Occup Environ Med. 2014 Sep 8. doi:10.1136/oemed-2014-102095


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