New paths for drug-resistant bacteria in North Carolina hog farms

The children of people who work in industrial hog farms in North Carolina, the second-largest hog-producing state in the country, are much more likely to be carrying drug-resistant bacteria than children whose households have no swine-farm contact, according to a new study.

Children were especially likely to be carrying drug-resistant bacteria if the adults in their households brought their protective gear home from work with them, raising the possibility that it is dust or manure on boots and clothing that pose a greater risk to children than whether parents have picked up bacteria themselves.

The study is by researchers at Johns Hopkins University and the University of North Carolina at Chapel Hill, along with a farmworker-advocacy group called the Rural Empowerment Association for Community Help. It doesn’t measure whether children or adults connected to industrial hog farms have higher rates of drug-resistant infections. But in the case of the bacteria the researchers were looking for — MRSA, or drug-resistant staph — people were more likely to develop infections if they previously had carried the bacteria. So the study measures, indirectly, whether farmworkers’ children are at greater risk because of their parents’ work.

North Carolina produces more hogs than any state except Iowa. In Duplin County, which was once a tobacco-farming region and embraced hogs when the tobacco economy collapsed, there are 32 pigs for every person. That is a potentially risky density, because in the United States, most intensive hog-raising operations use routine doses of antibiotics to fatten animals and protect them from diseases that spread in close confinement. In other countries — such as the Netherlands and Canada — that routine antibiotic use created a new strain of MRSA that scientists traced back, via its genetic signatures, to antibiotics given to hogs.

Since the first sightings of the new strain on a Dutch farm in 2004, researchers have been trying to understand the risk that staph in pigs poses to pig-farm workers and the rest of society. That is a tricky thing to quantify.

Unlike other organisms that become resistant to farm antibiotic use and then jump to humans — Salmonella, say, or E. coli — staph does not live in animals’ intestines and become a contaminant of meat when it is slaughtered. Staph is a skin-dwelling organism, and it hitches a ride on the skin or in the nostrils of farmworkers who have touched animals or breathed in farm dust. Plus, staph is a common health problem, whether or not farms are nearby, so sorting out the fraction of cases that can be attributed to farms is challenging.

In the new study — published in the journal Environmental Health Perspectives and based on data gathered between March and October 2014 — researchers attempted to solve that problem by choosing households in which at least one adult worked in an industrial hog operation. They then matched those households to ones in the near area in which no one had a farm job. In each case they chose households in which there was at least one child, in order to estimate the effect on children, whose undeveloped immune systems may be more at risk of infection. Researchers ended up with 198 households in which someone worked in a hog operation, and 202 nearby households where no one did.

To their surprise, among those households, there was no real difference in the presence of MRSA among the adults, whether they worked on hog farms or not; the rates of RSA carriage, as it’s called, were 2 percent for hog-farm workers and 3 percent among those who had other jobs.

But there was a significant difference in MRSA carriage, as it’s called, among those households’ children. Among the children living in households with a hog-farm workers, 49 percent were carrying staph in their noses, 14 percent were carrying drug-resistant staph, and 23 percent were carrying staph that was resistant to three or more classes of antibiotics. For children not living with a hog-farm worker, the rates were 31 percent, 6 percent and 8 percent.

The study wasn’t designed to show how children might be picking up the staph bacteria they were carrying — but given that adult rates were similar no matter their jobs, the researchers think it is unlikely the adults passed bacteria directly to their children. After scouring their data for another explanation, the researchers identified a factor they did not expect. The households where children were more likely to be carrying drug-resistant staph were also ones in which hog workers tended to bring their work gear home with them — posing the possibility that MRSA was traveling, not just by taking up residence in the adults’ noses, but in dust or manure smears on coveralls or hats or boots.

This isn’t the first time that working at an industrial livestock facility has been implicated in the spread of resistant bacteria. Ten years ago, a separate group at Johns Hopkins showed that poultry workers were 32 times more likely than people with other jobs to be carrying E. coli that was resistant to gentamicin, an antibiotic injected into eggs before they hatch into meat chickens — and that 88 percent of those poultry workers were bringing their work clothes home to wash.

In the past several years, researchers have shown that industrial livestock operations pose risks that reach beyond their property. Some of the researchers on this study earlier discovered that workers at industrial hog farms carry multi-drug resistant staph as a result of their work, but workers on antibiotic-free farms do not. Other studies have shown that people living near industrial farms carry higher rates of resistant bacteria, even if they have no connection to the farm. This includes studies done in Pennsylvania and in Germany. In Iowa, patients at the Iowa City VA Medical Center were 2.76 times more likely to be carrying MRSA if they lived within a mile of a farm that held 2,500 pigs or more.

The researchers who wrote the new paper say that more needs to be done to tease out the routes by which resistant bacteria move into farmworkers’ households. But their work already makes it clear that antibiotic-using farms can’t be considered separate from their surrounding environments, and that the risks farms take when they choose to use routine antibiotics reach far beyond a farm’s fence.

Maryn McKenna is the author of Superbug and Beating Back the Devil. She last wrote for FERN about how the Netherlands cut antibiotic use on farms. Her new book on antibiotics in agriculture will be published by National Geographic Books/Penguin Random House in 2017She writes regularly about antibiotics and agriculture for Ag Insider.