Community-spread MRSA infections related to pig manure

Liz Szabo | USA TODAY

Living near a hog farm or a field fertilized with pig manure significantly increases the risk of being infected with a dangerous superbug, new research finds.

Two new studies published Monday in JAMA Internal Medicine focus on a bacteria called methicillin-resistant Staphylococcus aureus , or MRSA, which caused more than 80,000 invasive infections in the USA in 2011.

These infections, which invade the body deeper than the skin, can be deadly for patients in hospitals or nursing homes who have weakened immune systems. The first new study, led by the Centers for Disease Control and Prevention, suggests that hospital efforts to reduce infections are paying off; the number of hospital-based invasive MRSA infections fell by more than half from 2005 to 2011.

In 2011, for the first time since officials began tracking invasive MRSA infections, more Americans were infected with MRSA in the community than in the hospital, one of the studies shows.

In the second study, researchers found that exposure to hog manure is related to 11% of MRSA infections, even among people who don't work on farms.

Authors of that study, from the Johns Hopkins Bloomberg School of Public Health, compared infection rates of people who lived near one or more farms with those who had no exposure to hog manure.

People with the greatest exposure to hog farms -- because they lived close to a large farm or several smaller ones -- were 25% more likely to develop a MRSA infection, compared to those with the lowest exposure, says lead author Joan Casey, a graduate student at Johns Hopkins.

But people didn't have to live near hogs to be at risk. Just living near any farm field fertilized with hog manure increased the risk of a MRSA infection, Casey says.

Those with the greatest exposure to fields treated with pig waste were about 38% more likely to develop a MRSA infection or a skin and muscle infection of any kind, according to the study, based on infections treated in eastern Pennsylvania.

Although hospitals have mounted major efforts to prevent MRSA infections among their patients, doctors have had fewer clues what causes MRSA outside of hospitals, Casey says.

People are known to be at higher risk of MRSA if they inject IV drugs or if they spend time in close contact with others, such as in the military, at schools or sports facilities or in prisons, she says.

Modern farming practices make their manure more of a problem than in the past. That's because pigs are commonly fed antibiotics to fatten them up, Casey says. About 80% of the antibiotics used in the USA, in fact, are used in food animals, according to the Food and Drug Administration.

While scientists don't completely understand why antibiotics make animals fat, Casey says it's possible that the drugs kill off natural bacteria in the animals' guts that would otherwise help prevent weight gain.

About 75% of antibiotics given to livestock end up in their manure, research shows.

In addition to those antibiotics, manure also contains bacteria that have evolved to resist those antibiotics, as well as free-floating antibiotic resistance genes, Casey says.

Casey says her study is consistent with earlier research, which has found that 45% of people who work on hog farms carry MRSA, mostly on the skin. That's 30 times higher than the general population. Carrying MRSA on the skin puts people at risk of an infection, which can develop when bacteria enter the body through a wound, she says.

"Farmers can purchase antibiotics at the feed store that you or I would need a prescription for," Casey says. "We probably should be doing more to protect public health."

A pork industry group takes issues with the new findings.

Liz Wagstrom, chief veterinarian at the National Pork Producers Council, says the study doesn't prove that exposure to hog manure causes MRSA infections; only that a link exists. Wagstrom notes that none of the patients in the study was infected with the strain of MRSA most commonly found in pigs.

"We take our responsibility to protect public health very seriously," Wagstrom says. "In our work with CDC, they have not observed a single case of livestock-associated MRSA. . . Much of the slight increase in MRSA incidence could be explained by other factors, and is not well explained solely by proximity to livestock or crop fields."

In an accompanying editorial, Columbia University's Franklin Lowy writes that the new study "provides yet another reason to be concerned" about feeding antibiotics to livestock. Lowy, an infectious disease specialist, says the study also supports the need for legislation restricting the use of antibiotics in animal feed.

Last year, the Food and Drug Administration recommended phasing out the use of "medically important" antibiotics in animal feed, to help ensure that these drugs will remain effective in people. According to the FDA's voluntary guidelines, veterinarians should have to write prescriptions for antibiotics in livestock.

Environmentalists and others criticized the FDA guidelines as too weak, arguing that limits on antibiotic use on farms should be set into law.