Pennsylvania residents living near pig farms or fields fertilized with pig manure were up to 38 percent more likely to acquire infections of methicillin-resistant Staphylococcus aureus, commonly known as MRSA, according to a new study by researchers from the John Hopkins Bloomberg School of Public Health and Geisinger’s Henry Hood Center for Health Research.
The study used the health records of MRSA-infected patients between 2005 and 2010 in the Geisinger Health System, a Pennsylvania-based medical network that helped fund the study. MRSA has been found throughout swine livestock populations and their manure in past studies, and a study published earlier this year isolated MRSA from the nasal swabs of 21 percent of pig-farm workers.
Researchers looked at the records of nearly 3,000 patients infected with MRSA and 3,000 controls, mapping out their proximity to swine operations and fertilized fields. What they found was a “significant” association between proximity to swine manure and rates of MRSA infection, according to Joan Casey, an environmental health scientist at John Hopkins.
They found that living close to pigs or their manure accounted for increased risks of 30 percent for healthcare-associated MRSA, 38 percent for community-associated MRSA, and 37 percent for skin and soft-tissue cases. MRSA infections are often categorized according to healthcare-associated risk factors, such as recent surgeries or hospital visits. If someone’s medical history includes a healthcare-associated risk factor, their MRSA infection automatically qualifies as healthcare-associated; otherwise, it’s community-associated, even though the lines between the two types may blur.
MRSA can colonize on the skin or inside the nostrils of humans without infecting them, but if those colonies are able to enter the body, such as through a wound, they can cause severe illness. The antibiotic methicillin has been known as a “last resort” antibiotic to treat Staphycoccolus aureus infections, and so the methicillin-resistant bacteria have garnered much attention, though other antibiotics may work to treat MRSA.
Casey and the study’s other authors did not examine the causal chain from pig pen to human infection, but she said that would be an important next step in MRSA research.
“We know this manure has antibiotic-resistant bacteria, and we also know MRSA is found on swine,” she told Food Safety News. “It’s also in the air and water near livestock operations, so there’s certainly a plausible environmental pathway.”
Methicillin is not used in livestock production, according to Purdue University’s Food and Animal Education Network. Methicillin does, however, belong to a group of antibiotics that are used in animal agriculture, such as penicillin G and ampicillin. Purdue says it is unclear if the use of penicillin G can promote methicillin resistance in Staphylococcus, and that it is more likely MRSA in livestock comes from other virulence factors.
The study’s abstract highlights the statistic that nearly 80 percent of antibiotics sold in the U.S. are used in livestock production.
“Unfortunately, no one is required to report what antibiotics they use,” Casey said. “Requiring that disclosure would be an excellent policy change.”
The study also looked at 200 samples of MRSA isolated from patients in the Geisinger system in 2012. Among those samples, they did not find any MRSA classified as clonal complex 398 (CC398), a strain commonly associated with livestock.
Casey said the lack of CC398 among the samples was noteworthy but not surprising. CC398 is often found colonizing on the skin of livestock, but it is not often found to cause infections in humans, and other types of MRSA persist on livestock as well.
Next, the team plans to perform more in-depth molecular analyses of the samples from 2012.
“It would also be wonderful to see this study replicated in Iowa or North Carolina, where there are a lot more pigs,” Casey said.© Food Safety News