Exposure to pig manure Study links human infections with livestock, but researchers have not identified strains
A new study adds to growing concern about methicillin-resistant staphylococcus aureus infections associated with livestock operations.
A recently published paper links people living near pig farms or fields fertilized with pig manure with higher rates of the infection.
“I think there’s a major concern that some of these infections are becoming harder to treat and they’re be-coming resistant to additional antibiotics and that we don’t really have new antibiotics coming on the market to replace those that are no longer working,” said researcher Joan Casey with the Johns Hopkins Bloomberg School of Public Health.
Staphylococcus aureus are bacteria commonly found on skin and in noses.
Infections have typically occurred through contact with vulnerable patients in hospitals. Cases are treatable, but with MRSA, the bacteria are resistant to antibiotics, and cases are more serious.
Casey referred to an “MRSA epidemic” in the United States, with more cases of infection occurring outside of health-care facilities, as a costly burden.
An increase in cases in Canadian hospitals has been noted.
The issue raises questions about antibiotic use in livestock production. MRSA has been identified in several animals, including dairy cattle, pigs and veal calves.
“We know that MRSA, especially in the U.S., is rampant in the population and some of the issue gets clouded because the strains that we find in livestock aren’t just that livestock strain,” said Scott Weese of the University of Guelph, Ont., who studies infections.
“We find human strains in pigs and horses and other species as well because we’re infecting them probably as much as they’re infecting us sometimes.”
Manure shed by livestock can contain the antibiotic-resistant bacteria and spread to the human population. Contact with the animals has been identified as a risk factor.
“We basically conducted the study because we wondered if the general population might be at risk of MRSA infection due to exposure to high-density livestock production,” said Casey.
Other researchers have identified the presence of MRSA on pig farms and in farm workers.
In Canada, issues with beef production haven’t been identified.
MRSA has been found in a small percentage of ground beef samples, but a 2009 study was unable to find the bacteria in an Alberta feedlot, raising the possibility that contamination occurs at the processing or retail level.
For her study, Casey surveyed Pennsylvania health records from 2005-10, finding 1,539 cases of community-acquired MRSA, compared to 1,335 cases of health-care associated infections.
Of the community cases, 11 percent can be linked to areas where crops are fertilized with pig manure.
Weese said the report fails to distinguish the strains of MRSA that are associated with livestock from the ones associated with humans.
One strain, USA300, is common, he said, while a separate one is linked with livestock cases.
“That’s one of the problems with studies like this. If you’re looking at MRSA infections, you need to differentiate the sequence types …” he said.
“Certainly if a person has that, we need to be thinking it came from an animal or at least animals … if they have USA300, then odds are incredibly high it came from another person.”
Casey said it’s a tough issue and more research needs to be conducted.
“We never want to leap to any strong conclusions right now. We think this is adding to the body of evidence that these livestock practices might be bad for public health, but we’re not ready to make any sweeping conclusions on this,” said Casey.
“We’d really like to see it replicated in another population, maybe in another area of the United States where there’s high pig density.”