High exposure to swine manure in crop fields linked with increased risk of MRSA, soft-tissue infection

High exposure to swine manure spread in crop fields and proximity to high-density swine livestock operations appear to be associated with increased risk of methicillin-resistant Staphylococcus aureus (MRSA) and skin and soft-tissue infection in humans, according to a study published by JAMA Internal Medicine, a JAMA Network publication.

Most of the antibiotics used in animal feed to promote livestock growth in high-production livestock facilities are not absorbed by the animals and end up in manure. In addition to the antibiotics, antibiotic-resistant bacteria and resistance genes have been found in manure, so applying manure in crop fields close to residential homes could increase the risk of antibiotic-resistant infections, the authors write in the study background.

Joan A. Casey, M.A., of the Johns Hopkins Bloomberg School of Public Health, Baltimore, and colleagues examined the association between Pennsylvania residents' residential proximity to high-density swine and dairy/veal operations and to manure applied to crop fields, and their risk for community-associated MRSA (CA-MRSA), health care-associated MRSA (HA-MRSA) and skin and soft-tissue infections (SSTI).

The study focused on 1,539 patients with CA-MRSA, 1,335 with HA-MRSA and 2,895 with SSTI, along with 2,914 control patients without MRSA infection cared for through a single health care system in Pennsylvania from 2005 to 2010.

Researchers found higher odds of CA-MRSA, HA-MRSA and SSTI with higher swine manure exposure in crop fields. High exposure to high-density swine livestock operations also was associated with increased odds of CA-MRSA and SSTI, the results indicate.

"Proximity to swine manure application to crop fields and livestock operations each was associated with MRSA and skin and soft-tissue infection. These findings contribute to the growing concern about the potential public health impacts of high-density livestock production," the study concludes.

In a commentary, Franklin D. Lowy, M.D., of Columbia University, New York, writes: "This study is the first to demonstrate an association between MRSA infections and proximity to high-density livestock farms or to antibiotic-exposed manure."

"The authors speculate that aerosols of bacteria from the manure-containing fields might account for the spread and ultimately for the infections of those living close to the fields. Alternatively, it is possible that there are as yet unrecognized risks of infection for those living nearest the farms," Lowy continues.

"Future studies that would strengthen the identified associations include (1) determining the types and concentrations of antibiotics in manure; (2) determining the prevalence of genes mediating antimicrobial resistance found in the manure; and (3) comparing human infection isolates with those in the manure. Even without these additional studies, this investigation provides yet another reason to be concerned regarding the use of antibiotics as growth enhancers in animal feed and argues for legislation that restricts the use of antibiotics in this setting," Lowy concludes.