Researchers from the United States have found an association between living in proximity to high-density livestock production and community-acquired infections with methicillin-resistant Staphylococcus aureus, commonly known as MRSA.

The research, by scientists from Johns Hopkins Bloomberg School of Public Health in Baltimore, follows similar findings from the Netherlands, where researchers discovered higher rates of people exposed to MRSA in livestock farming communities, even if members of the community had no direct contact with animals. Dr Ellen Silbergeld one of the Dutch study's authors, said, "In the past, MRSA had been largely associated with hospitals and other health care facilities, but in the last decade the majority of infections have been acquired in the community outside of a health care setting."

The Baltimore researchers' analysis concluded that approximately 11 percent of community-acquired MRSA and soft tissue infections in the study population could be attributed to crop fields fertilised with swine manure. Although similar research has been conducted in Europe and elsewhere in the United States, The study is the first to examine the association between high-density livestock operations, manure-applied crop fields and MRSA infections in the community.

According to the U.S. Food and Drug Administration, nearly 80 percent of antibiotics in the United States are sold for use in livestock feeds. The manure produced by these livestock and applied to crop fields contains antibiotic-resistant bacteria, resistance genes, and about 75 percent of the antibiotics consumed by the animals.

For the study, researchers identified patients with MRSA infections and skin and soft tissue infections reported in electronic medical records provided by Geisinger Health System. The two case groups were compared to patients who never had a MRSA infection. Patients received an exposure score based on their distance from areas of livestock production, the number of animals at livestock operations, the amount of manure spread on crop fields, and the size of fields.

The cases came from more than 446,000 Pennsylvania residents between 2005 and 2010. The researchers noted that between 2005 and 2010 there were about 3,000 patients with MRSA and 50,000 with skin and soft tissue infections who were diagnosed and treated in the Geisinger Health System.

The study identified 1,539 cases of community-associated MRSA (that had been contracted outside of a hospital or healthcare facility) and 1,335 cases of health-care-associated MRSA (those contracted in hospital). The researchers found a significant association between community-associated MRSA and application of swine manure to crop fields. A similar but weaker association was found between swine operations and community-associated MRSA. No association was found between dairy farms and MRSA infections.

lead author Joan A. Casey commented on the findings, "This is the first study that has linked MRSA infection and skin and soft tissue infections in the community to high-density livestock production. The results require replication, but we believe we have provided additional strong evidence that these livestock practices have important public health consequences."