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The sight of just one boot coming through the doorway cues the clatter of tiny hoofs as 500 piglets scramble away from Mike Male. “That’s the sound of healthy pigs,” shouts Male, a veterinarian who has been working on pig farms for more than 30 years. On a hot June afternoon, he walks down the central aisle of a nursery in eastern Iowa, scoops up a piglet and dangles her by her hind legs. A newborn piglet’s navel is an easy entry point for bacterial infections, he explains. If this pig were infected, she would have an abscess, a lump of inflamed tissue, just below the navel. “In human terms, she’d be an outie instead of an innie,” he says, rubbing the pig’s healthy, pink belly button.

Nearly six years ago, an outbreak of ‘outies’ at this nursery marked the first known infection with methicillin-resistant Staphylococcus aureus (MRSA) in pigs in the United States. MRSA has troubled hospitals around the world for more than four decades and has been infecting people outside of health-care settings since at least 1995 (see
Nature 482, 23-25; 2012). It causes around 94,000 infections and 18,000 deaths annually in the United States. In the European Union, more than 150,000 people are estimated to contract MRSA each year. Its first appearance on a US farm signaled the expansion of what many believe is a dangerous source of human infection.

Male investigated the infections with Tara Smith, an epidemiologist at the University of Iowa in Iowa City, who has since launched one of the most comprehensive investigations yet of where MRSA lives and how it spreads into and out of agricultural settings. She has surveyed farms and grocery stores as well as people’s homes, noses and pets. Her findings could help to end a raging debate about whether farms’ use of antibiotics is contributing to the rise of drug-resistant bacterial infections in humans.