Rowles is using a vaccine to fight the outbreak, but he is also dosing the pigs with two antibiotics — chlorotetracycline and Denagard — to prevent secondary bacterial infections. The combination is also routinely used to prevent bacterial diarrhoea and other common ailments in piglets.
Such practices have been common for decades. But few CAFOs have veterinarians on staff to advise on antibiotic use. They are not required to: veterinary antibiotics are generally available over the counter. And some CAFO operators use antibiotics much more liberally than Rowles does. Small doses of antibiotics in feed curb low-grade infections that might otherwise stymie livestock growth. Studies have found that certain antibiotics can increase pigs' growth rate by 2.5%, enough to make the difference for farmers between profit and loss. In the current US market, a farmer might get around US$1 per pound for a pig that costs about $0.94 per pound to produce. Although farm owners do not always reveal the quantities or types of antibiotics they use, an analysis of FDA data by researchers at the Johns Hopkins Center for a Livable Future in Baltimore found that in 2009, some 13.1 million kilograms — 80% of the antibiotics sold in the United States that year — were used on farms.
Antibiotic use on such a broad scale leads to resistant microbes. In a 1976 study, Stuart Levy, a microbiologist at Tufts University School of Medicine in Boston, Massachusetts, found that when farmers started using tetracycline, the numbers of tetracycline-resistant bacteria on the farms spiked. Within months, resistance had spread to microbes in farmworkers' intestinal tracts. “You don't have to look that far to see resistant bacteria moving to the environment,” Levy says.
In humans, S. aureus generally lives peacefully on the skin and in the nose. But if the bacterium enters the body through a wound, for example, it can become an aggressive pathogen and eventually make its way into the bloodstream to cause deadly infections. Most infections succumb to antibiotics, but resistant varieties, including MRSA, can be difficult if not impossible to cure. There are 270,000 strains, each potentially harmful.
Smith and her colleagues are distinguishing the strains of S. aureus around Iowa City in part by sequence type (ST), a categorization based on DNA sequences from several places in the genome. The sequence type that Male and Smith found in the Iowa nursery in 2007 was ST398. Before then, researchers had seen ST398 mostly in Europe, where it was found in livestock and farmworkers but usually did not cause infection.
But two years earlier, ST398 had been reported in a hospital in Hong Kong, from patient samples dating back to the early 2000s. For Smith and her collaborator Lance Price, an epidemiologist at George Washington University in Washington DC, it was a sign that the boundaries between animal and human infections were blurring.
In February 2012, Price, Smith and their colleagues published a genetic analysis of strains related to ST398 isolated from animals and humans around the world. They found that the lineage that gave rise to ST398 originated in humans. At some point, it crossed into livestock, where it acquired genes conferring drug resistance and a preference for pigs before jumping back to humans. As of 2012, ST398 was the cause of up to 20% of human cases of MRSA in the Netherlands, although the infections are generally mild.