Across the world, the antibiotics that farmers use to prevent illness in their animals are losing effectiveness as bacteria develop antibiotic resistance. According to new research, it’s a huge problem, one that’s been masked by a longstanding focus on the risk that resistant bacteria pose to humans instead.

This trend in the animal world carries a double danger. Long term, these resistant bacteria could travel to people, creating untreatable, hard-to-contain infections. But even now, within the meat industry, routine use of antibiotics is harming farmers’ ability to raise animals and treat them if they get sick.

The problem isn’t universal. The global hot spots for antibiotic resistance in animals are China and India, Brazil and Turkey, Iran and Kenya, and a handful of other emerging economies—exactly the places where rising demand for meat is spurring huge expansions in industrial-scale animal farming.

This is all revealed in a complex analysis published today in Science, the product of four years in which a multinational team of researchers hunted down more than 900 studies published between 2000 and 2018. Across those years, the proportion of bacteria that no longer responded to many of the drugs used to treat them tripled. Those bacteria were harvested from pigs and chickens, the most important meat animals in the developing world.

“Everyone talks about antibiotic resistance in humans, but no one has been talking about antibiotic resistance in animals,” says Ramanan Laxminarayan, the director of the Center for Disease Dynamics, Economics & Policy in Washington, DC, and the paper’s senior author. “Yet there are far more animals than humans on the planet, and they are essential for livelihoods across the developing world. If we are not able to treat sick animals, that will have a huge impact on global poverty.”

The problem arises from a way of using antibiotics that is both common and surprisingly little known. Antibiotics are added to animals’ feed, accelerating their growth and preventing them from getting sick in crowded barns and feedlots. Possibly three-fourths of all the antibiotics dispensed in the world are used this way—which is not at all how they are used in humans, where the point of the drugs is to cure infections, not prevent them.

This indiscriminate use of antibiotics dates back to the drugs’ earliest days, and it has been a touchy policy point for most of that time. The practice is risky because whenever antibiotics are deployed, the microbial world reacts to them with defensive mutations that protect bacteria from being killed. It’s also troubling because the bacteria present in animals are the same ones—Salmonella, Campylobacter, E. coli—that cause illness in humans. If a drug loses its effectiveness on farms, it won’t work to cure a person’s infection either.

Most of the attention paid to antibiotic use in animals has focused on the threat that farm-created resistance causes to humans. One example: As many drugs have lost their effectiveness, medicine has reached further back on the shelf, to antibiotics that went out of favor years ago. About a decade ago, doctors needing a last-resort antibiotic turned to an old but still effective antibiotic called colistin to cure highly resistant infections. Then in 2015, researchers discovered that colistin resistance was spreading around the world. The reason turned out to be agriculture: China and Europe had been using thousands of tons of the drugs to prop up pig production. Since then, the Chinese government has banned colistin from its agriculture—but the gene creating colistin resistance has spread to dozens of countries worldwide.