Much of the meat on our dinner plates contains a not-so-secret ingredient: antibiotics.
When these bacteria-fighting compounds were first introduced nearly a century ago, they were hailed as miracle drugs, capable of halting the spread of human infections that once spelled certain death. Nowadays, however, around three-quarters of the antibiotics in use are administered to mostly healthy livestock and poultry, which collectively consume about 150,000 tons of these drugs each year.
This pharmaceutical feeding frenzy comes with dire consequences. Where use of antibiotics increases, so too does the number of bacteria that can resist them—including in some of the countries least equipped to combat obstinate infections.
Today, in the journal Science, a first-of-its kind global analysis shows that, in the past 20 years, the number of antibiotic-resistant pathogens has skyrocketed in food animals—especially in certain regions of Asia, which is home to more than half of the world’s chickens and pigs. The study, which compiled data from more than 900 surveys from around the world, portends the possibility that multidrug-resistant infections may once again spread unchecked in animals and humans alike.
“[These authors] have done a terrific job of making sense of the available data,” says Karin Hoelzer, an expert in foodborne pathogens at the Pew Charitable Trusts who wasn’t involved in the study. “The key takeaway is that antimicrobial resistance is a really big threat [around the world]...and animal agriculture clearly has an important role to play.”
Soon after antibiotics first came onto the scene in 1928, they were introduced into farm animal feed. At first, the drugs served the same therapeutic purpose they did in humans, combating illness on an as-needed basis. But farmers were quick to notice antibiotics’ surprising side effect of triggering weight gain in healthy livestock and poultry—a biological quirk that beefed up the yield of marketable meat.
Nowadays, only a minority of agricultural antibiotics are administered as a treatment for disease, while the rest are consumed by healthy animals as growth enhancers and infection-preventing prophylactics, often in place of good hygiene, says study author Thomas Van Boeckel, an epidemiologist at the Swiss Federal Institute of Technology in Zurich.
These practices do more than cut agricultural corners. Any time an antibiotic is used, it gives microbes the opportunity to develop resistance—the ability to subvert a drug’s lethal effects. Decades of research have shown that bacteria can evolve resistance far faster than scientists can discover or synthesize new drugs: In the world of antimicrobial warfare, humans are dramatically outpaced.
Increasingly untreatable infections imperil food animals and, by extension, the world’s meat supply. And if and when these “superbugs” transfer the genes that confer resistance to microbes that do harm to humans, the repercussions can be even more severe.
“The health of animals, humans, and the environment are intricately linked,” says Karen Tang, a social epidemiologist and medical researcher at the University of Calgary in Canada who was not involved in the study.
For these reasons and more, several world powers, including the United States, Canada, and the European Union, have passed legislation curtailing the use of antibiotics in livestock and poultry. But in most low- and middle-income countries in Asia, Africa, and South America, where disease surveillance remains sparse, there are no formal measures in place to tabulate the true extent of antibiotic resistance—let alone regulations to curb its spread, Van Boeckel says. That’s especially concerning, he says, considering the appetite for meat in these regions is expanding at an unprecedented rate.
To fill in some of these gaps, a team led by Van Boeckel and João Pires, a clinical microbiologist at the Swiss Federal Institute of Technology, decided to appraise antibiotic resistance on a global scale. In the absence of surveillance data, the researchers turned to point prevalence surveys, which document snapshots of infection rates at a single point in time.
Using data from 901 of these surveys, the team tabulated the number of animal antibiotics to which at least 50 percent of bacterial strains were resistant—a cutoff that indicated a drug would likely fail the majority of the time. The analysis focused on four common foodborne pathogens: E. coli, Salmonella, Campylobacter, and Staphylococcus aureus.
The results weren’t surprising, Van Boeckel says, but they were grim. Between 2000 and 2018, the proportion of “failing” antibiotics nearly tripled in chickens and pigs and doubled in cattle. Among these drugs were several that also play critical roles in human medicine, such as ciprofloxacin and erythromycin. “I’m very worried about the future of these drugs,” Pires says.
The drugs most commonly used in food production, such as penicillin, also tended to harbor the highest rates of resistance. That trend hints strongly at the link between antibiotic use and resistance—something that’s never been demonstrated on such a large scale over a long period of time, says Catrin Moore, a microbiologist at the University of Oxford’s Big Data Institute who authored a perspective piece on the study, but didn’t contribute directly to its findings. Where drugs go, she says, resistance follows.
When the team superimposed its data onto a global map, they noted several “hotspots” of multidrug resistance in Asia overlapped with areas where antibiotic use is expected to increase. Many of these hotspots were also close to major urban centers, likely reflecting how local demand for meat, as well as accessibility to veterinary drugs, boost antibiotic use, Van Boeckel says.
Though these regions warrant further attention, the paper’s resistance map is far from comprehensive, Moore says. “These models are only ever as good as the data that goes into them,” she says. “This is a platform on which to build even better maps of antimicrobial resistance.”
Still, it’s clear that “a huge amount of work” went into this study, says Tang, who praised the paper as exciting and important.
Van Boeckel acknowledges that there’s no replacement for systematic surveillance, which is the gold standard for tracking infection. But point prevalence surveys, he says, “are the best thing we have available. The alternative would be nothing.”
And what’s been presented so far, he says, is already cause for immediate action. Regulations passed in high-income countries “become pointless if similar interventions aren’t implemented in low- and middle-income countries,” he says.
Of course, there’s no silver bullet against a problem this widespread. Two years ago, Van Boeckel co-authored another publication outlining three potential strategies to wean the world’s livestock and poultry off antibiotics: enforcing caps on the use of antibiotics in agriculture, taxing their use in veterinary contexts, and reducing meat consumption. The third recommendation, which calls for a potentially dramatic shift in diet, is an especially delicate one, Van Boeckel says. Moore agrees, highlighting the importance of cultural sensitivity in guiding such transitions.
All this will also require coordinated assistance from high-income countries—especially from those that have had success in cutting antibiotics out of animal feed, Van Boeckel says. But he and Pires both hope that the global stakes are clear: No one will be protected if we don’t act together, they say.
The urgency of the situation can’t be overstated, Hoelzer says. As antibiotic resistance grows, the number of treatment options for both animals and humans rapidly dwindles. “Our options are limited,” she says. “This is a global issue that concerns us all.”