Each year, humans produce, prescribe, and ingest more antibiotics than they did the year before. Those drugs have done wonders for public health, saving millions from infections that might otherwise have killed them.

But the drugs' influence persists in the environment long after they've done their duty in human bodies. They leach into the outside world, where their presence can spur the development of “antibiotic resistant” strains of bacteria. In a new study that surveyed 91 rivers around the world, researchers found antibiotics in the waters of nearly two-thirds of all the sites they sampled, from the Thames to the Mekong to the Tigris.

That's a big deal, says Alistair Boxoll, the study's co-lead scientist and an environmental chemist at the University of York, in the U.K. “These are biologically active molecules, and we as a society are excreting tons of them into the environment,” he says.

That leads to the potential for huge effects on the ecology of the rivers—as well as on human health.

Resistance is growing

Antibiotics prevent harmful infections, saving millions of lives each year. But the populations of the bacteria they fight against can evolve in response, morphing and changing in ways that let them evade death by the drugs designed to kill them. That means an infection by one of these “resistant” bacteria strains is harder, and sometimes impossible, to treat. The U.K. Chief Medical Officer, Professor Dame Sally Davies, says the problem is getting worse each year, and poses a "catastrophic threat" to doctors' ability to treat basic infections in the future.

A 2016 report found that each year around 700,000 people worldwide die of infections that are resistant to the antibiotics we have today. Scientists, medical experts, and public health officials worry that number could skyrocket as resistance to commonly used medicines increases. In 2014, a U.K.-commissioned study warned that by 2050, antimicrobial-resistant infections could be the leading cause of death worldwide.

And antibiotic “pollution,” in which excess antibiotics enter natural systems and influence the bacteria living there, helps speed along the development of resistant strains. It also disrupts the delicate ecological balances in rivers and streams, changing the makeup of bacterial communities.

That can affect all kinds of ecological processes, says Emma Rosi, an aquatic ecologist at the Cary Institute of Ecosystem Studies, in Millbrook, New York, because many bacteria play critical roles in river ecosystems, like helping to cycle nutrients like carbon or nitrogen.

One big problem for scientists is that no one has had a good picture of exactly where, when, and how many antibiotics are flowing into the natural world. Many countries have little or no data about antibiotic concentrations in their rivers. So Boxall and his colleagues decided to start mapping out the scope of the problem.

Fishing for antibiotics

The team—which presented their results on Monday at the Society of Environmental Toxicology and Chemistry in Helsinki—gathered a group of collaborators from around the world, each of whom sampled their nearby rivers: 72 in all, on all continents but Antarctica. The scientists would go out on a bridge or jetty and dangle a bucket into the river water, pull up a sample, carefully push some through a filter, freeze their sample and airmail it back to the U.K. to be analyzed.

The samples were screened for 14 different types of commonly used antibiotics. No continent was immune: They found traces of at least one drug in 65 percent of all the samples they studied.

“The problem really is global,” says Boxall.

That’s not particularly surprising, says Rosi, because “anywhere people use pharmaceuticals in their everyday lives, we see the evidence downstream.”

Bodies don’t break down the drugs, so the excess comes out in urine or waste. In many developed countries, the waste—and its load of antibiotics—passes through a wastewater treatment plant, but even the state-of-the-art plants don’t clear away all of the drugs. In places with no treatment plants, the antibiotics can flow even more directly into rivers and streams.

The data matched up with those expectations. The concentrations of many of the antibiotics were highest downstream of treatment plants and river-adjacent trash dumps, and in places where sewage was routed directly into river waters.

In one river, in Bangladesh, concentrations of metronidazole, a commonly prescribed treatment for skin and mouth infections, was 300 times higher than a recently determined limit deemed “safe” for the environment. In the Danube, the second-longest river in Europe, the researchers detected seven different types of antibiotics. They found one—clarithromycin, which is used as a treatment for respiratory tract infections like bronchitis—in concentrations four times higher than “safe” levels.

“In many ways it's like the plastic pollution problem,” says Boxall. “The issue is we don't think about where our waste goes, and that it has a life beyond us.”

Even faint traces of antibiotics could have big effects on the development of resistance, says William Gaze, a microbial ecologist at the University of Exeter. Bacteria are particularly good at swapping genes around in ways that let them quickly evolve in response to a threat, like an antibiotic. That evolution can happen in the presence of even very low concentrations of the drugs, concentrations like those the research team found in rivers worldwide.

Gaze stresses that there is much more research to be done before scientists understand exactly how the evolution of antibiotic resistance works. But, he says, now is the time for communities to find solutions that will keep antibiotics from flooding into rivers, because the potential outcomes for human health are so serious.