One river, 19 test sites, and a warning sign for all of Europe’s waterways

One river in Spain has just become a case study for a problem researchers say extends far beyond its own banks. A year-long investigation into the Tagus River, the longest in the country, has found antibiotic-resistant bacteria and signs of faecal contamination across multiple monitoring points, and the scientists behind it believe the pattern is likely playing out in rivers well beyond Spain.

Researchers from the University of Castilla-La Mancha tested 19 stations along the Tagus over nearly a full year, publishing their findings in the journal Environmental Research. The contamination varied in intensity by location but was not limited to a single spot, suggesting it stems from a combination of ongoing human activities across the river basin rather than one isolated source.

Sewage, agricultural runoff and livestock waste were all identified as contributors, with antibiotics used in animal farming reaching the water through manure and runoff. The researchers say this points to inadequately treated sewage still entering the system in places, even where modern treatment infrastructure exists.

The stakes attached to this kind of contamination are already well documented. A Lancet analysis estimated that bacterial antimicrobial resistance was linked to close to 5 million deaths globally in 2019, with 1.27 million deaths directly attributable to resistant infections, and a Nature Reviews Microbiology study has separately identified rivers and wastewater systems as environments where resistance genes circulate between bacteria far more freely than in hospitals.

While the Tagus was the focus of this particular study, the researchers argue the underlying pressures, growing urban populations, expanding agriculture, and shifting river flows tied to climate change, apply broadly across European rivers, not just in Spain.

Their recommendations are straightforward: stronger routine monitoring to catch resistant bacteria early, upgraded wastewater treatment, stricter pollution controls, and more responsible antibiotic use in both medicine and farming. As the study’s authors put it, keeping rivers from becoming reservoirs of resistance may be just as critical to public health as the search for new antibiotics.

Wikimedia Commons/by Antonio Soler