What scientists just found flowing through Toledo should worry more than Spain

The river that flows past Toledo’s old town has just delivered an uncomfortable finding. Scientists tracking water quality along Spain’s Tagus River, the longest in the country, spent nearly a year testing for antibiotic-resistant bacteria and signs of faecal contamination, and found both at multiple points along its course.

The study, carried out by researchers at the University of Castilla-La Mancha and published in the journal Environmental Research, covered 19 separate monitoring stations along the river. The contamination did not cluster around a single source; it turned up in varying degrees at sites across the basin, suggesting several overlapping causes rather than one fixable leak.

Investigators point to a combination of human sewage, agricultural runoff and livestock waste as the likely drivers, with antibiotics used on farm animals entering waterways through manure. The presence of resistant bacteria alongside faecal markers suggests sewage that has not been fully treated is still reaching the river in places.

That matters well beyond Spain. A Lancet analysis has linked bacterial antimicrobial resistance to nearly 5 million deaths globally in 2019 alone, with 1.27 million deaths directly attributable to resistant infections, and separate research in Nature Reviews Microbiology has identified rivers and wastewater systems as environments where resistance genes can pass between bacteria far more easily than inside a hospital.

Treatment plants do remove most pollutants from wastewater, but the Tagus findings show that resistant organisms can still slip through before water re-enters the river. The researchers stress that this does not mean people are at risk from casual contact with the water, but it does mean resistance genes are actively circulating in the open environment.

For a river that has shaped Spain’s history, economy and culture for centuries, the researchers say the study should be a signal to tighten monitoring, modernise sewage treatment, and use antibiotics more carefully in both healthcare and farming, before the problem spreads any further across the wider Iberian Peninsula.

Wikimedia Commons/by Antonio Soler