Sewage, farms and a river under strain: Spain’s Tagus tests positive for resistant bacteria

A year of water testing along Spain’s Tagus River has turned up a problem that goes beyond typical pollution readings. Researchers have detected antibiotic-resistant bacteria and signs of faecal contamination at multiple sites, and they are pointing squarely at sewage systems and farming practices as the likely sources.

The University of Castilla-La Mancha team behind the study sampled 19 locations along the river over close to a year, with the results published in the journal Environmental Research. Contamination levels differed from site to site, but the pattern repeated across the basin rather than staying confined to one problem area, which the researchers say points to multiple sources feeding the river at once.

Human sewage, urban runoff, and livestock farming all came up as contributors, with manure from animals treated with antibiotics identified as one of the main routes by which resistance-linked bacteria and drug residues reach freshwater systems. Even where treatment plants are in place, the study suggests they are not catching every resistant organism before water returns to the river.

The bigger worry, according to the researchers, is how easily resistance genes can move between bacteria once they are out in a river rather than contained in a hospital environment. A Lancet study already estimated bacterial antimicrobial resistance was tied to nearly 5 million deaths globally in 2019, with 1.27 million deaths directly attributable to resistant infections, and separate research in Nature Reviews Microbiology has flagged rivers and wastewater systems as key reservoirs for that spread.

The scientists are careful to note that detecting these bacteria in the water does not automatically put people at risk through casual contact, but it does confirm that resistance genes are actively circulating outside clinical settings, in an ecosystem millions of people rely on.

Given the Tagus River’s long-standing ecological and economic importance to Spain, the study’s authors are calling for stronger routine monitoring, upgraded wastewater treatment technology, and more careful antibiotic use across both healthcare and agriculture, arguing that protecting rivers from becoming resistance reservoirs is now as urgent as developing new antibiotics.

Wikimedia Commons/by Antonio Soler