A banned antibiotic is showing up in Brazilian river fish — and the plant that cleans the water might make it worse

Patrícia Alexandre Evangelista was tracking antibiotics in one of São Paulo state's major waterways when her team found something they weren't expecting: chloramphenicol — an antibiotic banned from use in food-producing animals in Brazil, the EU, and the United States because it can cause fatal aplastic anemia in humans — turning up inside lambari fish bought from local fishermen near the Barra Bonita reservoir.

The fish were destined for dinner tables.

According to a study published in Environmental Sciences Europe (DOI: 10.1186/s12302-025-01275-7), Evangelista's team at CENA-USP (the Center for Nuclear Energy in Agriculture at the University of São Paulo) identified residues of 12 antibiotics in the Piracicaba River — in the water, in the sediment, and in the fish. But the findings diverged sharply by matrix. During the rainy season, most antibiotics fell below detection limits across all three. In the dry season, when river volume drops and pollutants concentrate, multiple compounds emerged in water and sediment. In the fish, the picture was narrower: chloramphenicol was the only antibiotic detected at quantifiable levels during the dry season, appearing in lambari at concentrations in the range of tens of micrograms per kilogram.

The Piracicaba River drains a basin that receives treated sewage, household wastewater, aquaculture runoff, pig farming effluent, and agricultural chemicals. Samples were collected near the Santa Maria da Serra dam, where contaminants from the entire basin accumulate. Sediment levels of enrofloxacin — a fluoroquinolone widely used in aquaculture and veterinary medicine — exceeded concentrations reported in comparable international studies, according to the FAPESP press release. The sediment's high organic matter content means it can store these compounds and release them back into the water over time — a slow-motion reservoir of pharmaceutical contamination.

Why chloramphenicol changes the calculus

The chloramphenicol finding is what makes this notable beyond the environmental monitoring frame. The drug has been banned for use in food-producing animals in the European Union under Commission Regulation (EU) No 37/2010, which assigns no maximum residue limit — the regulatory signal for a prohibited substance. Brazil and the United States have similarly prohibited its use in food animals. The concern is a specific human health risk: even low-level exposure to chloramphenicol can cause aplastic anemia, a rare but potentially fatal destruction of bone marrow. The risk is not dose-dependent, meaning there is no safe threshold.

The EPA's reference point for action for chloramphenicol in food animals is 0.15 micrograms per kilogram — a level set orders of magnitude below what Evangelista's team found in the lambari. The detected concentrations, described in the study as reaching into the tens of micrograms per kilogram, exceed that threshold by a wide margin.

The plant that helps — and doesn't

The study's second act involves Salvinia auriculata, a floating aquatic plant common in tropical freshwater systems and often dismissed as invasive. Evangelista's team tested whether it could serve as a natural filter for antibiotic-contaminated water — a low-cost alternative to advanced treatment methods like ozonation that are economically out of reach in much of rural Brazil.

The results were promising on paper: in controlled experiments using carbon-14-radiolabeled compounds (which allow precise tracking of where the antibiotics end up), Salvinia removed more than 95% of enrofloxacin from water within days, cutting its half-life to two to three days. For chloramphenicol, the plant managed 30–45% removal, with half-lives of 16 to 20 days. Autoradiography showed the antibiotics concentrating primarily in the plant's roots.

But the dynamics between water concentration and fish absorption proved less straightforward than the removal numbers suggest. In some experimental conditions, the presence of Salvinia actually increased the rate at which lambari took up chloramphenicol from the water. The researchers hypothesize that the plant may partially transform the antibiotic into more bioavailable forms — cleaning the water while potentially making the remaining contamination easier for fish to absorb.

"This shows that using plants as 'sponges' for contaminants is not a trivial matter," Evangelista said. "The presence of the macrophyte changes the entire system, including the way the organism comes into contact with the contaminant."

The two antibiotics diverged sharply in how the fish handled them. Enrofloxacin stayed dissolved in water and was eliminated relatively quickly by the lambari, with a half-life of about 21 days and low tissue accumulation. Chloramphenicol behaved very differently: it persisted in fish tissue with a half-life exceeding 90 days and showed a strong tendency to bioaccumulate. That is a three-month residence time in an animal that people catch and eat from these rivers.

There was one genuinely encouraging result. When Salvinia was present, the DNA damage caused by chloramphenicol in fish — measured by micronuclei and other blood cell abnormalities — decreased to near-control levels. The researchers suggest the plant may generate fewer genotoxic byproducts or release antioxidant compounds from its roots that reduce oxidative stress. For enrofloxacin, the plant offered no such protective effect.

What this means

This is a single study on a single river in São Paulo state. But the Piracicaba River is not unusual — it is representative of waterways across Brazil and much of the developing world that receive the combined runoff of agriculture, aquaculture, and inadequate sewage treatment. The antibiotics found here are the same ones used globally. The seasonal concentration pattern — dilute during floods, concentrated during droughts — likely applies wherever rivers serve as both waste conduit and food source.

The broader context is antimicrobial resistance. A systematic analysis published in The Lancet by the Global Research on Antimicrobial Resistance Project forecasts 39.1 million deaths directly attributable to bacterial AMR between 2025 and 2050. Every waterway carrying subtherapeutic antibiotic concentrations is a potential incubator for resistant bacteria.

But the chloramphenicol finding adds a second and more immediate concern: direct human ingestion of a banned drug through contaminated fish. Chloramphenicol was detected at levels that exceed regulatory action thresholds by orders of magnitude. The contamination route — from agricultural and sewage runoff into a river, into sediment, into fish, onto a local dinner plate — bypasses the monitoring infrastructure that would catch the drug at any point in the legal food supply chain. This is not a story about resistant bacteria. It is a story about a banned substance reaching people through a pathway nobody is watching.

Phytoremediation remains a genuinely interesting approach for low-resource settings. But Evangelista's work is a useful corrective to the instinct to treat nature-based solutions as simple wins. Biology is a system, not a filter. Change one variable and the others shift in ways that may take radiolabeled compounds and autoradiography to detect.

The study was supported by FAPESP. Radiolabeled antibiotic compounds were provided by the IAEA.