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Hidden antibiotics in river fish spark new food safety fears

Antibiotics are quietly building up in rivers and fish—and even nature’s cleanup tools may come with surprising side effects.

Date:

March 21, 2026

Source:

Fundação de Amparo à Pesquisa do Estado de São Paulo

Summary:

Antibiotics are accumulating in a major Brazilian river, especially during the dry season when pollution becomes more concentrated. Scientists even detected a banned drug inside fish sold for food, raising concerns about human exposure. A common aquatic plant showed promise in removing these chemicals from water—but it also altered how fish absorb them, creating unexpected risks.

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FULL STORY

X-ray of a lambari fish contaminated with carbon-14-radiolabeled enrofloxacin. The color indicates the radiative intensity of the signal, ranging from low (blue) to high (red). Credit: Patrícia Alexandre Evangelista

Researchers from the Center for Nuclear Energy in Agriculture at the University of São Paulo (CENA-USP) have identified multiple classes of antibiotics in the Piracicaba River, a major waterway in São Paulo state, Brazil. Their findings, published in Environmental Sciences Europe, show that these substances are not only present in the water but also accumulate in fish. The team also examined whether a common aquatic plant in the region, Salvinia auriculata, could help reduce this contamination.

The research was led by Patrícia Alexandre Evangelista with support from FAPESP. It combined several approaches, including environmental monitoring, studies of how pollutants build up in organisms, analyses of genetic damage in aquatic life, and experiments using plants to remove contaminants. This broad strategy allowed the team to better understand both the scale of the problem and possible ways to address pollution linked to human and veterinary drug use.

Pollution Sources and Seasonal Patterns

Samples were collected near the Santa Maria da Serra dam, close to the Barra Bonita reservoir, where contaminants from across the river basin tend to gather. This region receives inputs from treated sewage, household wastewater, aquaculture operations, pig farming, and agricultural runoff.

The researchers analyzed water, sediment, and fish during both the rainy and dry seasons. They monitored 12 commonly used antibiotics from groups such as tetracyclines, fluoroquinolones, sulfonamides, and phenols. "The results showed a clear pattern of seasonality. During the rainy season, most antibiotics had concentrations below detection limits. In the dry season, however, when water volume decreases and contaminants become concentrated, different compounds were detected," says Evangelista.

Measured levels ranged from nanograms per liter in water to micrograms per kilogram in sediment. Some antibiotics, including enrofloxacin and certain sulfonamides, were found in sediment at higher levels than those reported in similar studies worldwide. Because the sediment is rich in organic matter and nutrients like phosphorus, calcium, and magnesium, it can store these compounds and potentially release them back into the environment over time.

Banned Antibiotic Found in Fish

"One of the most significant findings of the study was the detection of chloramphenicol in lambari fish (Astyanax sp.) collected from local fishermen in the Barra Bonita region. Chloramphenicol is an antibiotic whose use in livestock is prohibited in Brazil precisely because of the risks associated with its toxicity," the researcher states.

This substance appeared only during the dry season, at levels of tens of micrograms per kilogram. Since lambari fish are widely consumed in the region, this raises concerns about possible exposure to antibiotics through food.

Evangelista explains that chloramphenicol and enrofloxacin were selected for detailed lab experiments because of their importance to both environmental and human health. "Enrofloxacin is widely used in animal husbandry, including aquaculture, as well as in human medicine. Chloramphenicol, on the other hand, is still used in humans despite being banned for food-producing animals and serves as a historical marker of persistent contamination," she explains.

Can Aquatic Plants Remove Antibiotics?

The team also explored whether Salvinia auriculata, a floating plant often considered invasive, could help clean contaminated water.

In controlled experiments, the plant was exposed to both typical environmental concentrations and levels 100 times higher for enrofloxacin and chloramphenicol. Carbon-14-radiolabeled compounds were used to precisely track how the antibiotics moved through the water, plant, and fish.

"The results showed the high efficiency of Salvinia in removing enrofloxacin. In treatments with higher plant biomass, more than 95% of the antibiotic was removed from the water within a few days. The half-life