A highly toxic insecticide used on cats and dogs to kill fleas is poisoning rivers and streams across the United States and the United Kingdom, according to two recent studies. The pollution is directly affecting water insects and the fish and birds that depend on them, the researchers warned.

Both studies focused on fipronil, a pesticide commonly used as an anti-flea substance for petsin many parts of the world. It has several properties that make it an attractive pest control agent (including high toxicity towards invertebrates and water solubility) — but those same properties also make it a nasty pollutant.

Despite being banned for agricultural use, fipronil is still commonly used in pets to treat fleas and ticks. In the UK alone, there are 66 licensed veterinary products that contain fipronil, including spot-on solutions, topical sprays, and collars impregnated with the active ingredient. Some require prescription and others don’t.

Researchers in the UK found fipronil in 99% of samples from 20 rivers and the average level of one particularly toxic breakdown product of the pesticide was 38 times above the safety limit. There are about 10 million dogs and 11 million cats in the UK, with an estimated 80% receiving flea treatments.

“Fipronil is one of the most commonly used flea products and recent studies have shown it degrades to compounds that are more toxic to most insects than fipronil itself,” Rosemary Perkins at the University of Sussex, who led the study, told The Guardian. “Our results are extremely concerning.”

This isn’t the first time researchers have sounded the alarm on this type of pollution. A study in 2017 by the conservation group Buglife had already warned over high levels of insecticides in rivers but didn’t include fipronil. Aquatic insects are highly vulnerable to such substances. Previous studies shown chronic waterway pollution led to sharp drops in insect numbers and falls in bird numbers.

With that in mind, Perkins and her team decided to review 4,000 analyses on samples in 20 English rivers. They found fipronil in 99% of the samples as well as a highly toxic breakdown product called fipronil sulfone in 97% of them. Average concentrations were 5 and 38 times higher than their chronic toxicity limits, respectively.

“I couldn’t quite believe the pesticides were so prevalent. Our rivers are routinely and chronically contaminated with these chemicals,” Dave Goulson, part of the study, told The Guardian. “The problem is these chemicals are so potent, even at tiny concentrations. We would expect them to be having significant impacts on insect life in rivers.”

Similar results were found in a recent study in the US. Researchers from Colorado State University. Researchers learned that fipronil and other related compounds were more toxic to stream communities than previous research had suggested, especially in the relatively urbanized Southeast region.

The insecticide is likely affecting stream insects and impairing aquatic ecosystems across the country. To make matters even worse, fipronil degrades into new compounds, some of which the study found to be more toxic than fipronil itself.

The researchers also found delayed or altered timing of when these insects emerged from streams, and the effects of this can cascade across the entire food chain.

“The emerging insects serve as an important food source,” Janet Miller, the lead researcher of the study, said in a statement. “When we see changes, including a drop in emergence rates or delayed emergence, it’s worrisome. The effects can reverberate beyond the banks of the stream.”

Miller said fipronil compounds were detected at unsafe concentrations in 16% of streams sampled across the U.S. and were most prevalent in streams of the Southeast region of the country. Scientists found fipronil compounds much less widespread in other regions, suggesting use patterns of the insecticide differ across the country.

The study in the UK was published in the journal Science of the Total Environment and the study in the US was published in the journal Science Advances.