Toxic pet flea treatment chemicals found at 'damaging' levels in rivers

Toxic chemicals used in pet flea treatments are widespread in rivers across Wales and are harming wildlife, scientists say.

A study found the problem was worse in urban areas, with pesticides reaching streams through misconnected sewers and discharges from waste-water treatment plants after pets and their bedding were washed.

More than 3.5 million doses of spot-on flea treatment, where liquid chemicals are applied to the back of an animal's neck, are sold in the UK each year. Senior vets are now changing their advice to pet owners on the routine use of the treatments.

Natural Resources Wales (NRW) said the findings would help to inform its work to monitor rivers and reduce pollution.

Welsh Water said the issue "did not impact the quality of drinking water".

"It was quite staggering to see [the chemicals] in these concentrations that are ecologically significant and toxic enough to start harming river wildlife," said Molly Hadley of Cardiff University's Water Research Institute.

The team spent three years sampling 62 different locations on nine rivers across Wales, trying to establish how the pesticides used in flea treatments - imidacloprid and fipronil - were reaching waterways.

Both insecticides have been banned for outdoor agricultural use in the UK, but remain widely used in products for pets.

It follows growing evidence of contamination in rivers across England, with the publication in 2025 of a UK-wide plan to try to better understand what was happening.

The chemicals were detected in over three quarters of Welsh river water samples, exceeding safe levels in almost half of the samples from urban locations.

The problem was worse downstream in built-up areas, where waterways received discharges from sewage treatment plants and misconnected sewers.

The team tested for caffeine alongside the flea treatments, becoming the first study to establish a "down the drain" pathway through which the chemicals reach rivers, rather than it just being a matter of dogs swimming in rivers and lakes.

It is estimated that as many as 500,000 properties across the UK may have sewer misconnections, with appliances or bathrooms incorrectly plumbed into surface water drains instead of foul sewers, often as a result of extensions or DIY.

Prof Steve Ormerod, who led the research, said: "What we think happens… is that people who are applying spot-on treatments to dogs, cats, rabbits or ferrets then either wash their animal in the shower, wash their animal's bedding or they wash their hands.

"Through those down the drain routes the chemicals reach small urban streams in particular."

The highest levels occurred in Roath Brook, a small Cardiff stream.

Concentrations of the chemicals were sometimes up to 45 times above safe levels, with the situation shown to be killing aquatic insects.

"The community of insects present in the stream changes as soon as the pet flea treatment chemicals are reaching toxic thresholds," Ormerod explained.

"In some cases we've got a 90% reduction in one particular mayfly species and also in one particular caddisfly, and a 30% reduction in two other mayfly species."

Hadley said: "This could have a very big impact.

"Imidacloprid and fipronil are universal pesticides – if you start putting those things into river environments then you're going to start seeing the collapse of ecosystems because invertebrates are the foundation of those ecosystems."

Rosie de Lloyd, walking her dog Meg near Roath Brook, said she was "amazed" to hear about the issue.

"I only ever shower her bottom half, not where the flea treatment goes," she said.

Linda Papadakis said she does not use the spot-on treatments on her dog Betsy and prefers tablets.

"I hate any kind of pollution to be honest, there's blue and green algae here, there's bird flu, ducks and swans dying."

In light of growing concerns about the environmental impact, the British Veterinary Association (BVA) recently updated its advice on the use of spot-on flea treatments.

Dr Gwen Rees, junior vice president of the BVA in Wales, said it was "something vets are thinking about a lot".

Whereas in the past the advice to pet owners would have been to use the treatments routinely throughout the year, vets were now urging more of a "risk-based approach".

"Each animal is going to have different circumstances and be at different risk of picking up fleas, for example it depends on the weather, whether your cat goes outside and so on," Rees said.

Talking to a vet could help an owner "decide whether for that animal at that point treatment is the right option" or "is there a better way to approach this, to be more responsible around our use of medicines", she added.

"For animals that swim a lot or who are bathed a lot and that water gets washed down the sink... it may be appropriate to move away from spot-on treatments and towards something like a tablet treatment," she said.

There were "usually options that are not spot-on" treatments for most issues, she said, but "they are different chemicals and it is best to have a conversation with your vet about the specific needs of your pet and the specific parasites that are a risk for your pet".

Flea treatments were "important medicines", she cautioned.

"It is an animal health and welfare issue if you have animals with flea infestations," she added.

"It's not [that we're saying] never use [them], but rather use them when appropriate."

Ormerod said it was the responsibility of veterinary bodies, regulators and ecologists to find a way to "address what is quite a significant environmental problem".

"For several decades, rivers in Britain were recovering ecologically from the gross pollution problems of the 20th Century, but this trend has slowed and might even be reversing," he added.

"Human and veterinary pharmaceuticals could be partly responsible."

A spokesperson for NRW, which was part of the research, said "improving the long-term water quality of Welsh rivers remains firmly at the top of our agenda and we continue to drive improvements through robust regulation and responsive action on pollution incidents".

They added that the research "strengthens the growing evidence on emerging pollutants in our rivers" and "helped to build a clearer picture of the pressures facing Welsh waterways".

Welsh Water said it was part of a wider water industry research programme looking into chemical substances in the water environment.

Washing pet bedding, disposing of leftover flea treatments and hand washing after use could all introduce these chemicals into the wastewater system, a spokesperson said - adding that letting dogs swim in rivers, particularly after recent flea treatment also had an impact.

"Redirecting misconnections to the correct drainage system can reduce environmental impact," the company said, pointing to advice on how to spot misconnections.

"It is important to highlight this issue does not impact the quality of drinking water," they added.