Toxic flea treatment chemicals found in rivers across Wales

Scientists have found that toxic chemicals used in pet flea treatments are now widespread in rivers across Wales, raising serious concerns about the impact on freshwater wildlife and aquatic ecosystems.

A major study revealed that pesticides commonly found in spot-on flea treatments for dogs and cats are entering rivers and streams, particularly in urban areas, where they can harm insects and other river life.

The findings add to growing concern over the role of pet parasite treatments in UK river pollution, following the publication in 2025 of a UK-wide plan, external to try to better understand what was happening.

Researchers are warning that these chemicals are becoming a hidden but significant environmental threat.

“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.

How flea treatment chemicals are getting into Welsh waterways

Researchers discovered that the pesticides are reaching rivers through a “down the drain” pathway.

This happens when:

• pets are washed after treatment
• pet bedding is cleaned
• owners wash their hands after applying treatment
• leftover flea products are rinsed away
• misconnected household drains send wastewater directly into streams
• wastewater treatment plants discharge contaminated water into rivers

To help confirm this route, the team also tested for caffeine, which is commonly used as an indicator of household wastewater contamination.

This is the first study to clearly show how pet flea treatment chemicals are being washed into rivers through domestic wastewater systems, rather than only through pets swimming in rivers and lakes.

Which chemicals were found in the rivers?

The two main chemicals identified were:

• Imidacloprid
• Fipronil

Both are insecticides that have already been banned for outdoor agricultural use in the UK because of the risks they pose to the environment.

However, despite these restrictions, they are still widely used in pet flea and tick treatments.

More than 3.5 million doses of spot-on flea treatments are sold in the UK each year, making them one of the most common sources of pesticide exposure for companion animals.

Study finds widespread contamination in Welsh rivers

Over a three-year period, researchers collected water samples from 62 locations across nine rivers in Wales to understand how these harmful substances were spreading through the environment.

The results were alarming:

• The chemicals were found in more than three-quarters of river water samples
• In urban locations, safe environmental levels were exceeded in almost half of samples
• Pollution levels were highest downstream of towns and cities
• The worst-affected site was Roath Brook in Cardiff

At some locations, chemical concentrations were recorded at up to 45 times above safe levels.

Researchers say these pollution levels are already killing aquatic insects, which are essential to healthy river ecosystems and food chains.

Urban rivers are facing the greatest risk

The study found that urban rivers and streams are under the most pressure, largely because they receive water from:

• sewage treatment works
• surface water drains
• misconnected household plumbing
• runoff from built-up areas

Across the UK, it is estimated that up to 500,000 properties may have sewer misconnections, often caused by DIY work, extensions, or incorrectly installed bathrooms and appliances.

This means wastewater that should enter the foul sewer system can instead flow directly into rivers and streams.

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.”

For wildlife, the consequences can be severe.

Why this matters for wildlife and river health

Healthy rivers depend on a wide range of aquatic insects, invertebrates, fish, birds, and plants.

When pesticides such as imidacloprid and fipronil enter freshwater environments, they can damage the species that form the foundation of river food webs.

Even low levels of these chemicals can have serious consequences for:

• aquatic insects
• freshwater biodiversity
• fish populations
• bird species that rely on river insects for food
• overall ecosystem resilience

This is why the issue matters far beyond pet care — it is now a river conservation and biodiversity crisis.

“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.”

Veterinary advice on flea treatments is beginning to change

In response to growing environmental concerns, the British Veterinary Association (BVA) has updated its advice on the use of spot-on flea treatments.

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.

Veterinary professionals are now moving away from recommending these treatments as a routine year-round default and are instead encouraging a more risk-based approach.

This means flea treatments should be used:

• only when needed
• based on the pet’s lifestyle and exposure risk
• with greater awareness of environmental impact

Dr Gwen Rees, junior vice president of the BVA in Wales, said the issue is now something that vets are thinking about seriously.

“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.”

However, vets also stress that flea treatments remain important medicines, especially where pets genuinely need protection.

The challenge is finding a balance between animal welfare and environmental protection.

Linda Papadakis said she now doesn’t use any 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.”

What water authorities are saying

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

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”.

Meanwhile, Welsh Water said the issue does not affect the quality of drinking water, but confirmed it is involved in wider research into chemical substances in the water environment.

“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.

Water companies and environmental regulators are increasingly recognising that pollution from household products — including pet treatments — is an emerging issue that needs closer attention.

How pet owners can help reduce river pollution

Pet owners can play an important role in reducing contamination from flea treatments.

Simple actions include:

1. Speak to your vet before using routine flea treatments

Ask whether your pet genuinely needs treatment, and whether there are lower-impact alternatives.

2. Avoid washing pets immediately after treatment

This helps reduce the amount of pesticide washed into drains and waterways.

3. Wash pet bedding carefully

Try to reduce unnecessary washing soon after treatment has been applied.

4. Dispose of leftover products responsibly

Never rinse unused treatments down the sink, toilet, or outdoor drain.

5. Keep treated pets out of rivers and streams

Avoid allowing dogs to swim shortly after flea treatment has been used.

6. Check household drainage

If possible, make sure sinks, washing machines, and bathrooms are connected to the correct sewer system.

A growing pollution problem that can no longer be ignored

The discovery of flea treatment pesticides in Welsh rivers highlights a wider and often overlooked pollution issue affecting waterways across the UK.

As concern grows over the decline of freshwater wildlife, this research makes one thing clear: what we use in our homes — and on our pets — can have a major impact on the natural world.

Reducing this form of pollution will require action from:

• pet owners
• vets
• water companies
• regulators
• government
• conservation organisations

Ormerod agreed, saying that it was the responsibility of veterinary bodies, regulators and ecologists to find a solution 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.”

Protecting rivers means looking not only at industrial or agricultural pollution, but also at the everyday chemicals entering the environment through modern life.

Support healthier rivers and wildlife in Wales and across the UK

At Natural World Fund, we believe healthy rivers are essential for wildlife, biodiversity, and future generations.

By raising awareness of issues like pet flea treatment pollution, we can help drive practical changes that protect both animals and ecosystems.

If you care about river conservation, wildlife protection, and reducing pollution, support the work of Natural World Fund today.