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Toxic chemicals used to kill fleas on hundreds of thousands of pets have contaminated more than 50 rivers across Scotland at levels that could harm wildlife, according to the Scottish Government’s green watchdog.
The Scottish Environment Protection Agency (Sepa) has found concentrations of two common flea treatment chemicals up to 25 times higher than those considered safe. Rivers that have been polluted include the Clyde, the Forth, the Almond, the Tweed, the Ayr and the Annan.
The Ferret has also discovered a complaint that Sepa is failing to combat the pollution is under “detailed review” by the government agency that watches the watchdogs, Environmental Standards Scotland.
Campaigners have condemned the pollution as “unacceptable” and “environmentally destructive”. They called for the two chemicals to be banned, and urged Sepa to do more to limit the pollution they cause.
Sepa described the chemicals as “substances of increasing concern”, and argued its monitoring could inform regulatory action “where appropriate”. It was “happy” to work with Environmental Standards Scotland, it said.
The companies that make the chemicals stressed they were “essential products for animal health”. Decisions about them should be guided by “robust scientific evidence,” they said.
One of the chemicals is called fipronil, a member of a group of hazardous “forever chemicals” which can persist in the environment for many years. The other is imidacloprid, a neonicotinoid insecticide known to harm bees and other insects.
Both chemicals have been banned for agricultural use since 2018 because of the dangers they pose to wildlife. But they are still widely and legally used by pet owners to treat fleas on dogs and cats.
The problem is that they end up in rivers via sewers, after they have been washed off pets. Rivers can also be contaminated by dogs going for a swim after they have been treated for fleas.
In a new report, Sepa has released data revealing how widespread the pollution is in Scotland. Representative sampling in 2024 detected harmful levels of fipronil in 16 per cent of rivers tested (43 out of 274), and of imidacloprid in four per cent of rivers (11 of 271).
The rivers polluted with fipronil included the Clyde near Rutherglen, the Esk near Musselburgh, the Carron and Bonny Water in Falkirk, the Tyne in East Linton and the Braid Burn in Edinburgh. The worst contamination – 25 times above safe levels – was detected in May 2024 in Quhomery Burn at Inverquhomery in Aberdeenshire.
The rivers polluted with imidacloprid included Padovan Burn in West Lothian, the Orchy in Argyll and Bute and three waters on the islands of Jura and Islay. The worst contamination – 12 times above safe levels – was found in the River Annan in Dumfries and Galloway in February 2024.
According Sepa, discharges from sewage works were the “potential source” in 32 instances and nearby dog-walking in eight cases. In the absence of any statutory safe levels for the two chemicals, the agency says the pollution is above levels thought to cause “no effects”.
Earlier sampling by Sepa in 2021-22 also found hazardous levels of fipronil in two rivers, and of imidacloprid in four rivers. These were the Forth near Stirling, the Almond near Edinburgh, the Tweed on the border with England and the Ayr in Ayrshire.
"It is unacceptable that nearly a decade after I discovered this problem these highly toxic chemicals are being emitted at environmentally destructive levels" - Matt Shardlow, environmental campaigner
He argued that the failure to control discharges of fipronil and imidacloprid from sewage works over the last ten years meant that “unlawful” pollution had become routine. “This offends against Sepa’s statutory purposes,” he said.
ESS confirmed to The Ferret that it had begun a review of Shardlow’s complaint so it could decide whether or not to launch a full investigation. It would “assess aspects of compliance with environmental law and the implementation and effectiveness of it,” said a spokesperson.
Shardlow, who used to be the chief executive of the campaign group, Buglife, welcomed the review. “It is unacceptable that nearly a decade after I discovered this problem these highly toxic chemicals are being emitted at environmentally destructive levels from pet homes, vets, kennels and many other establishments, and are pouring through sewage works into rivers,” he said.
“Action is overdue. If the UK government won't halt their sale, then Sepa must control the pollution levels. Healthy rivers full of insect life are essential.”
The campaign group, WildFish, described fipronil and imidacloprid as “potent insecticides” which could harm invertebrates fundamental to healthy waters. “This pollution has been allowed to continue for far too long,” said head of science, Dr Janina Gray.
“These chemicals should be removed from veterinary medicines without delay. In the meantime, Sepa must use the regulatory tools available to it to control these pollution pathways and protect Scotland’s rivers.”
The Scottish Greens urged ESS to conduct a “rigorous” review that resulted in “meaningful” action. “These pesticides do not simply disappear after they are used on pets,” said Green MSP, Mark Ruskell.
“They can enter the water environment, and regulators need to do much more to understand the impact and tackle this pollution. This is not about blaming pet owners.”
Sepa told The Ferret that it had not so far been contacted by ESS, but would “as always be happy to assist” with any review. “The 2024 monitoring marks an important step forward in establishing a new baseline that we can build on through continued monitoring,” said chief scientific advisor, Richard Allan.
“The findings provide evidence on occurrence in Scotland’s water environment and potential pathways, which can inform future monitoring and research and, where appropriate, environmental and regulatory action.”
Sepa pointed out that most of the pollution it detected was less than five times safe levels. Higher levels were found “in a small number of samples”, it said.
Waterways were also monitored for fipronil and imidacloprid in 2025, and Sepa is aiming to publish another report in “early 2027”. Sampling and analysis for 2026 are also underway.
The National Office of Animal Health (NOAH), which represents companies that make flea treatment chemicals, insisted it was committed to minimising their environmental impact.
“NOAH wants pet owners across Scotland to be able to do the very best for their animals, while also looking after the rivers, lochs and wildlife that make Scotland so special,” said chief executive, Dawn Howard.
“Anti-parasitic veterinary medicines are essential products for animal health and welfare, treating and preventing parasites that can negatively impact animal and human health,” she told The Ferret.
“It is important that any decisions on the use of such products are based on robust scientific evidence about their sources and environmental impact, however. Fipronil and imidacloprid have a range of uses, so their presence in Scotland’s waterways cannot automatically be attributed to pet parasite medicines.”
Rob has more than 40 years’ experience specialising in reporting on environmental issues. He has co-authored three books about nuclear power, produced radio and television programmes. He likes muckraking.
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