Highly toxic insecticides used on cats and dogs to kill fleas are poisoning rivers across England, a study has revealed. The discovery is “extremely concerning” for water insects, and the fish and birds that depend on them, the scientists said, who expect significant environmental damage is being done.

The research 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. Fipronil and another nerve agent called imidacloprid that was found in the rivers have been banned from use on farms for some years.

There are about 10 million dogs and 11 million cats in the UK, with an estimated 80% receiving flea treatments, whether needed or not. The researchers said the blanket use of flea treatments should be discouraged and that new regulation is needed. Currently, the flea treatments are approved without an assessment of environmental damage.

“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,” said Rosemary Perkins at the University of Sussex, who led the study. “Our results are extremely concerning.”

Prof Dave Goulson, also at the University of Sussex and part of the team, said: “I couldn’t quite believe the pesticides were so prevalent. Our rivers are routinely and chronically contaminated with both of these chemicals.

“The problem is these chemicals are so potent,” he said, even at tiny concentrations. “We would expect them to be having significant impacts on insect life in rivers.” One flea treatment of a medium-sized dog with imidacloprid contains enough pesticide to kill 60 million bees, he said.

The first report of high levels of neonicotinoids such as imidacloprid in rivers came in 2017 from the conservation group Buglife, although that study did not include fipronil. Aquatic insects are known to be vulnerable to neonicotinoids and Dutch research has shown chronic waterway pollution led to sharp drops in insect numbers and falls in bird numbers. Aquatic insects are also declining due to other pollution from farms and sewage, with just 14% of English rivers in good ecological health.

The new study, published in the journal Science of the Total Environment, includes almost 4,000 analyses on samples gathered by the Environment Agency in 20 English rivers between 2016-18. These ranged from the River Test in Hampshire to the River Eden in Cumbria.

Fipronil was detected in 99% of samples and a highly toxic breakdown product called fipronil sulfone was found in 97%. The average concentrations were 5 and 38 times higher than their chronic toxicity limits, respectively. The UK has no official limit for these chemicals so the scientists used a 2017 assessment produced for a water quality control board in California. Imidacloprid was found in 66% of the samples and was above toxicity limits in seven of the 20 rivers.

Fipronil was banned from use on farms in 2017, but was little used before that. Imidacloprid was banned in 2018 and was also relatively little used in recent years. The researchers found the highest levels of the pesticides downstream from water treatment plants, showing that urban areas were the main source and not farmland.

The washing of pets was already known to flush fipronil into sewers and then rivers, while dogs swimming in rivers provides another pathway for contamination. “It has to be the flea treatments causing the pollution,” Goulson said. “Really, there’s no other conceivable source.”

There are 66 licensed veterinary products containing fipronil and 21 containing imidacloprid in the UK, many of which are sold without prescriptions. Many pets are treated every month, whether the flea treatment is needed or not.

This needs to be reconsidered, the scientists said, particularly in winter when fleas are less common. New regulation should also be considered, they said, such as requiring prescriptions for the treatments and assessing the environmental risk before they are approved for use.

“When you start large scale use of any sort of pesticide, there are often unanticipated consequences,” said Goulson. “Clearly, something has gone wrong. There isn’t a regulatory process for this particular risk and clearly there needs to be.”

Matt Shardlow, at Buglife, said: “Three years have passed since we first highlighted the risk to wildlife from flea treatments and no regulatory action has been taken. The massive over-pollution of all waterbodies with fipronil is shocking and there is an urgent need for the government to ban the use of fipronil and imidacloprid as flea treatments.” He said tonnes of these insecticides were being applied to pets every year.

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