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Pet flea treatments cause toxic chemical pollution in English rivers

November 17, 2020
Image: Nathan Rupert | Flickr Creative Commons

Toxic pesticides found in veterinary flea treatments used on domestic cats and dogs have been detected at potentially harmful levels in English rivers. Researchers have found widespread contamination of two neurotoxic chemicals – fipronil and the neonicotinoid imidacloprid – in 20 sampled rivers, from the Test on the south coast to the Eden in Cumbria. Fipronil was found in 98% of samples, and the average level of its highly toxic breakdown product fipronil sulfone was 38 times above the recommended environmental safety limit.

Both fipronil and imidacloprid are banned for agricultural use due to their harmful environmental effects, including significant reductions in populations of both aquatic and pollinating insects and disruptions to the food webs that depend on them. However, there has been little attention given to the environmental impacts of their use in veterinary flea treatments for cats and dogs. The researchers behind the new study, published in the journal Science of the Total Environment, aimed to address this shortfall by analysing river water samples collected by the Environment Agency between 2016 and 2018.

“The use of pet parasite products has increased over the years, with millions of dogs and cats now being routinely treated multiple times per year,” said lead author Rosemary Perkins, a PhD researcher at the University of Sussex, and a qualified vet. “Fipronil is one of the most commonly used flea products, and recent studies have shown that it degrades to compounds that are more persistent in the environment, and more toxic to most insects, than fipronil itself. Our results, showing that fipronil and its toxic breakdown products are present in nearly all of the freshwater samples tested, are extremely concerning.”

There are 66 licensed veterinary flea treatment products containing fipronil and 21 containing imidacloprid available in the UK. There are around 10 million dogs and 11 million cats in the UK, and the new study suggests that the widespread use of routine flea treatment products has the potential to significantly affect the health of aquatic life in English freshwaters.

“Fipronil and imidacloprid are both highly toxic to all insects and other aquatic invertebrates,” said co-author Professor Dave Goulson from the University of Sussex. “Studies have shown both pesticides to be associated with declines in the abundance of aquatic invertebrate communities. The finding that our rivers are routinely and chronically contaminated with both of these chemicals and mixtures of their toxic breakdown products is deeply troubling.” Speaking to The Guardian, Professor Goulson added that one imidacloprid-based flea treatment for a medium-sized dog contains enough pesticide to kill 60 million bees.

The researchers found that the highest levels of pollution were found immediately downstream of wastewater treatment works. They suggest that this supports the hypothesis that significant quantities of environmentally-harmful pesticides are entering rivers via the washing of treated pets, their bedding, and other surfaces they come into contact with. It is also possible that swimming and rainfall wash-off from treated pets could be additional pathways for the flea treatment pesticides to reach waterways.

“We’ve identified a number of steps that can be taken to minimise or avoid environmental harm from pet flea and / or tick treatments,” outlined Rosemary Perkins. “These range from introducing stricter prescription-only regulations, to considering a more judicious and risk-based approach to the control of parasites in pets, for example by moving away from blanket year-round prophylactic use. We’d recommend a re-evaluation of the environmental risks posed by pet parasite products, and a reappraisal of the risk assessments that these products undergo prior to regulatory approval.”


Perkins R, et al (2020), “Potential role of veterinary flea products in widespread pesticide contamination of English rivers”, Science of The Total Environment,

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