When dogs given spot treatments for fleas go swimming, they release levels of pesticides dangerous to aquatic life for at least a month after the treatment
By Michael Le Page
2 June 2025
Dogs treated for fleas release insect-killing chemicals into water when they swim
Natalya_Maisheva/Shutterstock
If your dog will jump in the nearest river, pond or lake given half a chance, don’t use spot-on treatments for fleas and ticks, say researchers.
A study has shown that when dogs are immersed in water, their skin and fur can release levels of the active ingredients harmful to aquatic wildlife and the animals that eat them – including birds – for up to 28 days after treatment.
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“If your dog swims regularly, you shouldn’t be treating it with spot-on,” says Rosemary Perkins at the University of Sussex, UK.
When spot-on treatments were first introduced, they were wrongly assumed not to have any consequences for the wider environment. Only in 2011 did a European Medicines Agency paper suggest animals be kept out of water for 48 hours afterwards, and this suggestion was not based on any experiments, says Perkins. “I could find absolutely no supporting evidence for that. It’s just a thumb-suck figure.”
She started to suspect there was an issue after finding fipronil, one of the pesticides used in spot-on treatments, in rivers in the UK. “We found astonishingly high levels,” says Perkins.