A new study reveals that handwashing in the weeks after spot-on flea and tick treatments is the largest source of pet pesticide pollution in rivers.

The study’s authors, from the University of Sussex and Imperial College London, are calling for a review of the regulatory framework and prescribing practices to address toxic pet pesticides washing into rivers.

Fipronil and imidacloprid are highly toxic pesticides that are no longer approved for use in outdoor agriculture but continue to be widely used in pet flea treatments, typically applied to the back of the pet’s neck (known as spot-ons).

The researchers found that wastewater from sewage treatment works is a major source of fipronil and imidacloprid pollution in rivers. They conclude that pesticides used in flea products on domestic pets are washing down-the-drain from households, in concentrations exceeding safe limits for wildlife.