Researchers advise against using spot-on flea and tick treatments if your dog has access to any water bodies.

Studies indicate that dogs submerged in water can release harmful levels of active ingredients into aquatic ecosystems, affecting wildlife and animals that consume them, including birds, for up to 28 days post-treatment.

“If your dog frequently swims, it’s best to avoid spot-on treatments,” says Rosemary Perkins from the University of Sussex, UK.

When spot-on treatments were introduced, it was mistakenly believed they posed no threat to the environment. It wasn’t until 2011 that a European Medicines Agency paper suggested protecting animals from water for 48 hours, yet Perkins notes that there was no substantial evidence backing this guideline. “It seems to be just a finger-in-the-air estimate,” she says.

Her concern grew after detecting fipronil, a pesticide used in these treatments, in a UK river. “We noted alarmingly high concentrations,” Perkins recalls.

Her team administered spot-on treatments containing either fipronil or neonicotinoids like imidacloprid to 25 and 24 dogs, respectively. After 5, 14, or 28 days, dogs were immersed up to their shoulders in a plastic tub for 5 minutes to measure pesticide levels in the water.

The findings revealed that even after 28 days, a single large dog’s chemical residue could exceed safe levels in 100 cubic meters of water—similar to the volume of a small pond. Regular swimming by treated dogs can further escalate contamination in larger bodies of water, warns Perkins.

She advocates for regulatory changes globally but fears that such adjustments may require significant time. In the meantime, dog owners should limit spot-on treatments to necessary occasions and keep their pets away from water for at least a month following treatment. “The key takeaway is that if your dog swims during this period, there are associated risks,” she emphasizes.

Currently, alternatives exist in the form of oral tablets, but Perkins remains uncertain of their environmental impact, noting that the active ingredients can persist in feces and contaminate soil. “We still lack clarity on their effects.”