Veterinary Parasiticides (Imidacloprid and Fipronil): Contaminants of Concern

In addition to nutrients and microbial indicators in our ponds and streams, there is growing concern about a class of contaminants that are not routinely monitored in standard water quality programmes: veterinary parasiticides used in companion-animal flea and tick treatments.

Two of the most widely used active ingredients, imidacloprid (a neonicotinoid) and fipronil (a phenylpyrazole insecticide), are of particular interest because of their high toxicity to invertebrates and increasing detection in UK surface waters.

Remember Friends of the Earth’s Bee Cause campaign? Recall the outrage at widespread public information about bee deaths being attributed to insecticides. Both compounds were previously subject to regulatory restriction in agricultural contexts because of their demonstrated impacts on pollinators and aquatic ecosystems. However, their continued use in veterinary products, particularly in over-the-counter spot-on treatments and collars for cats and dogs, has created a diffuse but significant pathway into the environment.

Unlike agricultural applications, which are constrained, cat and dog treatments are applied continuously by their owners, resulting in chronic, low-level release.

Environmental entry pathways include the washing of treated animals and bedding, direct transfer during swimming, and runoff of residues shed from fur and skin. Recent UK monitoring studies have reported widespread detection of these compounds in rivers and streams, frequently at concentrations exceeding established ecotoxicological thresholds for aquatic invertebrates. These organisms, including mayflies, caddisflies, and other key taxa, are highly sensitive to toxic insecticides and form the foundation of freshwater food webs. Rest assured, we have seen plenty of mayflies caddis flies and dragonflies in our New Forest waters!

Detection of imidacloprid and fipronil requires advanced analytical techniques such as liquid chromatography or tandem mass spectrometry, These methods operate at parts-per-trillion sensitivity and are not accessible within the scope of our monitoring programme. However, as someone has pointed out, we cannot really claim that a pond that is free of nutrients such as phosphate and nitrates and free from bacterial contamination is ‘clean’ if it contains insecticides. As such, the absence of direct measurements in our analysis should not be interpreted as absence of risk.

Instead, we can only consider the potential presence and impact of these compounds through the interpretation of biological indicators. We could monitor aquatic invertebrates to determine whether there are reductions in sensitive invertebrate groups, or shifts toward more pollution-tolerant assemblages that may reflect exposure to these neurotoxic contaminants.

The issue highlights a broader limitation in current monitoring frameworks such as ours, where analytical and resource constraints prevent routine assessment of emerging contaminants despite increasing evidence of ecological relevance. New Forest Water Watch relies entirely on volunteers, not salaried professionals. They give of their time freely and generously to the cause of ensuring good water quality in the New Forest.

Emerging policy responses focus not only on regulatory restriction, but also on reformulation of products, changes in veterinary prescribing practices, and reduction of routine prophylactic (preventative) use.

Ongoing national-level investigations and policy discussions may lead to future changes in the regulation and monitoring of veterinary parasiticides. Meanwhile, their contribution to diffuse chemical pressure on freshwater ecosystems can only be acknowledged as part of our interpretation of water quality data.

That said, it is important to recognise that our work is being carried out carefully, transparently and within the limits of the methods and resources available to us. The absence of specialist pesticide analysis is a limitation of scope, not of commitment.  What word would you use to describe water that is free from nutrients and bacterial contamination, one that replaces the word ‘clean’? If you have a suggestion, we would love to hear it.  What we are able to show is already genuinely excellent: a clear, volunteer-led picture of local water quality that is honest about its boundaries, scientifically useful, and valuable for informing public understanding and future action