Parasiticides are essential medicines for the treatment of animal ectoparasites (e.g. ticks, fleas, mites) and endoparasites (e.g. tapeworms, roundworms, protozoa), and for prophylaxis both in the UK and in travelling pets. These parasites can have direct animal health and welfare impacts, indirect impacts through the transmission of disease, and pose zoonotic risks. In some cases, parasiticide treatment is legally mandated (e.g. tapeworm treatment of travelling pets).

Overuse of parasiticide products can lead to resistance developing in target parasites, and in grazing animals, resistance, to anthelmintics used to treat endoparasites in particular, is serious and increasing. In contrast, parasiticide resistance in UK small animal practice is not currently considered a major concern, although limited research has been conducted in this area. Nevertheless, overuse of these agents may lead to increased prevalence of resistant organisms and reduced efficacy of parasiticide treatments.

Parasiticides are pesticides and therefore toxic to a wide range of non-target invertebrates. There is increasing recognition of environmental contamination from parasiticides used to treat companion animals, posing risks to wildlife and ecosystems. Such contamination may occur through transfer of active ingredients from hair and skin (e.g. down-the-drain passage to wastewater following the bathing of topically treated animals) or through urine and faecal excretion after systemic absorption. Veterinary surgeons should be mindful of these concerns, reduce use where possible, and encourage responsible use to limit environmental exposure.

Recommendations for responsible parasiticide use include:

• Prescribing parasiticide treatments based on risk rather than providing blanket treatment. This should take account of health risks (animal, human and environmental), lifestyle and environmental factors, season, geographical prevalence and the results of parasite testing (e.g. faecal examination)
• Ensuring correct dosing and application, as well as avoiding spillage
• Disposing of unused products, packaging and faeces from treated animals responsibly
• Avoiding bathing or swimming after topical application. Where possible and reasonable, avoiding topical products in animals likely to enter waterways
• Treating parasites as needed for the individual animal, including using narrow spectrum products where appropriate and available
• Encouraging targeted treatment through laboratory testing (e.g. faecal examination) and owner recognition of parasites (e.g. via regular examination for fleas, and for ticks after walks in high prevalence areas). Early treatment of parasites with a significant domestic environmental lifecycle (e.g. fleas) is likely to result in the use of less parasiticide than if severe infestations are allowed to develop
• Considering non-pharmacological interventions to reduce preventative treatment requirements (such as vacuuming and cleaning of bedding, regular bathing of untreated animals, removing faeces from the environment, and avoiding scavenging, hunting or raw feeding)
• Educating owners about parasites and responsible parasiticide use to ensure adherence to guidance and early recognition of parasites.