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 parasiticides in farm animal and equine practice has led to widespread resistance, particularly within endoparasites. Whilst resistance is not currently a significant concern in UK small animal practice, overuse of these agents may lead to increased prevalence of resistant organisms and reduced efficacy of parasiticide treatments.

Parasiticides are harmful to a wide range of invertebrates. In addition, there is increasing recognition of environmental contamination from products used to treat companion animals, with potentially significant detrimental impact on wildlife ecosystems. Significant declines in terrestrial, including bee, and aquatic invertebrate populations are likely worsened by widespread environmental pesticide contamination, including from companion animal parasiticides. Environmental contamination can occur from topical products (such as seen with bathing after treatment), urine and faecal excretion, wastewater from homes and from incorrect disposal of products. Although significant uncertainty surrounds the proportional impact of companion animal parasiticides, veterinary surgeons should be mindful of these concerns and reduce use where possible.

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. Avoiding topical products in animals likely to contaminate water
• Treating parasites as needed for the individual animal, including using narrow spectrum products where appropriate
• 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 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.