Received: 8 August 2023 | Accepted: 21 December 2023 DOI: 10.1111/eve.13944
R E V I EW A R T I C L E
Environmental impacts of equine parasiticide treatment: The UK perspective
Callum J. Haseler 1 | Julia L. Shrubb 2 | Hannah G. D. Davies 3 | David I. Rendle 4 Polly C. Rathbone 5 | Timothy S. Mair 6
1 Rossdales Equine Hospital , Newmarket , UK 2 Ashbrook Equine Hospital , Allostock , UK 3 School of Veterinary Medicine , University of Surrey , Guildford , UK 4 EMT Consulting Ltd , Tiverton , UK 5 Blackdown Equine Clinic , Haslemere , UK 6 Bell Equine Veterinary Clinic , Maidstone , UK
Correspondence: Timothy Mair Email:
tim.mair@
btinternet.com
Summary While there are limited data on the environmental impact of administering equine parasiticide drugs, evidence from other species indicates significant negative ecological effects. Anthelmintic drugs are excreted unchanged or metabolised to other active and/or toxic metabolites that enter the environment through direct excretion. These chemicals can have significant toxic effects on insects, such as dung beetles, earthworms and aquatic animals. Of the frequently used equine anthelmintics, ivermectin is the most ecotoxic; available evidence indicates that moxidectin is less toxic and fenbendazole appears to have little impact on dung- colonising insects but may be toxic to aquatic organisms and fungi. There are little data regarding the ecotoxicity of pyrantel and praziquantel, although their ecotoxic effects are thought to be low. Pasture hygiene reduces pharmaceutical contamination and also helps to break the endoparasitic cycle of infectivity, thus reducing reliance on anthelmintics. Judicious and targeted use of endoparasiticides, along with pasture hygiene measures, will limit the ecotoxic effects of these drugs as well as reduce the selection pressure that drives anthelmintic resistance. Anthelmintics may also negatively impact the equine gastrointestinal microbiota. Ectoparasiticides (such as fipronil, permethrin and cypermethrin) may also have significant negative ecological effects, with a risk of contamination of both the immediate environment and water courses following topical use of these drugs. The half- life of fipronil in the environment is variable, but it degrades into compounds which are more toxic; for example, it is highly toxic to bees and is reported to bioaccumulate in fish and can be toxic to birds. Of the synthetic pyrethroids, permethrin degrades at a faster rate than cypermethrin and may therefore have a lower ecotoxic effect. The ecotoxic effects of injectable doramectin are likely to be similar to oral ivermectin, although persistence in faeces may be significantly prolonged compared to the oral treatment route for ivermectin.
INTRODUCTION
There is growing concern over the environmental impact of veteri- nary parasiticide treatments. Relatively little research has been pub- lished, and the studies that do exist have tended to focus on small animals and livestock. However, the scale of parasiticide use in the equine industry merits further consideration of the associated envi- ronmental impacts, and knowledge of the underlying principles will help to reduce this impact in clinical practice.
Equine Vet Educ. 2024;36:381–392.
THE SCALE OF ANTHELMINTIC USE IN HORSES IN THE UNITED KINGDOM
The British Equestrian Trade Association ' s 2019 National Equestrian Survey estimated that there are 847,000 horses in the UK (BETA, 2019 ), and data collected by the British Equine Veterinary Association (BEVA) working group on anthelmintic use suggested that approximately 1.1 million equine anthelmintic doses were sold in the UK in 2018 (Rendle et al., 2021 ). To the authors ' knowledge,
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