Environmental impacts of equine parasiticide treatment: The UK perspective

Abstract

SummaryWhile 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.