Abstract
Whilst there are limited data on the environmental impact of
administering parasiticide drugs to horses, evidence from other species
treated with identical drugs indicates significant negative ecological
effects. Anthelmintic drugs may be excreted unchanged or metabolised to
other active and/or toxic metabolites that enter the environment through
direct excretion during grazing. These chemicals can have significant
toxic effects on non-target insects, such as dung beetles, earthworms
and aquatic animals. Of the anthelmintics frequently used in horses,
ivermectin appears to be the most ecotoxic; available evidence indicates
that moxidectin is less toxic. Fenbendazole appears to have little
impact on dung-colonising insects but may be toxic to aquatic organisms
and fungi. There is very 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 use of endoparasiticides, along
with pasture hygiene measures, will limit the ecotoxic effects of these
drugs and reduce the selection pressure that drives anthelmintic
resistance. Anthelmintics may also impact the equine gastrointestinal
microbiota.Following topical treatment with ectoparasiticides (such as
fipronil, permethrin or cypermethrin), there is a risk of contamination
of both the immediate environment and water courses. The half-life of
fipronil in the environment is variable, but it degrades into compounds
which are more toxic; 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.