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TABLE 2 Management strategies to reduce the use of anthelmintics and reduce the parasite infection pressure on pastures.
Management strategy Recommendations
Regular removal of faeces At least twice a week during grazing period
Lowering stocking density Resting pastures
Grazing pastures with other species
PYRANTEL
Pyrantel belongs to the tetrahydropyrimidine group of anthelmintics (Horvat et al., 2012 ). The pamoate salt (also known as embonate) of pyrantel is practically insoluble in water, which offers the advantage of reduced absorption from the gastrointestinal
tract of horses
and allows the drug to reach the microenvironmental sites of the target parasites. Most of the drug passes unchanged in the faeces (Arundel, 1983 ). To the authors ' knowledge, there is only one study on the ecotoxicity and photodegradation of pyrantel embonate (Pelko et al., 2010 ). While the impact is thought to be limited, the degree of environmental contamination is unknown, and therefore, the risk of environmental harm posed by pyrantel administered to horses is difficult to quantify.
PRAZIQUANTEL
Praziquantel, a member of the hexahydropyrazine group of anthelmintics, is frequently used for the treatment of the commonest equine
tapeworm, Anoplocephala perfoliata . Praziquantel is
extensively metabolised by the liver; however, the exact chemical structure of most metabolites, including in the horse, is unknown (Horvat et al., 2012 ). Praziquantel has been shown to be photo- labile when directly exposed to sunlight and loses about 50% of its initial concentration over a period of 4 months (Horvat et al., 2012 ). It is unclear how this might translate to its rate of degradation when protected from sunlight, such as when protected within equine faeces on pasture, or in a muck heap. Very little is known of the environmental impacts of praziquan-
tel. However, in one experimental laboratory study of the effect of four anthelmintics on the dung beetle Aphodius constans , praziquan- tel showed very low toxicity compared to ivermectin and moxidectin (Hempel et al., 2006 ).
EFFECT ON PASTURE ECOLOGY
Widespread use of parasiticides to control internal and external parasites in grazing animals has raised concerns that their residues
One horse to at least 0.4–0.6 hectares (1–1.5 acres)
End of one grazing period to middle of next grazing period
Co- graze with sheep or cattle (consider liver fluke)
HASELER ET AL.
adversely affect pasture ecology (Floate et al., 2005 ; Herd, 1995 ; Lumaret & Errouissi, 2002 ). The impacts of veterinary parasiticide residues in dung on important decomposer insect communities such as dung beetles and flies are well studied (Finch et al., 2020 ; Floate et al., 2005 ; Junco et al., 2021 ). Even at sub- lethal concentrations, macrocyclic lactones and other anthelmintics can have a variety of other effects on dung- colonising insects, including loss of water bal- ance, disruption of feeding and reduced fat accumulation, delayed ovarian development, decreased fecundity and impaired mating (Strong & Wall, 1994 ; Weaving et al., 2020 ). These lethal and nonlethal effects have an impact on faecal deg-
radation of pasture. In one field trial of calves fitted with ruminal bo- luses delivering ivermectin, faeces of calves treated with ivermectin failed to degrade in the same way as faeces from untreated calves, and this failure was associated with the absence of dung- degrading insects (Wall & Strong, 1987 ). With respect to production animals (cattle and sheep), routine
use of ivermectin for treating parasitic infections is therefore likely to have a significant impact on ecosystems due to reduced faecal degradation and thus accumulation of dung on pasture, reducing the available grazing area, impacting grazing husbandry and management, and facilitating the spread of pests (including resistant populations of parasites) and disease. Beynon et al. ( 2015 ) estimated that cattle dung accumulation on pasture due to a lack of breakdown by dung beetles could account for up to 4.8% loss of permanent pasture in England each year, and that cessation of blanket treatment of cattle with an- thelmintics could save the UK cattle industry an additional £6.2 mil- lion each year due to preservation of these ecosystem services. These effects are likely to be much less significant in horses
due to the widespread implementation of pasture hygiene strate- gies (removing faeces from the pasture—‘poo- picking’). However, in systems where faeces are left on the pasture, or where faeces are incompletely removed, faecal degradation is likely to be impaired, with an associated reduction in available grazing area and increased spread of disease.
NONCHEMICAL MANAGEMENT AND THE ROLE OF PASTURE HYGIENE
A summary of the nonchemical management measures to reduce the parasite infection pressure on pastures and reduce the use of an- thelmintics is shown in Table 2 . This approach will lower the parasite burden in grazing horses and, if combined with appropriate diagnos- tics, will lead to substantial reductions in anthelmintic use, which in turn reduces both the environmental toxicity of the drugs and the selection pressures that drive anthelmintic resistance. Conditions will vary greatly between premises and obtaining basic information on the management parameters should always be performed as part of a general risk assessment before providing any advice on parasite control. Alongside the judicious use of anthelmintics, pasture hygiene (‘poo- picking’) has long been acknowledged as a highly efficient
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