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in the summer and, under these conditions, it may have a half- life of 14 days or less (Halley et al., 1989 ). In winter, however, the half- life can be as long as 217 days and, when stored as a muck heap, the ma- nure is only in contact with sunlight at the surface, thereby limiting the extent of photodegradation. Little is known about the potential for animal anthelmintics to
contaminate groundwater. Mooney et al. ( 2021 ) conducted a study in the Republic of Ireland to explore this and found anthelmintic residues at 8% of groundwater sites and 28% of surfacewater sites, with albendazole (most commonly used in cattle) being the most fre- quently detected residue. Higher levels were identified in February/ March and August/September, coinciding with periods of increased use of anthelmintics in livestock. The stocking density also has a major effect on the levels of
pasture contamination by equine parasites, and therefore, the fre- quency of anthelmintics required. Although one horse per 0.4–0.6 ha of permanent grazing (1–1.5 acres/horse) is considered appro- priate (Matthews, 2024 ), this is often not achieved. Several other factors also affect the impact of stocking rate, including whether or not dung is removed and how frequently and effectively this is undertaken; the age, size and number of horses that graze the pas- ture; the amount of time that horses spend on the pasture; the sea- son; and how well the pasture is managed (Singer et al., 2002 ). A recent study (Joó et al., 2022 ) found that strongyle egg shedding levels of horses kept at high stocking densities (>30 horses/ha) were significantly higher than in horses kept at low (1–2 horses/ha) or at moderate (3–10 horses/ha) densities. There is, however, a lack of published data on associations between stocking density, pasture hygiene measures and pasture infectivity and how these relate to anthelmintic requirements and ecotoxicity.
THE ROLE OF TARGETED ANTHELMINTIC TREATMENT
In horses, the concept of targeted worming based on a risk assess- ment and only treating horses with faecal strongyle worm egg counts above a predetermined threshold (e.g., 200 eggs/g) is universally accepted (Gomez & Georgi, 1991 ; Kaplan & Nielsen, 2010 ; Rendle et al., 2019 ). This approach takes account of the over- dispersed na- ture of intestinal helminths in their hosts, particularly in adult animals, where the majority of the burden is harboured by a small proportion of the population. Regular (interval) blanket treatments are no longer recommended as these exert a strong selection pressure for anthel- mintic resistance (Pfister & van Doorn, 2018 ). Leathwick et al. ( 2019 ) suggest that in order to delay the development of anthelmintic re- sistance in cyathostomins, in most situations, the average number of treatments administered annually across a herd of horses needs to be two or fewer per horse. Reducing the number of parasites exposed to anthelmintics and maintaining a population of parasites unexposed to anthelmintics (refugia) slows the development of anthelmintic resist- ance (Kenyon et al., 2009 ; Leathwick et al., 2019 ; Van Wyk, 2001 ), reduces the impacts of drug residues on nontarget invertebrates and
HASELER ET AL.
prevents a cycle of increasing demand for anthelmintics as their ef- ficacy is compromised. Currently, there remains a large discrepancy between the num-
ber of doses of anthelmintic sold and the number of faecal worm egg counts performed, with sales data from the UK between 2015 and 2018 suggesting that approximately one test was performed per 11 anthelmintic doses sold (Rendle et al., 2021 ). However, if the decision to administer an anthelmintic was being made appropriately, based on a risk assessment and targeted approach, it would be logical to expect more than two faecal tests performed for each anthelmintic dose administered (Rendle et al., 2021 ). There is therefore consid- erable scope to make equine anthelmintic treatment more targeted, and future work should focus on how this might be achieved. A sum- mary of recommendations regarding endoparasiticide use in horses is shown in Table 4 .
ANTHELMINTICS AND THE EQUINE GASTR OIN TES TINAL MICROBIOTA
An additional area of concern over excessive/indiscriminate use of anthelmintics is the effect on the equine gastrointestinal mi- crobiota. The interaction between parasitic helminths and gas- trointestinal microbiota is an important,
although incompletely
understood, factor in the regulation of immunity, inflammation and a range of diseases (Boisseau et al., 2023 ). Infection with intestinal helminths is ubiquitous in grazing horses, with cyathostomins pre- dominating (Love et al., 1999 ). The interaction between parasites and the gut microbiome can be altered by anthelmintic treatment, a phenomenon which was initially explored in horses by Goachet et al. ( 2004 ), who reported a reduction in cellulolytic bacteria and caecal pH followed by an increase in Lactobacilli and Streptococci after treatment with moxidectin. Subsequent studies have con- firmed significant alterations in the equine micobiota and microbi- ome following the administration of anthelminitics, although with individual variations (Daniels et al., 2020 ; Kunz et al., 2019 ; Peachey et al., 2018 ; Sirois, 2013 ; Walshe et al., 2019 ). The interplay between resident cyathostomin populations and the bacterial microbiota of the equine large intestine is not only important in maintaining ho- meostasis but, in addition, disturbance of this ecology can lead to gut dysbiosis and may play a role in the aetiology of inflammatory conditions in the horse, including acute larval cyathostominosis. It is noteworthy that anthelmintic administration has previously been identified as a risk factor for the development of acute larval cy- athostominosis (Lawson et al., 2023 ; Reid et al., 1995 ).
THE ENVIRONMENTAL IMPACT OF ECTOP ARA SIT ICI DES
Less information is available on the use of ectoparasiticides in horses; however, blanket treatment of at- risk animals is much less commonly performed in horses compared to small animals. Resistance is not
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