ECOTOXICITY OF PARASITICIDES IN THE UK


method of controlling equine parasites (Herd, 1986a , 1986b ; Matthee et al., 2002 ; Rendle et al., 2019 ). Faeces on pasture not only harbour parasites but also act as a breeding site for flies and can drastically reduce the available grazing area. The objective of pasture hygiene is to eliminate the source of strongyle eggs before they develop into infectious third- stage larvae, thereby breaking the infection cycle. Helminth eggs are excreted into the environment via faeces and hatching of these eggs in the faeces is a significant source for re- infection. Removing infective larvae from the environment is there- fore essential to reducing pasture burden and mechanically breaking the cycle of reinfection. Strongyle eggs develop into infective third stage larvae within 3–4 days at optimal temperatures (25–33°C) (Mfitilodze & Hutchinson, 1988 ), so removal of faeces at least twice a week during the summer grazing period is recommended in tem- perate regions such as the UK; such measures have also been shown to increase the effective grazing area by approximately 50% by re- ducing the degree of separation of pastures into ‘roughs’ and ‘lawns’ (Herd, 1986a ). At lower temperatures, such as during the autumn and winter in the UK, the rate of larval development slows, with lim- ited hatching observed at 7.5–10°C, and no larval development at temperatures less than 4°C, so less frequent removal of faeces can be practised at these times. The beneficial effects of dung removal from the pasture need to


be balanced against the negative impacts on invertebrates, including dung beetles. As dung beetles prefer dung up to 48 h, uncontaminated dung can be left on pasture for 3–4 days before clearing; poo- picking twice a week during the summer should concentrate on removing the older dung piles and dung from horses that have recently received an- thelmintics, while leaving the fresher, uncontaminated faecal piles for the beneficial activities of dung beetles (Table 3 ). Pasture hygiene is not only important for control of the common-


est nematode parasites infecting grazing horses (the cyathostomins) but also for control of Paracaris species (a common parasite of foals), whose eggs are very resistant and can persist in the environment over the winter and infect the following season ' s crop of foals. Consequently, in order to reduce infectivity, wherever possible foals should not be grazed on the same pastures for consecutive seasons. The commonest equine tapeworm, A. perfoliata , differs from the


nematodes in several ways (Nielsen, 2012 ), including the involve- ment of an intermediate host and the relatively greater pathogenic- ity of the adult stages compared to the larval stages. Further studies are needed to elucidate tapeworm epidemiology in order to fully integrate tapeworms into a sustainable parasite control strategy; however, pasture hygiene is likely to be as equally important in con- trolling tapeworms as it is for nematodes. Manure/muck heaps may be contaminated with the residues of


veterinary pharmaceuticals. For example, in Australia, commercial beef feedlot manure has been found to contain parasiticides at con- centrations toxic to Scarabaeidae dung beetles and dung- breeding Diptera spp. flies and their larvae (Khan et al., 2008 ; Lumaret & Errouissi, 2002 ) as well as steroidal hormones and antibiotics. The persistence of anthelmintic drugs during manure storage, and the impact on invertebrate biodiversity after spreading of


TABLE 3 Recommendations for pasture hygiene in equine pastures in the UK.


Poo- pick twice a week during the grazing period (March–October) Poo- pick once a week during the winter (November–February)


Where feasible, remove all faeces contaminated by anthelmintics for 7–8 days following treatment


Remove older faecal piles (older than 3–4 days) and leave more recent faecal piles until the next collection


Position muck heap at least 3 m outside of the grazing area Position muck heap at least 10 m away from water courses


Cover muck heap (e.g. tarpaulin) to reduce the risk of rainwater run- off


Compost muck for at least 6 months before spreading on the fields


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manure on pastures, is not well understood. Sands and Noll ( 2022 ) investigated the persistence of ivermectin residues in aged livestock manure used as a fertiliser on fields, and the impact these residues may have on terrestrial and freshwater invertebrate biodiversity and ecosystem function. Fresh cattle dung was spiked with known con- centrations of ivermectin or an excipient only (control) and aged in the field for 4 months. Rainwater runoff from the manure contam- inated with ivermectin was highly toxic to D. magna for the entire 4 months of storage. Coleoptera spp. and Diptera spp. emergence was lower from mesocosms spread with ivermectin- contaminated ma- nure compared with control manure. Pasture productivity was sig- nificantly lower in mesocosms spread with ivermectin- contaminated manure. The results indicated that ivermectin residues with toxic effects can persist in manure for at least 4 months of storage, could reduce pasture productivity, and may have pervasive impacts on in- vertebrate biodiversity in agricultural systems. Although there are a lack of data relating to ivermectin contam-


ination in equine faeces, ivermectin is excreted in the faeces for at least 40 days following oral dosing of horses (Pérez et al., 2001 ). One study in horses investigating the breakdown of ivermectin during the composting process found an exponential decay of ivermec- tin at a rate of 1.8% per day with a half- life of 3.6 days (Schwarz & Bonhotal, 2016 ). After 175 days of composting, ivermectin concen- tration had decreased to 0.6 mg/kg in the composted material, just less than the median lethal concentration calculated for the dung beetle A. constans by Hempel et al. ( 2006 ). It should also be noted that Schwarz and Bonhotal ( 2016 ) found a reduction in potentially mineralisable nitrogen in the soil below the pile, indicating reduced microbial activity. Current UK legislation regarding storage of animal manures in


nitrate- vulnerable zones considers the risk of leaching and run- off from areas of agricultural land which drain into waters that could be polluted by nitrates. Manure should not be stored within 10 m of any surface water or 30 m if the land slopes (Defra, 2015 ). It is not clear whether this would be sufficient to prevent ivermectin resi- dues in run- off or sediment from equine muck heaps from reach- ing waterbodies. Rapid photodegradation of ivermectin has been demonstrated when it was placed outdoors in faeces/soil mixtures


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