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| Doramectin
Injectable doramectin (10 mg/mL solution for injection) is also used frequently off- licence in horses in the UK as a treatment for chorioptic mange (there is no licensed drug for the treatment of this condition in the horse). Rendle et al. ( 2007 ) found no difference in the efficacy of doramectin and fipronil in the treatment of this disease. Two injections of doramectin
(subcutaneous or intramuscular)
2 weeks apart are often given. As the Chorioptes bovis and Chorioptes equi (the usual causes of
chorioptic mange) are not host- specific and can survive in the envi- ronment for several weeks, these aspects should be considered by treating all in- contact animals (Gerber et al., 2023 ). Stable hygiene (such as removing bedding and decontamination of the stable sur- faces) is another important component of the control of chorioptic mange to prevent re- infection, and antiparasitic drugs should not be relied on as the sole treatment. There is evidence that doramectin is excreted in the faeces for
several weeks at potentially toxic levels following subcutaneous or intramuscular administration in cattle and sheep (Summary of Product Characteristics data sheet for Dectomax®). Furthermore, Suarez et al. ( 2003 ) reported the persistence of doramectin for up to 180 days in faeces of cattle following intramuscular administration, with concentrations lethal to dung fauna during this time. In horses, following intramuscular injection (0.2 mg/kg bwt), the peak faecal concentration was detected at 5.6 days (Pérez et al., 2010 ), and the drug was detected in the faeces for 60 days (sampling was terminated at 60 days). As with ivermectin, doramectin is highly toxic to fish and invertebrates and, even at low levels, has negative effects on co- prophagous invertebrates such as dung beetles (Erzen et al., 2005 ). It is therefore likely that systemic administration of doramectin to horses results in significant environmental contamination via faeces and associated ecotoxicity, with effects on biodiversity that may be equally as significant as oral ivermectin. As with oral endoparasite treatments, this environmental contamination could be reduced by regularly removing faeces from the pasture. Injectable formulations of ivermectin and moxidectin have been
used to treat chorioptic mange in horses, since oral dosing is less effective. Similar issues with regards to excretion and ecotoxicity of the injectable preparations apply. Moxidectin, doramectin and ivermectin are also frequently applied topically to horses with this problem (Brys et al., 2023 ), with the associated risks of direct envi- ronmental contamination.
Pyrethrins
Cypermethrin and permethrin are synthetic pyrethrins with a wide range of uses including application to arable crops, wool processing, livestock treatment and wood preservation. Cypermethrin sheep dips have not been available since 2010. However, these agents are commonly administered topically to horses to treat lice and repel biting flies (such as horn flies, face flies, stable flies, house flies, horse
HASELER ET AL.
flies, black flies, etc.), and also for the management of insect- bite hypersensitivity (‘sweet itch’, ‘summer eczema’). In most cases, these drugs are marketed as fly repellants for use on the premises around horses rather than authorised for use on horses themselves, so they are most commonly used ‘off- licence’; there are, however, a few formulations of cypermethris and permethrin that are authorised for the control of lice and flies (including the control/prevention of sweet itch) in horses. They are also used off licence for the treatment of chorioptic mange. One study in cattle demonstrated reduced reproductive per-
formance of the dung beetle Onthophagus similis on farms that use synthetic pyrethroids compared to those that do not (Weaving et al., 2020 ). Pyrethrins have very limited toxicity to birds and earth- worms but are highly toxic to bees, fish and aquatic invertebrates. The Environment Agency highlights the concern associated with contamination of water courses, even with small amounts of cyper- methrin (Environment Agency, 2021 ). Cypermethrin and permethrin are not very persistent in the en-
vironment and are not easily leached from the soil. Permethrin is a type 1 pyrethroid, which breaks down in the presence of sunlight much faster than cypermethrin, a type 2 pyrethroid. The degrada- tion half- life is 1 day in sunlight and 13 days in soil for permethrin and 7.8 days in sunlight and 22 days in soil for cypermethrin (Lewis et al., 2016 ). For this reason, permethrin may have a lesser ecotoxic effect than cypermethrin. The main environmental concern of using pyrethrins in livestock
is water contamination, most likely due to spillages and run- off from hard standing where treatment takes place; a similar situation probably exists in equids, although there is no published informa- tion that documents this route of environmental contamination. As with fipronil, treated horses should not be washed or hosed down following treatment, although there is no clear guidance on how long this should apply. Any spills should be mopped up with dispos- able material rather than washed down the drain. Packaging should not be rinsed out and should be disposed of carefully. Disposable gloves should be worn during application, reducing contamination of wastewater from handwashing. A summary of recommendations regarding ectoparasiticide use in horses is shown in Table 5 .
DISPOSAL OF USED/UNUSED/ OUT- OF- DATE MEDICATIONS
The use of veterinary medicines such as anthelmintics inevitably generates waste, the inappropriate disposal of which can have a detrimental impact on environmental, animal and human health (Murphy, 2022 ). When not disposed of correctly, drug residues pre- sent in the immediate packaging of veterinary medicinal products can leach out into the environment (e.g. via domestic rubbish or when poured down the drain). Both prescribers and users of veteri- nary medicinal products are responsible for this waste management, which is becoming a complex environmental issue as the number
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