ECOTOXICITY OF PARASITICIDES IN THE UK
TABLE 4 Summary of recommendations regarding endoparasiticide use in horses.
1 . Judicious use of endoparasiticides will help limit the ecotoxic effects of these drugs, as well as reducing the selection pressure which drives anthelmintic resistance
2 . Adoption of diagnostic- led (targeted) treatment programmes will help identify at- risk/high- shedding horses and therefore minimise unnecessary use of anthelmintics
3 . Ivermectin is the most toxic anthelmintic to natural invertebrates 4 . Moxidectin is less toxic than ivermectin
5 . Fenbendazole appears to have little impact on dung- colonising insects but may be toxic to aquatic organisms and fungi
6 . There is insufficient evidence to draw any conclusions regarding the ecotoxicity of pyrantel and praziquantel, although the ecotoxic effects are thought to be low
7 . Pasture hygiene (‘poo- picking’) reduces pharmaceutical contamination of pasture and also helps to break the endoparasitic cycle of infectivity, thus reducing reliance on anthelmintics
8 . Anthelmintics degrade in muck heaps over time, and dung beetles are not generally attracted to muck heaps in the as same way they are to faeces in the field a. Best practice is to put faeces onto a muck heap and allow it to decompose
b. Current evidence suggests that faeces should be stored for at least 6 months, or longer if possible, before being spread on fields as fertiliser
9 . Muck heaps should be situated as far as possible from water courses/ponds/drains (at least 10 m on flat ground, 30 m if the land slopes) and, if possible, covered with a roof/tarpaulin to reduce the risk of run- off
10 . Anthelmintics may also alter the equine gastrointestinal microbiota
yet widely reported in equine ectoparasites; however, resistance to ivermectin and synthetic pyrethroids has been widely documented in the cattle tick, Rhipicephalus microplus , in Asia and South America (Rodríguez- Vivas et al., 2014 ; Sindhu et al., 2022 ). As with endoparasite treatment, a targeted approach is advised
in order to reduce selective pressure for resistance genes as well as decrease the effect on nontarget invertebrates. Specific environ- mental concerns, and actions to mitigate harm, are detailed below.
Fipronil
Fipronil belongs to the fiprole group of insecticides, which disrupt the central nervous system in insects via interference with the gamma- aminobutyric
acid- regulated chloride channel
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more persistent in the environment and more toxic than fipronil itself (Tingle et al., 2003 ). Fipronil is highly toxic to bees, both via contact and ingestion, and its agricultural use is thought to have been responsible for the reduction in honey bees in the 1990s (Holder et al., 2018 ). Fipronil was banned for agricultural use in 2017. Fipronil has variable toxicity to noninsects, with widely varying toxic doses reported in animals within the same taxonomic group. It is reported to bioaccumulate in fish and can be toxic to gallinaceous birds (Jackson et al., 2009 ; Tingle et al., 2003 ). Although it is not authorised for use in equids, topical fipronil
is commonly used off- licence in horses as a treatment for choriop- tic mange (‘leg mange’) (Gerber et al., 2023 ), as there are no drugs authorised for use in this disease in the horse. Dermatitis of the cannon and pastern regions is commonly encountered in feathered horses incuding Cobs and Heavy Horses, and a proportion of these animals will be affected by chorioptic mange (‘feather mites’) (Gerber et al., 2023 ; Yu, 2013 ). The environmental impact of the veterinary use of fipronil in dogs and cats has received recent media attention with a joint policy statement from the BVA, BSAVA and BVZS issued in 2021 (BVA, 2021 ) urging an end to blanket use in small animals. In one study in the UK, fipronil and imidacloprid were found in 99% of river samples, and it has been suggested that small animal pets may contribute to this contamination through several routes such as dogs swimming in rivers or owners washing their hands after application of topical treatments (Preston- Allen et al., 2023 ). Traces of these com- pounds have also been isolated in birds, with the source thought to be from the collecting of dog hair for their nests (Diepens et al., 2023 ). Fipronil is relatively immobile in soil and has a low potential to leach
into groundwater (Tingle et al., 2003 ). Despite this, Perkins et al. ( 2021 ) analysed water in English waterways between 2016 and 2018 and de- tected fipronil in 98.6% of the 3861 samples tested. The mean concen- tration of fipronil was over 5 times the chronic toxicity limit, while the mean concentration of the persistent metabolite, fipronil sulfone, was over 38 times the chronic toxicity limit, indicating a high environmental risk to aquatic ecosystems. In this study, the highest levels were detected downstream of wastewater treatment works, suggesting washing of treated pets and pet bedding may well be a source of contamination, in addition to direct entry into waterways through swimming. The environmental impact of fipronil use in horses is difficult
(Hainzl
et al., 1998 ). First introduced in the 1990s for use in agriculture, these compounds have several properties that make them extremely effective insecticides, including high toxicity towards a wide range of invertebrate species and high environmental persistence (Perkins, 2020 ). Although the half- life of fipronil in the environment is very variable (36 h to 7.3 months) depending on conditions, it degrades into compounds (such as fipronil desulfinyl) which are
to quantify given the lack of data on equine ectoparasiticide sales and use. However, given its toxicity for bees and other insects, as well as the relatively high volumes needed to treat the equine limbs, its use in horses is likely to have a significant environmental cost. If possible, the use of fipronil in horses should be avoided. If there is no other appropriate option, it would be prudent to take measures to minimise contamination to both the immediate environment as well as contamination of water sources. It is advisable to avoid wash- ing horses after topical application of fipronil, although there is no clear evidence for how long washing should be avoided. Any spills should be mopped up with disposable material rather than washed down the drain. Packaging should not be rinsed out and should be disposed of carefully. Disposable gloves are advisable during appli- cation, reducing contamination of wastewater from handwashing.
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